# 27 SWIG and Java

- Overview
- Preliminaries
  - Running SWIG
  - Additional Commandline Options
  - Getting the right header files
  - Compiling a dynamic module
  - Using your module
  - Dynamic linking problems
  - Compilation problems and compiling with C++
  - Building on Windows
    - Running SWIG from Visual Studio
    - Using NMAKE
- A tour of basic C/C++ wrapping
  - Modules, packages and generated Java classes
  - Functions
  - Global variables
  - Constants
  - Enumerations
    - Anonymous enums
    - Typesafe enums
    - Proper Java enums
    - Type unsafe enums
    - Simple enums
  - Pointers
  - Structures
  - C++ classes
  - C++ inheritance
  - Pointers, references, arrays and pass by value
    - Null pointers
  - C++ overloaded functions
  - C++ default arguments
  - C++ namespaces
  - C++ templates
  - C++ Smart Pointers
    - The shared_ptr Smart Pointer
    - Generic Smart Pointers
- Further details on the generated Java classes
  - The intermediary JNI class
    - The intermediary JNI class pragmas
  - The Java module class
    - The Java module class pragmas
  - The Java constants interface
    - The Java constants interface pragmas
  - Java proxy classes
    - Memory management
    - Inheritance
    - Proxy classes and garbage collection
    - The premature garbage collection prevention parameter for proxy class marshalling
    - Single threaded applications and thread safety
  - Type wrapper classes
  - Enum classes
    - Typesafe enum classes
    - Proper Java enum classes
    - Type unsafe enum classes
  - Interfaces
- Cross language polymorphism using directors
  - Enabling directors
  - Director classes
  - Overhead and code bloat
  - Simple directors example
  - Director threading issues
  - Director performance tuning
  - Java exceptions from directors
    - Customizing director exceptions
  - Accessing virtual protected methods
  - Accessing non-virtual protected members
- Common customization features
  - C/C++ helper functions
  - Class extension with %extend
  - Class extension with %proxycode
  - Exception handling with %exception and %javaexception
  - Method access with %javamethodmodifiers
  - Java begin
- Tips and techniques
  - Input and output parameters using primitive pointers and references
  - Simple pointers
  - Wrapping C arrays with Java arrays
  - Unbounded C Arrays
  - Passing a string with length
  - Overriding new and delete to allocate from Java heap
- Java typemaps
  - Default primitive type mappings
  - Default typemaps for non-primitive types
  - Sixty four bit JVMs
  - What is a typemap?
  - Typemaps for mapping C/C++ types to Java types
  - Java typemap attributes
  - Java special variables
  - Typemaps for both C and C++ compilation
  - Java code typemaps
  - Director specific typemaps
- Typemap Examples
  - Simpler Java enums for enums without initializers
  - Handling C++ exception specifications as Java exceptions
  - NaN Exception - exception handling for a particular type
  - Converting Java String arrays to char **
  - Expanding a Java object to multiple arguments
  - Using typemaps to return arguments
  - Adding Java downcasts to polymorphic return types
  - Adding an equals method to the Java classes
  - Void pointers and a common Java base class
  - Struct pointer to pointer
  - Memory management for accessing member variables
    - Member variables via accessor methods
    - Direct access to member variables
  - Memory management for objects passed to the C++ layer
  - Date marshalling using the javain typemap and associated attributes
- Living with Java Directors
- Odds and ends
  - JavaDoc comments
  - Functional interface without proxy classes
  - Using your own JNI functions
  - Performance concerns and hints
  - Debugging
- Java Examples

This chapter describes SWIG's support of Java. It covers most SWIG features, but certain low-level details are covered in less depth than in earlier chapters.

## 27.1 Overview

The 100% Pure Java effort is a commendable concept, however in the real world programmers often either need to re-use their existing code or in some situations want to take advantage of Java but are forced into using some native (C/C++) code. The Java extension to SWIG makes it very easy to plumb in existing C/C++ code for access from Java, as SWIG writes the Java Native Interface (JNI) code for you. It is different to using the 'javah' tool as SWIG will wrap existing C/C++ code, whereas javah takes 'native' Java function declarations and creates C/C++ function prototypes. SWIG wraps C/C++ code using Java proxy classes and is very useful if you want to have access to large amounts of C/C++ code from Java. If only one or two JNI functions are needed then using SWIG may be overkill. SWIG enables a Java program to easily call into C/C++ code from Java. Historically, SWIG was not able to generate any code to call into Java code from C++. However, SWIG now supports full cross language polymorphism and code is generated to call up from C++ to Java when wrapping C++ virtual methods via the director feature.

Java is one of the few non-scripting language modules in SWIG. As SWIG utilizes the type safety that the Java language offers, it takes a somewhat different approach to that used for scripting languages. In particular runtime type checking and the runtime library are not used by Java. This should be borne in mind when reading the rest of the SWIG documentation. This chapter on Java is relatively self contained and will provide you with nearly everything you need for using SWIG and Java. However, the "SWIG Basics" chapter will be a useful read in conjunction with this one.

This chapter starts with a few practicalities on running SWIG and compiling the generated code. If you are looking for the minimum amount to read, have a look at the sections up to and including the tour of basic C/C++ wrapping section which explains how to call the various C/C++ code constructs from Java. Following this section are details of the C/C++ code and Java classes that SWIG generates. Due to the complexities of C and C++ there are different ways in which C/C++ code could be wrapped and called from Java. SWIG is a powerful tool and the rest of the chapter details how the default code wrapping can be tailored. Various customisation tips and techniques using SWIG directives are covered. The latter sections cover the advanced techniques of using typemaps for complete control of the wrapping process.

## 27.2 Preliminaries

SWIG 1.1 works with JDKs from JDK 1.1 to JDK1.4 (Java 2 SDK1.4) and should also work with any later versions. Given the choice, you should probably use the latest version of Sun's JDK. The SWIG Java module is known to work using Sun's JVM on Solaris, Linux and the various flavours of Microsoft Windows including Cygwin. The Kaffe JVM is known to give a few problems and at the time of writing was not a fully fledged JVM with full JNI support. The generated code is also known to work on vxWorks using WindRiver's PJava 3.1. The best way to determine whether your combination of operating system and JDK will work is to test the examples and test-suite that comes with SWIG. Run `make -k check` from the SWIG root directory after installing SWIG on Unix systems.

The Java module requires your system to support shared libraries and dynamic loading. This is the commonly used method to load JNI code so your system will more than likely support this.

Android uses Java JNI and also works with SWIG. Please read the Android chapter in conjunction with this one if you are targeting Android.

### 27.2.1 Running SWIG

Suppose that you defined a SWIG module such as the following:

    /* File: example.i */
    %module test
    %{
    #include "stuff.h"
    %}
    int fact(int n);

To build a Java module, run SWIG using the `-java` option :

    %swig -java example.i

If building C++, add the `-c++` option:

    $ swig -c++ -java example.i

This creates two different files; a C/C++ source file `example_wrap.c` or `example_wrap.cxx` and numerous Java files. The generated C/C++ source file contains the JNI wrapper code that needs to be compiled and linked with the rest of your C/C++ application.

The name of the wrapper file is derived from the name of the input file. For example, if the input file is `example.i`, the name of the wrapper file is `example_wrap.c`. To change this, you can use the `-o` option. It is also possible to change the output directory that the Java files are generated into using `-outdir`.

The module name, specified with `%module`, determines the name of various generated classes as discussed later. Note that the module name does not define a Java package and by default, the generated Java classes do not have a Java package. The `-package` option described below can specify a Java package name to use.

The following sections have further practical examples and details on how you might go about compiling and using the generated files.

### 27.2.2 Additional Commandline Options

The following table lists the additional commandline options available for the Java module. They can also be seen by using:

    $ swig -java -help

Java specific options
-nopgcpp | suppress the premature garbage collection prevention parameter
-noproxy | generate the low-level functional interface instead of proxy classes
-package <name> | set name of the Java package to <name>

Their use will become clearer by the time you have finished reading this section on SWIG and Java.

### 27.2.3 Getting the right header files

In order to compile the C/C++ wrappers, the compiler needs the `jni.h` and `jni_md.h` header files which are part of the JDK. They are usually in directories like this:

    /usr/java/include
    /usr/java/include/<operating_system>

The exact location may vary on your machine, but the above locations are typical.

### 27.2.4 Compiling a dynamic module

The JNI code exists in a dynamic module or shared library (DLL on Windows) and gets loaded by the JVM. Assuming you have code you need to link to in a file called `example.c`, in order to build a shared library file, you need to compile your module in a manner similar to the following (shown for Solaris):

    $ swig -java example.i
    $ gcc -fPIC -c example_wrap.c -I/usr/java/include -I/usr/java/include/solaris
    $ gcc -fPIC -c example.c
    $ ld -G example_wrap.o example.o -o libexample.so

The exact commands for doing this vary from platform to platform. However, SWIG tries to guess the right options when it is installed. Therefore, you may want to start with one of the examples in the `Examples/java` directory. If that doesn't work, you will need to read the man-pages for your compiler and linker to get the right set of options. You might also check the SWIG Wiki for additional information.

Important
If you are going to use optimisations turned on with gcc (for example -O2), ensure you also compile with -fno-strict-aliasing. The GCC optimisations have become more aggressive from gcc-4.0 onwards and will result in code that fails with strict aliasing optimisations turned on. See the C/C++ to Java typemaps section for more details.

The name of the shared library output file is important. If the name of your SWIG module is "`example`", the name of the corresponding shared library file should be "`libexample.so`" (or equivalent depending on your machine, see Dynamic linking problems for more information). The name of the module is specified using the `%module` directive or `-module` command line option.

### 27.2.5 Using your module

To load your shared native library module in Java, simply use Java's `System.loadLibrary` method in a Java class:

    // runme.java

    public class runme {
      static {
        System.loadLibrary("example");
      }

      public static void main(String argv[]) {
        System.out.println(example.fact(4));
      }
    }

Compile all the Java files and run:

    $ javac *.java
    $ java runme
    24
    $

If it doesn't work have a look at the following section which discusses problems loading the shared library.

### 27.2.6 Dynamic linking problems

As shown in the previous section the code to load a native library (shared library) is `System.loadLibrary("name")`. This can fail with an UnsatisfiedLinkError exception and can be due to a number of reasons.

You may get an exception similar to this:

    $ java runme
    Exception in thread "main" java.lang.UnsatisfiedLinkError: no example in java.library.path
            at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1312)
            at java.lang.Runtime.loadLibrary0(Runtime.java:749)
            at java.lang.System.loadLibrary(System.java:820)
            at runme.<clinit>(runme.java:5)

The most common cause for this is an incorrect naming of the native library for the name passed to the `loadLibrary` function. The string passed to the `loadLibrary` function must not include the file extension name in the string, that is .dll or .so. The string must be name and not libname for all platforms. On Windows the native library must then be called name.dll and on most Unix systems it must be called libname.so.

Another common reason for the native library not loading is because it is not in your path. On Windows make sure the path environment variable contains the path to the native library. On Unix make sure that your LD_LIBRARY_PATH contains the path to the native library. Adding paths to LD_LIBRARY_PATH can slow down other programs on your system so you may want to consider alternative approaches. For example you could recompile your native library with extra path information using `-rpath` if you're using GNU, see the GNU linker documentation (`ld` man page). You could use a command such as `ldconfig` (Linux) or `crle` (Solaris) to add additional search paths to the default system configuration (this requires root access and you will need to read the man pages).

The native library will also not load if there are any unresolved symbols in the compiled C/C++ code. The following exception is indicative of this:

    $ java runme
    Exception in thread "main" java.lang.UnsatisfiedLinkError: libexample.so: undefined
    symbol: fact
            at java.lang.ClassLoader$NativeLibrary.load(Native Method)
            at java.lang.ClassLoader.loadLibrary0(ClassLoader.java, Compiled Code)
            at java.lang.ClassLoader.loadLibrary(ClassLoader.java, Compiled Code)
            at java.lang.Runtime.loadLibrary0(Runtime.java, Compiled Code)
            at java.lang.System.loadLibrary(System.java, Compiled Code)
            at runme.<clinit>(runme.java:5)
    $

This error usually indicates that you forgot to include some object files or libraries in the linking of the native library file. Make sure you compile both the SWIG wrapper file and the code you are wrapping into the native library file. If you forget to compile and link in the SWIG wrapper file into your native library file, you will get a message similar to the following:

    $ java runme
    Exception in thread "main" java.lang.UnsatisfiedLinkError: exampleJNI.gcd(II)I
            at exampleJNI.gcd(Native Method)
            at example.gcd(example.java:12)
            at runme.main(runme.java:18)

where `gcd` is the missing JNI function that SWIG generated into the wrapper file. Also make sure you pass all of the required libraries to the linker. The `java -verbose:jni` commandline option is also a great way to get more information on unresolved symbols. One last piece of advice is to beware of the common faux pas of having more than one native library version in your path.

In summary, ensure that you are using the correct C/C++ compiler and linker combination and options for successful native library loading. If you are using the examples that ship with SWIG, then the Examples/Makefile must have these set up correctly for your system. The SWIG installation package makes a best attempt at getting these correct but does not get it right 100% of the time. The SWIG Wiki also has some settings for commonly used compiler and operating system combinations. The following section also contains some C++ specific linking problems and solutions.

### 27.2.7 Compilation problems and compiling with C++

On most machines, shared library files should be linked using the C++ compiler. For example:

    % swig -c++ -java example.i
    % g++ -c -fPIC example.cxx
    % g++ -c -fPIC example_wrap.cxx -I/usr/java/j2sdk1.4.1/include -I/usr/java/j2sdk1.4.1/include/linux
    % g++ -shared example.o example_wrap.o -o libexample.so

In addition to this, you may need to include additional library files to make it work. For example, if you are using the Sun C++ compiler on Solaris, you often need to add an extra library `-lCrun` like this:

    % swig -c++ -java example.i
    % CC -c example.cxx
    % CC -c example_wrap.cxx -I/usr/java/include -I/usr/java/include/solaris
    % CC -G example.o example_wrap.o -L/opt/SUNWspro/lib -o libexample.so -lCrun

If you aren't entirely sure about the linking for C++, you might look at an existing C++ program. On many Unix machines, the `ldd` command will list library dependencies. This should give you some clues about what you might have to include when you link your shared library. For example:

    $ ldd swig
            libstdc++-libc6.1-1.so.2 => /usr/lib/libstdc++-libc6.1-1.so.2 (0x40019000)
            libm.so.6 => /lib/libm.so.6 (0x4005b000)
            libc.so.6 => /lib/libc.so.6 (0x40077000)
            /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
    $

Finally make sure the version of JDK header files matches the version of Java that you are running as incompatibilities could lead to compilation problems or unpredictable behaviour.

### 27.2.8 Building on Windows

Building on Windows is roughly similar to the process used with Unix. You will want to produce a DLL that can be loaded by the Java Virtual Machine. This section covers the process of using SWIG with Microsoft Visual C++ 6 although the procedure may be similar with other compilers. In order for everything to work, you will need to have a JDK installed on your machine in order to read the JNI header files.

#### 27.2.8.1 Running SWIG from Visual Studio

If you are developing your application within Microsoft Visual studio, SWIG can be invoked as a custom build option. The Examples\java directory has a few Windows Examples containing Visual Studio project (.dsp) files. The process to re-create the project files for a C project are roughly:

- Open up a new workspace and use the AppWizard to select a DLL project.
- Add both the SWIG interface file (the .i file), any supporting C files, and the name of the wrapper file that will be created by SWIG (ie. `example_wrap.c`). Don't worry if the wrapper file doesn't exist yet--Visual Studio will keep a reference to it.
- Select the SWIG interface file and go to the settings menu. Under settings, select the "Custom Build" option.
- Enter "SWIG" in the description field.
- Enter "`swig -java -o $(ProjDir)\$(InputName)_wrap.c $(InputPath)`" in the "Build command(s) field"
- Enter "`$(ProjDir)\$(InputName)_wrap.c`" in the "Output files(s) field".
- Next, select the settings for the entire project and go to C/C++ tab and select the Preprocessor category. Add the include directories to the JNI header files under "Additional include directories", eg "C:\jdk1.3\include, C:\jdk1.3\include\win32".
- Next, select the settings for the entire project and go to Link tab and select the General category. Set the name of the output file to match the name of your Java module (ie. example.dll).
- Next, select the example.c and example_wrap.c files and go to the C/C++ tab and select the Precompiled Headers tab in the project settings. Disabling precompiled headers for these files will overcome any precompiled header errors while building.
- Finally, add the java compilation as a post build rule in the Post-build step tab in project settings, eg, "c:\jdk1.3\bin\javac *.java"
- Build your project.

Note: If using C++, choose a C++ suffix for the wrapper file, for example `example_wrap.cxx`. Use `_wrap.cxx` instead of `_wrap.c` in the instructions above and add -c++ when invoking swig.

Now, assuming all went well, SWIG will be automatically invoked when you build your project. When doing a build, any changes made to the interface file will result in SWIG being automatically invoked to produce a new version of the wrapper file.

The Java classes that SWIG output should also be compiled into .class files. To run the native code in the DLL (example.dll), make sure that it is in your path then run your Java program which uses it, as described in the previous section. If the library fails to load have a look at Dynamic linking problems.

#### 27.2.8.2 Using NMAKE

Alternatively, a Makefile for use by NMAKE can be written. Make sure the environment variables for MSVC++ are available and the MSVC++ tools are in your path. Now, just write a short Makefile like this :

    # Makefile for using SWIG and Java for C code

    SRCS          = example.c
    IFILE         = example
    INTERFACE     = $(IFILE).i
    WRAPFILE      = $(IFILE)_wrap.c

    # Location of the Visual C++ tools (32 bit assumed)

    TOOLS         = c:\msdev
    TARGET        = example.dll
    CC            = $(TOOLS)\bin\cl.exe
    LINK          = $(TOOLS)\bin\link.exe
    INCLUDE32     = -I$(TOOLS)\include
    MACHINE       = IX86

    # C Library needed to build a DLL

    DLLIBC        = msvcrt.lib oldnames.lib

    # Windows libraries that are apparently needed
    WINLIB        = kernel32.lib advapi32.lib user32.lib gdi32.lib comdlg32.lib winspool.lib

    # Libraries common to all DLLs
    LIBS          = $(DLLIBC) $(WINLIB)

    # Linker options
    LOPT      = -debug:full -debugtype:cv /NODEFAULTLIB /RELEASE /NOLOGO \
                 /MACHINE:$(MACHINE) -entry:_DllMainCRTStartup@12 -dll

    # C compiler flags

    CFLAGS        = /Z7 /Od /c /nologo
    JAVA_INCLUDE    = -ID:\jdk1.3\include -ID:\jdk1.3\include\win32

    java::
            swig -java -o $(WRAPFILE) $(INTERFACE)
            $(CC) $(CFLAGS) $(JAVA_INCLUDE) $(SRCS) $(WRAPFILE)
            set LIB=$(TOOLS)\lib
            $(LINK) $(LOPT) -out:example.dll $(LIBS) example.obj example_wrap.obj
            javac *.java

To build the DLL and compile the java code, run NMAKE (you may need to run `vcvars32` first). This is a pretty simplistic Makefile, but hopefully it's enough to get you started. Of course you may want to make changes for it to work for C++ by adding in the -c++ command line option for swig and replacing .c with .cxx.

## 27.3 A tour of basic C/C++ wrapping

By default, SWIG attempts to build a natural Java interface to your C/C++ code. Functions are wrapped as functions, classes are wrapped as classes, variables are wrapped with JavaBean type getters and setters and so forth. This section briefly covers the essential aspects of this wrapping.

### 27.3.1 Modules, packages and generated Java classes

The SWIG `%module` directive specifies the name of the Java module. When you specify ``%module example`', the module name determines the name of some of the generated files in the module. The generated code consists of a module class file `example.java`, an intermediary JNI class file, `exampleJNI.java` as well as numerous other Java proxy class files. Each proxy class is named after the structs, unions and classes you are wrapping. You may also get a constants interface file if you are wrapping any unnamed enumerations or constants, for example `exampleConstants.java`. When choosing a module name, make sure you don't use the same name as one of the generated proxy class files nor a Java keyword. Sometimes a C/C++ type cannot be wrapped by a proxy class, for example a pointer to a primitive type. In these situations a type wrapper class is generated. Wrapping an enum generates an enum class, either a proper Java enum or a Java class that simulates the enums pattern. Details of all these generated classes will unfold as you read this section.

The JNI (C/C++) code is generated into a file which also contains the module name, for example `example_wrap.cxx` or `example_wrap.c`. These C or C++ files complete the contents of the module.

The generated Java classes can be placed into a Java package by using the `-package` commandline option. This is often combined with the `-outdir` to specify a package directory for generating the Java files.

    $ swig -java -package com.bloggs.swig -outdir com/bloggs/swig example.i

SWIG won't create the directory, so make sure it exists beforehand.

### 27.3.2 Functions

There is no such thing as a global Java function so global C functions are wrapped as static methods in the module class. For example,

    %module example
    int fact(int n);

creates a static function that works exactly like you think it might:

    public class example {
      public static int fact(int n) {
        // makes call using JNI to the C function
      }
    }

The Java class `example` is the module class. The function can be used as follows from Java:

    System.out.println(example.fact(4));

### 27.3.3 Global variables

C/C++ global variables are fully supported by SWIG. Java does not allow the overriding of the dot operator so all variables are accessed through getters and setters. Again because there is no such thing as a Java global variable, access to C/C++ global variables is done through static getter and setter functions in the module class.

    // SWIG interface file with global variables
    %module example
    ...
    %inline %{
    extern int My_variable;
    extern double density;
    %}
    ...

Now in Java :

    // Print out value of a C global variable
    System.out.println("My_variable = " + example.getMy_variable());
    // Set the value of a C global variable
    example.setDensity(0.8442);

The value returned by the getter will always be up to date even if the value is changed in C. Note that the getters and setters produced follow the JavaBean property design pattern. That is the first letter of the variable name is capitalized and preceded with set or get. If you have the misfortune of wrapping two variables that differ only in the capitalization of their first letters, use %rename to change one of the variable names. For example:

    %rename Clash RenamedClash;
    float Clash;
    int clash;

If a variable is declared as `const`, it is wrapped as a read-only variable. That is only a getter is produced.

To make ordinary variables read-only, you can use the `%immutable` directive. For example:

    %{
    extern char *path;
    %}
    %immutable;
    extern char *path;
    %mutable;

The `%immutable` directive stays in effect until it is explicitly disabled or cleared using `%mutable`. See the Creating read-only variables section for further details.

If you just want to make a specific variable immutable, supply a declaration name. For example:

    %{
    extern char *path;
    %}
    %immutable path;
    ...
    extern char *path;      // Read-only (due to %immutable)

### 27.3.4 Constants

C/C++ constants are wrapped as Java static final variables. To create a constant, use `#define` or the `%constant` directive. For example:

    #define PI 3.14159
    #define VERSION "1.0"
    %constant int FOO = 42;
    %constant const char *path = "/usr/local";

By default the generated static final variables are initialized by making a JNI call to get their value. The constants are generated into the constants interface and look like this:

    public interface exampleConstants {
      public final static double PI = exampleJNI.PI_get();
      public final static String VERSION = exampleJNI.VERSION_get();
      public final static int FOO = exampleJNI.FOO_get();
      public final static String path = exampleJNI.path_get();
    }

Note that SWIG has inferred the C type and used an appropriate Java type that will fit the range of all possible values for the C type. By default SWIG generates runtime constants. They are not compiler constants that can, for example, be used in a switch statement. This can be changed by using the `%javaconst(flag)` directive. It works like all the other %feature directives. The default is `%javaconst(0)`. It is possible to initialize all wrapped constants from pure Java code by placing a `%javaconst(1)` before SWIG parses the constants. Putting it at the top of your interface file would ensure this. Here is an example:

    %javaconst(1);
    %javaconst(0) BIG;
    %javaconst(0) LARGE;

    #define EXPRESSION (0x100+5)
    #define BIG 1000LL
    #define LARGE 2000ULL

generates:

    public interface exampleConstants {
      public final static int EXPRESSION = (0x100+5);
      public final static long BIG = exampleJNI.BIG_get();
      public final static java.math.BigInteger LARGE = exampleJNI.LARGE_get();
    }

Note that SWIG has inferred the C `long long` type from `BIG` and used an appropriate Java type (`long`) as a Java `long` is the smallest sized Java type that will take all possible values for a C `long long`. Similarly for `LARGE`.

Be careful using the `%javaconst(1)` directive as not all C code will compile as Java code. For example neither the `1000LL` value for `BIG` nor `2000ULL` for `LARGE` above would generate valid Java code. The example demonstrates how you can target particular constants (`BIG` and `LARGE`) with `%javaconst`. SWIG doesn't use `%javaconst(1)` as the default as it tries to generate code that will always compile. However, using a `%javaconst(1)` at the top of your interface file is strongly recommended as the preferred compile time constants will be generated and most C constants will compile as Java code and in any case the odd constant that doesn't can be fixed using `%javaconst(0)`.

There is an alternative directive which can be used for these rare constant values that won't compile as Java code. This is the `%javaconstvalue(value)` directive, where `value` is a Java code replacement for the C constant and can be either a string or a number. This is useful if you do not want to use either the parsed C value nor a JNI call, such as when the C parsed value will not compile as Java code and a compile time constant is required. The same example demonstrates this:

    %javaconst(1);
    %javaconstvalue("new java.math.BigInteger(\"2000\")") LARGE;
    %javaconstvalue(1000) BIG;

    #define EXPRESSION (0x100+5)
    #define BIG 1000LL
    #define LARGE 2000ULL

Note the string quotes for `"2000"` are escaped. The following is then generated:

    public interface exampleConstants {
      public final static int EXPRESSION = (0x100+5);
      public final static long BIG = 1000;
      public final static java.math.BigInteger LARGE = new java.math.BigInteger("2000");
    }

Note: declarations declared as `const` are wrapped as read-only variables and will be accessed using a getter as described in the previous section. They are not wrapped as constants. The exception to this rule are static const integral values defined within a class/struct, where they are wrapped as constants, eg:.

    struct Maths {
      static const int FIVE = 5;
    };

Compatibility Note: In SWIG-1.3.19 and earlier releases, the constants were generated into the module class and the constants interface didn't exist. Backwards compatibility is maintained as the module class implements the constants interface (even though some consider this type of interface implementation to be bad practice):

    public class example implements exampleConstants {
    }

You thus have the choice of accessing these constants from either the module class or the constants interface, for example, `example.EXPRESSION` or `exampleConstants.EXPRESSION`. Or if you decide this practice isn't so bad and your own class implements `exampleConstants`, you can of course just use `EXPRESSION`.

### 27.3.5 Enumerations

SWIG handles both named and unnamed (anonymous) enumerations. There is a choice of approaches to wrapping named C/C++ enums. This is due to historical reasons as SWIG's initial support for enums was limited and Java did not originally have support for enums. Each approach has advantages and disadvantages and it is important for the user to decide which is the most appropriate solution. There are four approaches of which the first is the default approach based on the so called Java typesafe enum pattern. The second generates proper Java enums. The final two approaches use simple integers for each enum item. Before looking at the various approaches for wrapping named C/C++ enums, anonymous enums are considered.

#### 27.3.5.1 Anonymous enums

There is no name for anonymous enums and so they are handled like constants. For example:

    enum { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

is wrapped into the constants interface, in a similar manner as constants (see previous section):

    public interface exampleConstants {
      public final static int ALE = exampleJNI.ALE_get();
      public final static int LAGER = exampleJNI.LAGER_get();
      public final static int STOUT = exampleJNI.STOUT_get();
      public final static int PILSNER = exampleJNI.PILSNER_get();
      public final static int PILZ = exampleJNI.PILZ_get();
    }

The `%javaconst(flag)` and `%javaconstvalue(value)` directive introduced in the previous section on constants can also be used with enums. As is the case for constants, the default is `%javaconst(0)` as not all C values will compile as Java code. However, it is strongly recommended to add in a `%javaconst(1)` directive at the top of your interface file as it is only on very rare occasions that this will produce code that won't compile under Java. Using `%javaconst(1)` will ensure compile time constants are generated, thereby allowing the enum values to be used in Java switch statements. Example usage:

    %javaconst(1);
    %javaconst(0) PILSNER;
    enum { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

generates:

    public interface exampleConstants {
      public final static int ALE = 0;
      public final static int LAGER = 10;
      public final static int STOUT = LAGER + 1;
      public final static int PILSNER = exampleJNI.PILSNER_get();
      public final static int PILZ = PILSNER;
    }

As in the case of constants, you can access them through either the module class or the constants interface, for example, `example.ALE` or `exampleConstants.ALE`.

#### 27.3.5.2 Typesafe enums

This is the default approach to wrapping named enums. The typesafe enum pattern is a relatively well known construct to work around the lack of enums in versions of Java prior to JDK 1.5. It basically defines a class for the enumeration and permits a limited number of final static instances of the class. Each instance equates to an enum item within the enumeration. The implementation is in the "enumtypesafe.swg" file. Let's look at an example:

    %include "enumtypesafe.swg" // optional as typesafe enums are the default
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

will generate:

    public final class Beverage {
      public final static Beverage ALE = new Beverage("ALE");
      public final static Beverage LAGER = new Beverage("LAGER", exampleJNI.LAGER_get());
      public final static Beverage STOUT = new Beverage("STOUT");
      public final static Beverage PILSNER = new Beverage("PILSNER");
      public final static Beverage PILZ = new Beverage("PILZ", exampleJNI.PILZ_get());
      [... additional support methods omitted for brevity ...]
    }

See Typesafe enum classes to see the omitted support methods. Note that the enum item with an initializer (LAGER) is initialized with the enum value obtained via a JNI call. However, as with anonymous enums and constants, use of the `%javaconst` directive is strongly recommended to change this behaviour:

    %include "enumtypesafe.swg" // optional as typesafe enums are the default
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

will generate:

    public final class Beverage {
      public final static Beverage ALE = new Beverage("ALE");
      public final static Beverage LAGER = new Beverage("LAGER", 10);
      public final static Beverage STOUT = new Beverage("STOUT");
      public final static Beverage PILSNER = new Beverage("PILSNER");
      public final static Beverage PILZ = new Beverage("PILZ", PILSNER);
      [... additional support methods omitted for brevity ...]
    }

The generated code is easier to read and more efficient as a true constant is used instead of a JNI call. As is the case for constants, the default is `%javaconst(0)` as not all C values will compile as Java code. However, it is recommended to add in a `%javaconst(1)` directive at the top of your interface file as it is only on very rare occasions that this will produce code that won't compile under Java. The `%javaconstvalue(value)` directive can also be used for typesafe enums. Note that global enums are generated into a Java class within whatever package you are using. C++ enums defined within a C++ class are generated into a static final inner Java class within the Java proxy class.

Typesafe enums have their advantages over using plain integers in that they can be used in a typesafe manner. However, there are limitations. For example, they cannot be used in switch statements and serialization is an issue. Please look at the following references for further information: Replace Enums with Classes in Effective Java Programming on the Sun website, Create enumerated constants in Java JavaWorld article, Java Tip 133: More on typesafe enums and Java Tip 122: Beware of Java typesafe enumerations JavaWorld tips.

Note that the syntax required for using typesafe enums is the same as that for proper Java enums. This is useful during the period that a project has to support legacy versions of Java. When upgrading to JDK 1.5 or later, proper Java enums could be used instead, without users having to change their code. The following section details proper Java enum generation.

#### 27.3.5.3 Proper Java enums

Proper Java enums were only introduced in JDK 1.5 so this approach is only compatible with more recent versions of Java. Java enums have been designed to overcome all the limitations of both typesafe and type unsafe enums and should be the choice solution, provided older versions of Java do not have to be supported. In this approach, each named C/C++ enum is wrapped by a Java enum. Java enums, by default, do not support enums with initializers. Java enums are in many respects similar to Java classes in that they can be customised with additional methods. SWIG takes advantage of this feature to facilitate wrapping C/C++ enums that have initializers. In order to wrap all possible C/C++ enums using proper Java enums, the "enums.swg" file must be used. Let's take a look at an example.

    %include "enums.swg"
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

will generate:

    public enum Beverage {
      ALE,
      LAGER(10),
      STOUT,
      PILSNER,
      PILZ(PILSNER);
      [... additional support methods omitted for brevity ...]
    }

See Proper Java enum classes to see the omitted support methods. The generated Java enum has numerous additional methods to support enums with initializers, such as `LAGER` above. Note that as with the typesafe enum pattern, enum items with initializers are by default initialized with the enum value obtained via a JNI call. However, this is not the case above as we have used the recommended `%javaconst(1)` to avoid the JNI call. The `%javaconstvalue(value)` directive covered in the Constants section can also be used for proper Java enums.

The additional support methods need not be generated if none of the enum items have initializers and this is covered later in the Simpler Java enums for enums without initializers section.

#### 27.3.5.4 Type unsafe enums

In this approach each enum item in a named enumeration is wrapped as a static final integer in a class named after the C/C++ enum name. This is a commonly used pattern in Java to simulate C/C++ enums, but it is not typesafe. However, the main advantage over the typesafe enum pattern is enum items can be used in switch statements. In order to use this approach, the "enumtypeunsafe.swg" file must be used. Let's take a look at an example.

    %include "enumtypeunsafe.swg"
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

will generate:

    public final class Beverage {
      public final static int ALE = 0;
      public final static int LAGER = 10;
      public final static int STOUT = LAGER + 1;
      public final static int PILSNER = STOUT + 1;
      public final static int PILZ = PILSNER;
    }

As is the case previously, the default is `%javaconst(0)` as not all C/C++ values will compile as Java code. However, again it is recommended to add in a `%javaconst(1)` directive. and the `%javaconstvalue(value)` directive covered in the Constants section can also be used for type unsafe enums. Note that global enums are generated into a Java class within whatever package you are using. C++ enums defined within a C++ class are generated into a static final inner Java class within the Java proxy class.

Note that unlike typesafe enums, this approach requires users to mostly use different syntax compared with proper Java enums. Thus the upgrade path to proper enums provided in JDK 1.5 is more painful.

#### 27.3.5.5 Simple enums

This approach is similar to the type unsafe approach. Each enum item is also wrapped as a static final integer. However, these integers are not generated into a class named after the C/C++ enum. Instead, global enums are generated into the constants interface. Also, enums defined in a C++ class have their enum items generated directly into the Java proxy class rather than an inner class within the Java proxy class. In fact, this approach is effectively wrapping the enums as if they were anonymous enums and the resulting code is as per anonymous enums. The implementation is in the "enumsimple.swg" file.

Compatibility Note: SWIG-1.3.21 and earlier versions wrapped all enums using this approach. The type unsafe approach is preferable to this one and this simple approach is only included for backwards compatibility with these earlier versions of SWIG.

### 27.3.6 Pointers

C/C++ pointers are fully supported by SWIG. Furthermore, SWIG has no problem working with incomplete type information. Here is a rather simple interface:

    %module example

    FILE *fopen(const char *filename, const char *mode);
    int fputs(const char *, FILE *);
    int fclose(FILE *);

When wrapped, you will be able to use the functions in a natural way from Java. For example:

    SWIGTYPE_p_FILE f = example.fopen("junk", "w");
    example.fputs("Hello World\n", f);
    example.fclose(f);

C pointers in the Java module are stored in a Java `long` and cross the JNI boundary held within this 64 bit number. Many other SWIG language modules use an encoding of the pointer in a string. These scripting languages use the SWIG runtime type checker for dynamic type checking as they do not support static type checking by a compiler. In order to implement static type checking of pointers within Java, they are wrapped by a simple Java class. In the example above the `FILE *` pointer is wrapped with a type wrapper class called `SWIGTYPE_p_FILE`.

Once obtained, a type wrapper object can be freely passed around to different C functions that expect to receive an object of that type. The only thing you can't do is dereference the pointer from Java. Of course, that isn't much of a concern in this example.

As much as you might be inclined to modify a pointer value directly from Java, don't. The value is not necessarily the same as the logical memory address of the underlying object. The value will vary depending on the native byte-ordering of the platform (i.e., big-endian vs. little-endian). Most JVMs are 32 bit applications so any JNI code must also be compiled as 32 bit. The net result is pointers in JNI code are also 32 bits and are stored in the high order 4 bytes on big-endian machines and in the low order 4 bytes on little-endian machines. By design it is also not possible to manually cast a pointer to a new type by using Java casts as it is particularly dangerous especially when casting C++ objects. If you need to cast a pointer or change its value, consider writing some helper functions instead. For example:

    %inline %{
    /* C-style cast */
    Bar *FooToBar(Foo *f) {
      return (Bar *) f;
    }

    /* C++-style cast */
    Foo *BarToFoo(Bar *b) {
      return dynamic_cast<Foo*>(b);
    }

    Foo *IncrFoo(Foo *f, int i) {
      return f+i;
    }
    %}

Also, if working with C++, you should always try to use the new C++ style casts. For example, in the above code, the C-style cast may return a bogus result whereas as the C++-style cast will return a NULL pointer if the conversion can't be performed.

### 27.3.7 Structures

If you wrap a C structure, it is wrapped by a Java class with getters and setters for access to the member variables. For example,

    struct Vector {
      double x, y, z;
    };

is used as follows:

    Vector v = new Vector();
    v.setX(3.5);
    v.setY(7.2);
    double x = v.getX();
    double y = v.getY();

The variable setters and getters are also based on the JavaBean design pattern already covered under the Global variables section. Similar access is provided for unions and the public data members of C++ classes.

This object is actually an instance of a Java class that has been wrapped around a pointer to the C structure. This instance doesn't actually do anything--it just serves as a proxy. The pointer to the C object is held in the Java proxy class in much the same way as pointers are held by type wrapper classes. Further details about Java proxy classes are covered a little later.

`const` members of a structure are read-only. Data members can also be forced to be read-only using the `%immutable` directive. For example:

    struct Foo {
      ...
      %immutable;
      int x;        /* Read-only members */
      char *name;
      %mutable;
      ...
    };

When `char *` members of a structure are wrapped, the contents are assumed to be dynamically allocated using `malloc` or `new` (depending on whether or not SWIG is run with the -c++ option). When the structure member is set, the old contents will be released and a new value created. If this is not the behavior you want, you will have to use a typemap (described later).

If a structure contains arrays, access to those arrays is managed through pointers. For example, consider this:

    struct Bar {
      int  x[16];
    };

If accessed in Java, you will see behavior like this:

    Bar b = new Bar();
    SWIGTYPE_p_int x = b.getX();

This pointer can be passed around to functions that expect to receive an `int *` (just like C). You can also set the value of an array member using another pointer. For example:

    Bar b = new Bar();
    SWIGTYPE_p_int x = b.getX();
    Bar c = new Bar();
    c.setX(x);                    // Copy contents of b.x to c.x

For array assignment (setters not getters), SWIG copies the entire contents of the array starting with the data pointed to by `b.x`. In this example, 16 integers would be copied. Like C, SWIG makes no assumptions about bounds checking---if you pass a bad pointer, you may get a segmentation fault or access violation. The default wrapping makes it hard to set or get just one element of the array and so array access from Java is somewhat limited. This can be changed easily though by using the approach outlined later in the Wrapping C arrays with Java arrays and Unbounded C Arrays sections.

When a member of a structure is itself a structure, it is handled as a pointer. For example, suppose you have two structures like this:

    struct Foo {
      int a;
    };

    struct Bar {
      Foo f;
    };

Now, suppose that you access the `f` member of `Bar` like this:

    Bar b = new Bar();
    Foo x = b.getF();

In this case, `x` is a pointer that points to the `Foo` that is inside `b`. This is the same value as generated by this C code:

    Bar b;
    Foo *x = &b->f;       /* Points inside b */

Because the pointer points inside the structure, you can modify the contents and everything works just like you would expect. For example:

    Bar b = new Bar();
    b.getF().setA(3);   // Modify b.f.a
    Foo x = b.getF();
    x.setA(3);          // Modify x.a - this is the same as b.f.a

### 27.3.8 C++ classes

C++ classes are wrapped by Java classes as well. For example, if you have this class,

    class List {
    public:
      List();
      ~List();
      int  search(char *item);
      void insert(char *item);
      void remove(char *item);
      char *get(int n);
      int  length;
    };

you can use it in Java like this:

    List l = new List();
    l.insert("Ale");
    l.insert("Stout");
    l.insert("Lager");
    String item = l.get(2);
    int length = l.getLength();

Class data members are accessed in the same manner as C structures.

Static class members are unsurprisingly wrapped as static members of the Java class:

    class Spam {
    public:
      static void foo();
      static int bar;
    };

The static members work like any other Java static member:

    Spam.foo();
    int bar = Spam.getBar();

### 27.3.9 C++ inheritance

SWIG is fully aware of issues related to C++ inheritance. Therefore, if you have classes like this

    class Foo {
    ...
    };

    class Bar : public Foo {
    ...
    };

those classes are wrapped into a hierarchy of Java classes that reflect the same inheritance structure:

    Bar b = new Bar();
    Class c = b.getClass();
    System.out.println(c.getSuperclass().getName());

will of course display:

    Foo

Furthermore, if you have functions like this

    void spam(Foo *f);

then the Java function `spam()` accepts instances of `Foo` or instances of any other proxy classes derived from `Foo`.

Note that Java does not support multiple inheritance so any multiple inheritance in the C++ code is not going to work. A warning is given when multiple inheritance is detected and only the first base class is used.

Private and protected methods are not wrapped as they are inaccessible outside of the class. Protected methods can be made accessible though via the directors feature, see the Accessing virtual protected methods in the directors section.

### 27.3.10 Pointers, references, arrays and pass by value

In C++, there are many different ways a function might receive and manipulate objects. For example:

    void spam1(Foo *x);      // Pass by pointer
    void spam2(Foo &x);      // Pass by reference
    void spam3(Foo x);       // Pass by value
    void spam4(Foo x[]);     // Array of objects

In Java, there is no detailed distinction like this--specifically, there are only instances of classes. There are no pointers nor references. Because of this, SWIG unifies all of these types together in the wrapper code. For instance, if you actually had the above functions, it is perfectly legal to do this from Java:

    Foo f = new Foo();  // Create a Foo
    example.spam1(f);   // Ok. Pointer
    example.spam2(f);   // Ok. Reference
    example.spam3(f);   // Ok. Value.
    example.spam4(f);   // Ok. Array (1 element)

Similar behavior occurs for return values. For example, if you had functions like this,

    Foo *spam5();
    Foo &spam6();
    Foo  spam7();

then all three functions will return a pointer to some `Foo` object. Since the third function (spam7) returns a value, newly allocated memory is used to hold the result and a pointer is returned (Java will release this memory when the returned object's finalizer is run by the garbage collector).

#### 27.3.10.1 Null pointers

Working with null pointers is easy. A Java `null` can be used whenever a method expects a proxy class or typewrapper class. However, it is not possible to pass null to C/C++ functions that take parameters by value or by reference. If you try you will get a `NullPointerException`.

    example.spam1(null);   // Pointer - ok
    example.spam2(null);   // Reference - NullPointerException
    example.spam3(null);   // Value - NullPointerException
    example.spam4(null);   // Array - ok

For `spam1` and `spam4` above the Java `null` gets translated into a NULL pointer for passing to the C/C++ function. The converse also occurs, that is, NULL pointers are translated into `null` Java objects when returned from a C/C++ function.

### 27.3.11 C++ overloaded functions

C++ overloaded functions, methods, and constructors are mostly supported by SWIG. For example, if you have two functions like this:

    %module example

    void foo(int);
    void foo(char *c);

You can use them in Java in a straightforward manner:

    example.foo(3);           // foo(int)
    example.foo("Hello");     // foo(char *c)

Similarly, if you have a class like this,

    class Foo {
    public:
      Foo();
      Foo(const Foo &);
      ...
    };

you can write Java code like this:

    Foo f = new Foo();        // Create a Foo
    Foo g = new Foo(f);       // Copy f

Overloading support is not quite as flexible as in C++. Sometimes there are methods that SWIG cannot disambiguate as there can be more than one C++ type mapping onto a single Java type. For example:

    void spam(int);
    void spam(unsigned short);

Here both int and unsigned short map onto a Java int. Here is another example:

    void foo(Bar *b);
    void foo(Bar &b);

If declarations such as these appear, you will get a warning message like this:

    example.i:12: Warning 515: Overloaded method spam(unsigned short) ignored.
    Method spam(int) at example.i:11 used.

To fix this, you either need to either rename or ignore one of the methods. For example:

    %rename(spam_ushort) spam(unsigned short);
    ...
    void spam(int);
    void spam(unsigned short);   // Now renamed to spam_ushort

or

    %ignore spam(unsigned short);
    ...
    void spam(int);
    void spam(unsigned short);   // Ignored

### 27.3.12 C++ default arguments

Any function with a default argument is wrapped by generating an additional function for each argument that is defaulted. For example, if we have the following C++:

    %module example

    void defaults(double d=10.0, int i=0);

The following methods are generated in the Java module class:

    public class example {
      public static void defaults(double d, int i) { ... }
      public static void defaults(double d) { ... }
      public static void defaults() { ... }
    }

It is as if SWIG had parsed three separate overloaded methods. The same approach is taken for static methods, constructors and member methods.

Compatibility note: Versions of SWIG prior to SWIG-1.3.23 wrapped these with a single wrapper method and so the default values could not be taken advantage of from Java. Further details on default arguments and how to restore this approach are given in the more general Default arguments section.

### 27.3.13 C++ namespaces

SWIG is aware of named C++ namespaces and they can be mapped to Java packages, however, the default wrapping flattens the namespaces, effectively ignoring them. So by default, the namespace names do not appear in the module nor do namespaces result in a module that is broken up into submodules or packages. For example, if you have a file like this,

    %module example

    namespace foo {
      int fact(int n);
      struct Vector {
        double x, y, z;
      };
    };

it works in Java as follows:

    int f = example.fact(3);
    Vector v = new Vector();
    v.setX(3.4);
    double y = v.getY();

If your program has more than one namespace, name conflicts (if any) can be resolved using `%rename` For example:

    %rename(Bar_spam) Bar::spam;

    namespace Foo {
      int spam();
    }

    namespace Bar {
      int spam();
    }

If you have more than one namespace and you want to keep their symbols separate, consider wrapping them as separate SWIG modules. Each SWIG module can be placed into a separate package.

The default behaviour described above can be improved via the nspace feature. Note that it only works for classes, structs, unions and enums declared within a named C++ namespace. When the nspace feature is used (either the `nspace` feature flag or `%nspacemove`), the C++ namespaces are converted into Java packages of the same name. Proxy classes are thus declared within a package and this proxy makes numerous calls to the JNI intermediary class which is declared in the unnamed package by default. As Java does not support types declared in a named package accessing types declared in an unnamed package, the `-package` commandline option described earlier generally should be used to provide a parent package. So if SWIG is run using the `-package com.myco` option, a wrapped class, `MyWorld::Material::Color`, can then be accessed as `com.myco.MyWorld.Material.Color`. If you don't specify a package, you will get the following warning:

    example.i:16: Warning 826: The nspace feature is used on 'MyWorld::Material::Color' without -package. The generated code
    may not compile as Java does not support types declared in a named package accessing types declared in an unnamed package.

If it is undesirable to have a single top level package, the nspace feature may be used without the `-package` commandline option (and the resulting warning ignored) if all of the types exposed using SWIG are placed in a package using the nspace feature and the 'jniclasspackage' pragma is used to specify a package for the JNI intermediary class.

If the resulting use of the nspace feature and hence packages results in a proxy class in one package deriving or using a proxy class from another package, you will need to open up the visibility for the pointer constructor and `getCPtr` method from the default 'protected' to 'public' with the `SWIG_JAVABODY_PROXY` macro. See Java code typemaps.

### 27.3.14 C++ templates

C++ templates don't present a huge problem for SWIG. However, in order to create wrappers, you have to tell SWIG to create wrappers for a particular template instantiation. To do this, you use the `%template` directive. For example:

    %module example
    %{
    #include <utility>
    %}

    template<class T1, class T2>
    struct pair {
      typedef T1 first_type;
      typedef T2 second_type;
      T1 first;
      T2 second;
      pair();
      pair(const T1&, const T2&);
      ~pair();
    };

    %template(pairii) pair<int, int>;

In Java:

    pairii p = new pairii(3, 4);
    int first = p.getFirst();
    int second = p.getSecond();

Obviously, there is more to template wrapping than shown in this example. More details can be found in the SWIG and C++ chapter.

### 27.3.15 C++ Smart Pointers

#### 27.3.15.1 The shared_ptr Smart Pointer

The C++11 standard provides `std::shared_ptr` which was derived from the Boost implementation, `boost::shared_ptr`. Both of these are available for Java in the SWIG library and usage is outlined in the shared_ptr smart pointer library section.

#### 27.3.15.2 Generic Smart Pointers

In certain C++ programs, it is common to use classes that have been wrapped by so-called "smart pointers." Generally, this involves the use of a template class that implements `operator->()` like this:

    template<class T> class SmartPtr {
      ...
      T *operator->();
      ...
    }

Then, if you have a class like this,

    class Foo {
    public:
      int x;
      int bar();
    };

A smart pointer would be used in C++ as follows:

    SmartPtr<Foo> p = CreateFoo();   // Created somehow (not shown)
    ...
    p->x = 3;                        // Foo::x
    int y = p->bar();                // Foo::bar

To wrap this in Java, simply tell SWIG about the `SmartPtr` class and the low-level `Foo` object. Make sure you instantiate `SmartPtr` using `%template` if necessary. For example:

    %module example
    ...
    %template(SmartPtrFoo) SmartPtr<Foo>;
    ...

Now, in Java, everything should just "work":

    SmartPtrFoo p = example.CreateFoo(); // Create a smart-pointer somehow
    p.setX(3);                           // Foo::x
    int y = p.bar();                     // Foo::bar

If you ever need to access the underlying pointer returned by `operator->()` itself, simply use the `__deref__()` method. For example:

    Foo f = p.__deref__();               // Returns underlying Foo *

## 27.4 Further details on the generated Java classes

In the previous section, a high-level view of Java wrapping was presented. A key component of this wrapping is that structures and classes are wrapped by Java proxy classes and type wrapper classes are used in situations where no proxies are generated. This provides a very natural, type safe Java interface to the C/C++ code and fits in with the Java programming paradigm. However, a number of low-level details were omitted. This section provides a brief overview of how the proxy classes work and then covers the type wrapper classes. Finally enum classes are covered. First, the crucial intermediary JNI class is considered.

### 27.4.1 The intermediary JNI class

In the "SWIG basics" and "SWIG and C++" chapters, details of low-level structure and class wrapping are described. To summarize those chapters, if you have a global function and class like this

    class Foo {
    public:
      int x;
      int spam(int num, Foo* foo);
    };
    void egg(Foo* chips);

then SWIG transforms the class into a set of low-level procedural wrappers. These procedural wrappers essentially perform the equivalent of this C++ code:

    Foo *new_Foo() {
      return new Foo();
    }
    void delete_Foo(Foo *f) {
      delete f;
    }
    int Foo_x_get(Foo *f) {
      return f->x;
    }
    void Foo_x_set(Foo *f, int value) {
      f->x = value;
    }
    int Foo_spam(Foo *f, int num, Foo* foo) {
      return f->spam(num, foo);
    }

These procedural function names don't actually exist, but their functionality appears inside the generated JNI functions. The JNI functions have to follow a particular naming convention so the function names are actually:

    SWIGEXPORT jlong JNICALL Java_exampleJNI_new_1Foo(JNIEnv *jenv, jclass jcls);
    SWIGEXPORT void JNICALL Java_exampleJNI_delete_1Foo(JNIEnv *jenv, jclass jcls,
                                                        jlong jarg1);
    SWIGEXPORT void JNICALL Java_exampleJNI_Foo_1x_1set(JNIEnv *jenv, jclass jcls,
                                                        jlong jarg1, jobject jarg1_, jint jarg2);
    SWIGEXPORT jint JNICALL Java_exampleJNI_Foo_1x_1get(JNIEnv *jenv, jclass jcls,
                                                        jlong jarg1, jobject jarg1_);
    SWIGEXPORT jint JNICALL Java_exampleJNI_Foo_1spam(JNIEnv *jenv, jclass jcls,
                                                      jlong jarg1, jobject jarg1_, jint jarg2,
                                                      jlong jarg3, jobject jarg3_);
    SWIGEXPORT void JNICALL Java_exampleJNI_egg(JNIEnv *jenv, jclass jcls,
                                                jlong jarg1, jobject jarg1_);

For every JNI C function there has to be a static native Java function. These appear in the intermediary JNI class:

    class exampleJNI {
      public final static native long new_Foo();
      public final static native void delete_Foo(long jarg1);
      public final static native void Foo_x_set(long jarg1, Foo jarg1_, int jarg2);
      public final static native int Foo_x_get(long jarg1, Foo jarg1_);
      public final static native int Foo_spam(long jarg1, Foo jarg1_, int jarg2,
                                              long jarg3, Foo jarg3_);
      public final static native void egg(long jarg1, Foo jarg1_);
    }

This class contains the complete Java - C/C++ interface so all function calls go via this class. As this class acts as a go-between for all JNI calls to C/C++ code from the Java proxy classes, type wrapper classes and module class, it is known as the intermediary JNI class.

You may notice that SWIG uses a Java long wherever a pointer or class object needs to be marshalled across the Java-C/C++ boundary. This approach leads to minimal JNI code which makes for better performance as JNI code involves a lot of string manipulation. SWIG favours generating Java code over JNI code as Java code is compiled into byte code and avoids the costly string operations needed in JNI code. This approach has a downside though as the proxy class might get collected before the native method has completed. You might notice above that there is an additional parameters with a underscore postfix, eg `jarg1_`. These are added in order to prevent premature garbage collection when marshalling proxy classes.

The functions in the intermediary JNI class cannot be accessed outside of its package. Access to them is gained through the module class for globals otherwise the appropriate proxy class.

The name of the intermediary JNI class can be changed from its default, that is, the module name with JNI appended after it. The module directive attribute `jniclassname` is used to achieve this:

    %module(jniclassname="name") modulename

If `name` is the same as `modulename` then the module class name gets changed from `modulename` to `modulenameModule`.

#### 27.4.1.1 The intermediary JNI class pragmas

The intermediary JNI class can be tailored through the use of pragmas, but is not commonly done. The pragmas for this class are:

Pragma | Description
jniclassbase | Base class for the intermediary JNI class
jniclasspackage | Package in which to place the intermediary JNI class
jniclassclassmodifiers | Class modifiers and class type for the intermediary JNI class
jniclasscode | Java code is copied verbatim into the intermediary JNI class
jniclassimports | Java code, usually one or more import statements, placed before the intermediary JNI class definition
jniclassinterfaces | Comma separated interface classes for the intermediary JNI class

The pragma code appears in the generated intermediary JNI class where you would expect:

    [ jniclassimports pragma ]
    [ jniclassclassmodifiers pragma ] jniclassname extends [ jniclassbase pragma ]
                                              implements [ jniclassinterfaces pragma ] {
    [ jniclasscode pragma ]
    ... SWIG generated native methods ...
    }

The `jniclasscode` pragma is quite useful for adding in a static block for loading the shared library / dynamic link library and demonstrates how pragmas work:

    %pragma(java) jniclasscode=%{
      static {
        try {
          System.loadLibrary("example");
        } catch (UnsatisfiedLinkError e) {
          System.err.println("Native code library failed to load. \n" + e);
          System.exit(1);
        }
      }
    %}

Pragmas will take either `""` or `%{ %}` as delimiters. For example, let's change the intermediary JNI class access to just the default package-private access.

    %pragma(java) jniclassclassmodifiers="class"

All the methods in the intermediary JNI class will then not be callable outside of the package as the method modifiers have been changed from public access to default access. This is useful if you want to prevent users calling these low level functions.

### 27.4.2 The Java module class

All global functions and variable getters/setters appear in the module class. For our example, there is just one function:

    public class example {
      public static void egg(Foo chips) {
        exampleJNI.egg(Foo.getCPtr(chips), chips);
      }
    }

The module class is necessary as there is no such thing as a global in Java so all the C globals are put into this class. They are generated as static functions and so must be accessed as such by using the module name in the static function call:

    example.egg(new Foo());

The primary reason for having the module class wrapping the calls in the intermediary JNI class is to implement static type checking. In this case only a `Foo` can be passed to the `egg` function, whereas any `long` can be passed to the `egg` function in the intermediary JNI class.

#### 27.4.2.1 The Java module class pragmas

The module class can be tailored through the use of pragmas, in the same manner as the intermediary JNI class. The pragmas are similarly named and are used in the same way. The complete list follows:

Pragma | Description
modulebase | Base class for the module class
moduleclassmodifiers | Class modifiers and class type for the module class
modulecode | Java code is copied verbatim into the module class
moduleimports | Java code, usually one or more import statements, placed before the module class definition
moduleinterfaces | Comma separated interface classes for the module class

The pragma code appears in the generated module class like this:

    [ moduleimports pragma ]
    [ modulemodifiers pragma ] modulename extends [ modulebase pragma ]
                                          implements [ moduleinterfaces pragma ] {
    [ modulecode pragma ]
    ... SWIG generated wrapper functions ...
    }

See The intermediary JNI class pragmas section for further details on using pragmas.

### 27.4.3 The Java constants interface

C/C++ constants are generated as final static members in a constants interface as mentioned in the Constants section, such as the example in this section:

    public interface exampleConstants {
      public final static int EXPRESSION = (0x100+5);
      public final static long BIG = exampleJNI.BIG_get();
      public final static java.math.BigInteger LARGE = exampleJNI.LARGE_get();
    }

C/C++ enums can also be generated into the same constants interface as described in the Enumerations section.

#### 27.4.3.1 The Java constants interface pragmas

Scope for tailoring the generated interface is limited to one pragma, in the same manner as the intermediary JNI class pragmas and module class pragmas. The pragma details are:

Pragma | Description
constantsmodifiers | Class modifiers and class type for the constants interface, defaults to `public interface`

The pragma code appears in the generated constants interface like this:

    [ constantsmodifiers pragma ] constantsname {
    ... SWIG generated final static variables ...
    }

where `constantsname` is a name created by concatenating the module name and `Constants`, for example, `exampleConstants` for a module named `example`.

The only real use for this pragma is to change the visibility from public to default with:

    %pragma(java) constantsmodifiers="interface"

### 27.4.4 Java proxy classes

A Java proxy class is generated for each structure, union or C++ class that is wrapped. Proxy classes have also been called peer classes. The default proxy class for our previous example looks like this:

    public class Foo {
      private transient long swigCPtr;
      protected transient boolean swigCMemOwn;

      protected Foo(long cPtr, boolean cMemoryOwn) {
        swigCMemOwn = cMemoryOwn;
        swigCPtr = cPtr;
      }

      protected static long getCPtr(Foo obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }

      protected void finalize() {
        delete();
      }

      public synchronized void delete() {
        if(swigCPtr != 0 && swigCMemOwn) {
          swigCMemOwn = false;
          exampleJNI.delete_Foo(swigCPtr);
        }
        swigCPtr = 0;
      }

      public void setX(int value) {
        exampleJNI.Foo_x_set(swigCPtr, this, value);
      }

      public int getX() {
        return exampleJNI.Foo_x_get(swigCPtr, this);
      }

      public int spam(int num, Foo foo) {
        return exampleJNI.Foo_spam(swigCPtr, this, num, Foo.getCPtr(foo), foo);
      }

      public Foo() {
        this(exampleJNI.new_Foo(), true);
      }

    }

This class merely holds a pointer to the underlying C++ object (`swigCPtr`). It also contains all the methods in the C++ class it is proxying plus getters and setters for public member variables. These functions call the native methods in the intermediary JNI class. The advantage of having this extra layer is the type safety that the proxy class functions offer. It adds static type checking which leads to fewer surprises at runtime. For example, you can see that if you attempt to use the `spam()` function it will only compile when the parameters passed are an `int` and a `Foo`. From a user's point of view, it makes the class work as if it were a Java class:

    Foo f = new Foo();
    f.setX(3);
    int y = f.spam(5, new Foo());

#### 27.4.4.1 Memory management

Each proxy class has an ownership flag `swigCMemOwn`. The value of this flag determines who is responsible for deleting the underlying C++ object. If set to `true`, the proxy class's finalizer will destroy the C++ object when the proxy class is garbage collected. If set to false, then the destruction of the proxy class has no effect on the C++ object.

When an object is created by a constructor or returned by value, Java automatically takes ownership of the result. On the other hand, when pointers or references are returned to Java, there is often no way to know where they came from. Therefore, the ownership is set to false. For example:

    class Foo {
    public:
      Foo();
      Foo bar1();
      Foo &bar2();
      Foo *bar2();
    };

In Java:

    Foo f = new Foo();   //  f.swigCMemOwn = true
    Foo f1 = f.bar1();   // f1.swigCMemOwn = true
    Foo f2 = f.bar2();   // f2.swigCMemOwn = false
    Foo f3 = f.bar3();   // f3.swigCMemOwn = false

This behavior for pointers and references is especially important for classes that act as containers. For example, if a method returns a pointer to an object that is contained inside another object, you definitely don't want Java to assume ownership and destroy it!

For the most part, memory management issues remain hidden. However, there are situations where you might have to manually change the ownership of an object. For instance, consider code like this:

    class Obj {};
    class Node {
      Obj *value;
    public:
      void set_value(Obj *v) { value = v; }
    };

Now, consider the following Java code:

    Node n = new Node();    // Create a node
    {
      Obj o = new Obj();    // Create an object
      n.set_value(o);       // Set value
    }                       // o goes out of scope

In this case, the Node `n` is holding a reference to `o` internally. However, SWIG has no way to know that this has occurred. The Java proxy class still thinks that it has ownership of `o`. As `o` has gone out of scope, it could be garbage collected in which case the C++ destructor will be invoked and `n` will then be holding a stale-pointer to `o`. If you're lucky, you will only get a segmentation fault.

To work around this, the ownership flag of `o` needs changing to `false`. The ownership flag is a private member variable of the proxy class so this is not possible without some customization of the proxy class. This can be achieved by using a typemap to customise the proxy class with pure Java code as detailed later in the section on Java typemaps.

Sometimes a function will create memory and return a pointer to a newly allocated object. SWIG has no way of knowing this so by default the proxy class does not manage the returned object. However, you can tell the proxy class to manage the memory if you specify the `%newobject` directive. Consider:

    class Obj {...};
    class Factory {
    public:
      static Obj *createObj() { return new Obj(); }
    };

If we call the factory function, then we have to manually delete the memory:

    Obj obj = Factory.createObj();   // obj.swigCMemOwn = false
    ...
    obj.delete();

Now add in the %newobject directive:

    %newobject Factory::createObj();

    class Obj {...};
    class Factory {
    public:
      static Obj *createObj() { return new Obj(); }
    };

A call to `delete()` is no longer necessary as the garbage collector will make the C++ destructor call because `swigCMemOwn` is now true.

    Obj obj = Factory.createObj();   // obj.swigCMemOwn = true;
    ...

Some memory management issues are quite tricky to fix and may only be noticeable after using for a long time. One such issue is premature garbage collection of an object created from Java and resultant usage from C++ code. The section on typemap examples cover two such scenarios, Memory management for objects passed to the C++ layer and Memory management when returning references to member variables

#### 27.4.4.2 Inheritance

Java proxy classes will mirror C++ inheritance chains. For example, given the base class `Base` and its derived class `Derived`:

    class Base {
    public:
      virtual double foo();
    };

    class Derived : public Base {
    public:
      virtual double foo();
    };

The base class is generated much like any other proxy class seen so far:

    public class Base {
      private transient long swigCPtr;
      protected transient boolean swigCMemOwn;

      protected Base(long cPtr, boolean cMemoryOwn) {
        swigCMemOwn = cMemoryOwn;
        swigCPtr = cPtr;
      }

      protected static long getCPtr(Base obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }

      protected void finalize() {
        delete();
      }

      public synchronized void delete() {
        if(swigCPtr != 0 && swigCMemOwn) {
          swigCMemOwn = false;
          exampleJNI.delete_Base(swigCPtr);
        }
        swigCPtr = 0;
      }

      public double foo() {
        return exampleJNI.Base_foo(swigCPtr, this);
      }

      public Base() {
        this(exampleJNI.new_Base(), true);
      }

    }

The `Derived` class extends `Base` mirroring the C++ class inheritance hierarchy.

    public class Derived extends Base {
      private transient long swigCPtr;

      protected Derived(long cPtr, boolean cMemoryOwn) {
        super(exampleJNI.SWIGDerivedUpcast(cPtr), cMemoryOwn);
        swigCPtr = cPtr;
      }

      protected static long getCPtr(Derived obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }

      protected void finalize() {
        delete();
      }

      public synchronized void delete() {
        if(swigCPtr != 0 && swigCMemOwn) {
          swigCMemOwn = false;
          exampleJNI.delete_Derived(swigCPtr);
        }
        swigCPtr = 0;
        super.delete();
      }

      public double foo() {
        return exampleJNI.Derived_foo(swigCPtr, this);
      }

      public Derived() {
        this(exampleJNI.new_Derived(), true);
      }

    }

Note the memory ownership is controlled by the base class. However each class in the inheritance hierarchy has its own pointer value which is obtained during construction. The `SWIGDerivedUpcast()` call converts the pointer from a `Derived *` to a `Base *`. This is a necessity as C++ compilers are free to implement pointers in the inheritance hierarchy with different values.

It is of course possible to extend `Base` using your own Java classes. If `Derived` is provided by the C++ code, you could for example add in a pure Java class `Extended` derived from `Base`. There is a caveat and that is any C++ code will not know about your pure Java class `Extended` so this type of derivation is restricted. However, true cross language polymorphism can be achieved using the directors feature.

#### 27.4.4.3 Proxy classes and garbage collection

By default each proxy class has a `delete()` and a `finalize()` method. The `finalize()` method calls `delete()` which frees any malloc'd memory for wrapped C structs or calls the C++ class destructors. The idea is for `delete()` to be called when you have finished with the C/C++ object. Ideally you need not call `delete()`, but rather leave it to the garbage collector to call it from the finalizer. When a program exits, the garbage collector does not guarantee to call all finalizers. An insight into the reasoning behind this can be obtained from Hans Boehm's Destructors, Finalizers, and Synchronization paper. Depending on what the finalizers do and which operating system you use, this may or may not be a problem.

If the `delete()` call into JNI code is just for memory handling, there is not a problem when run on most operating systems, for example Windows and Unix. Say your JNI code creates memory on the heap which your finalizers should clean up, the finalizers may or may not be called before the program exits. In Windows and Unix all memory that a process uses is returned to the system on exit, so this isn't a problem. This is not the case in some operating systems like vxWorks. If however, your finalizer calls into JNI code invoking the C++ destructor which in turn releases a TCP/IP socket for example, there is no guarantee that it will be released. Note that with long running programs the garbage collector will eventually run, thereby calling any unreferenced object's finalizers.

Some not so ideal solutions are:

1. Call the `System.runFinalizersOnExit(true)` or `Runtime.getRuntime().runFinalizersOnExit(true)` to ensure the finalizers are called before the program exits. The catch is that this is a deprecated function call as the documentation says:

   This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.

   In many cases you will be lucky and find that it works, but it is not to be advocated. Have a look at Java web site and search for `runFinalizersOnExit`.

2. From jdk1.3 onwards a new function, `addShutdownHook()`, was introduced which is guaranteed to be called when your program exits. You can encourage the garbage collector to call the finalizers, for example, add this static block to the class that has the `main()` function:

         static {
           Runtime.getRuntime().addShutdownHook(
             new Thread() {
               public void run() { System.gc(); System.runFinalization(); }
             }
           );
         }

   Although this usually works, the documentation doesn't guarantee that `runFinalization()` will actually call the finalizers. As the shutdown hook is guaranteed you could also make a JNI call to clean up any resources that are being tracked by the C/C++ code.

3. Call the `delete()` function manually which will immediately invoke the C++ destructor. As a suggestion it may be a good idea to set the object to null so that should the object be inadvertently used again a Java `NullPointerException` is thrown, the alternative would crash the JVM by using a NULL pointer for the C++ "this" pointer. For example given a SWIG generated class A:

       A myA = new A();
       // use myA ...
       myA.delete();
       // any use of myA here would crash the JVM
       myA=null;
       // any use of myA here would cause a Java NullPointerException to be thrown

   The SWIG generated code ensures that the memory is not deleted twice, in the event the finalizers get called in addition to the manual `delete()` call.

   An even more robust solution would be to provide the 'check' typemap to check for a NULL C pointer and automatically convert it into a Java `NullPointerException` as described in the C++ "this" pointer section. Then the JVM crash would be avoided and instead a more user-friendly exception would occur, such as:

       A myA = new A();
       // use myA ...
       myA.delete();
       // any use of myA here would cause a Java NullPointerException to be thrown

4. Write your own object manager in Java. You could derive all SWIG classes from a single base class which could track which objects have had their finalizers run, then call the rest of them on program termination. The section on Java typemaps details how to specify a pure Java base class.

See the How to Handle Java Finalization's Memory-Retention Issues article for alternative approaches to managing memory by avoiding finalizers altogether.

#### 27.4.4.4 The premature garbage collection prevention parameter for proxy class marshalling

As covered earlier, the C/C++ struct/class pointer is stored in the proxy class as a Java long and when needed is passed into the native method where it is cast into the appropriate type. This approach provides very fast marshalling but could be susceptible to premature garbage collection. Consider the following C++ code:

    class Wibble {
    };
    void wobble(Wibble &w);

The module class contains the Java wrapper for the global `wobble` method:

    public class example {
      ...
      public static void wobble(Wibble w) {
        exampleJNI.wobble(Wibble.getCPtr(w), w);
      }
    }

where `example` is the name of the module. All native methods go through the intermediary class which has the native method declared as such:

    public class exampleJNI {
      ...
      public final static native void wobble(long jarg1, Wibble jarg1_);
    }

The second parameter, `jarg1_`, is the premature garbage collection prevention parameter and is added to the native method parameter list whenever a C/C++ struct or class is marshalled as a Java long. In order to understand why, consider the alternative where the intermediary class method is declared without the additional parameter:

    public class exampleJNI {
      ...
      public final static native void wobble(long jarg1);
    }

and the following simple call to `wobble`:

    {
      Wibble w = new Wibble();
      example.wobble(w);
    }

The hotspot compiler effectively sees something like:

    {
      Wibble w = new Wibble();
      long w_ptr = Wibble.getCPtr(w);
      // w is no longer reachable
      exampleJNI.wobble(w_ptr);
    }

The `Wibble` object is no longer reachable after the point shown as in this bit of code, the `Wibble` object is not referenced again after this point. This means that it is a candidate for garbage collection. Should `wobble` be a long running method, it is quite likely that the finalizer for the `Wibble` instance will be called. This in turn will call its underlying C++ destructor which is obviously disastrous while the method `wobble` is running using this object. Even if `wobble` is not a long running method, it is possible for the `Wibble` instance to be finalized. By passing the `Wibble` instance into the native method, it will not be finalized as the JVM guarantees not to finalize any objects until the native method returns. Effectively, the code then becomes

    {
      Wibble w = new Wibble();
      long w_ptr = Wibble.getCPtr(w);
      exampleJNI.wobble(w_ptr, w);
      // w is no longer reachable
    }

and therefore there is no possibility of premature garbage collection. In practice, this premature garbage collection was only ever observed in Sun's server JVM from jdk-1.3 onwards and in Sun's client JVM from jdk-1.6 onwards.

The premature garbage collection prevention parameter for proxy classes is generated by default whenever proxy classes are passed by value, reference or with a pointer. The implementation for this extra parameter generation requires the "jtype" typemap to contain `long` and the "jstype" typemap to contain the name of a proxy class.

The additional parameter does impose a slight performance overhead and the parameter generation can be suppressed globally with the `-nopgcpp` commandline option. More selective suppression is possible with the 'nopgcpp' attribute in the "jtype" Java typemap. The attribute is a flag and so should be set to "1" to enable the suppression, or it can be omitted or set to "0" to disable. For example:

    %typemap(jtype, nopgcpp="1") Wibble & "long"

Compatibility note: The generation of this additional parameter did not occur in versions prior to SWIG-1.3.30.

#### 27.4.4.5 Single threaded applications and thread safety

Single threaded Java applications using JNI need to consider thread safety. The same applies for the C# module where the .NET wrappers use PInvoke. Consider the C++ class:

    class Test {
      string str;
    public:
      Test() : str("initial") {}
    };

and the Java proxy class generated by SWIG:

    public class Test {
      private transient long swigCPtr;
      protected transient boolean swigCMemOwn;

      protected Test(long cPtr, boolean cMemoryOwn) {
        swigCMemOwn = cMemoryOwn;
        swigCPtr = cPtr;
      }

      protected static long getCPtr(Test obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }

      protected void finalize() {
        delete();
      }

      // Call C++ destructor
      public synchronized void delete() {
        if(swigCPtr != 0 && swigCMemOwn) {
          swigCMemOwn = false;
          exampleJNI.delete_Test(swigCPtr);
        }
        swigCPtr = 0;
      }

      // Call C++ constructor
      public Test() {
        this(exampleJNI.new_Test(), true);
      }

    }

It has two methods that call JNI methods, namely, `exampleJNI.new_Test()` for the C++ constructor and `exampleJNI.delete_Test()` for the C++ destructor. If the garbage collector collects an instance of this class, ie `delete()` is not explicitly called, then the C++ destructor will be run in a different thread to the main thread. This is because when an object is marked for garbage collection, any objects with finalizers are added to a finalization queue and the objects in the finalization queue have their `finalize()` methods run in a separate finalization thread. Therefore, if the C memory allocator is not thread safe, then the heap will get corrupted sooner or later, when a concurrent C++ delete and new are executed. It is thus essential, even in single threaded usage, to link to the C multi-thread runtime libraries, for example, use the /MD option for Visual C++ on Windows. Alternatively, lock all access to C++ functions that have heap allocation/deallocation.

Note that some of the STL in Visual C++ 6 is not thread safe, so although code might be linked to the multithread runtime libraries, undefined behaviour might still occur in a single threaded Java program. Similarly some older versions of Sun Studio have bugs in the multi-threaded implementation of the std::string class and so will lead to undefined behaviour in these supposedly single threaded Java applications.

The following innocuous Java usage of Test is an example that will crash very quickly on a multiprocessor machine if the JNI compiled code is linked against the single thread C runtime libraries.

    for (int i=0; i<100000; i++) {
      System.out.println("Iteration " + i);
      for (int k=0; k<10; k++) {
        Test test = new Test();
      }
      System.gc();
    }

### 27.4.5 Type wrapper classes

The generated type wrapper class, for say an `int *`, looks like this:

    public class SWIGTYPE_p_int {
      private transient long swigCPtr;

      protected SWIGTYPE_p_int(long cPtr, boolean bFutureUse) {
        swigCPtr = cPtr;
      }

      protected SWIGTYPE_p_int() {
        swigCPtr = 0;
      }

      protected static long getCPtr(SWIGTYPE_p_int obj) {
        return obj.swigCPtr;
      }
    }

The methods do not have public access, so by default it is impossible to do anything with objects of this class other than pass them around. The methods in the class are part of the inner workings of SWIG. If you need to mess around with pointers you will have to use some typemaps specific to the Java module to achieve this. The section on Java typemaps details how to modify the generated code.

Note that if you use a pointer or reference to a proxy class in a function then no type wrapper class is generated because the proxy class can be used as the function parameter. If however, you need anything more complicated like a pointer to a pointer to a proxy class then a typewrapper class is generated for your use.

Note that SWIG generates a type wrapper class and not a proxy class when it has not parsed the definition of a type that gets used. For example, say SWIG has not parsed the definition of `class Snazzy` because it is in a header file that you may have forgotten to use the `%include` directive on. Should SWIG parse `Snazzy *` being used in a function parameter, it will then generates a type wrapper class around a `Snazzy` pointer. Also recall from earlier that SWIG will use a pointer when a class is passed by value or by reference:

    void spam(Snazzy *x, Snazzy &y, Snazzy z);

Should SWIG not know anything about `Snazzy` then a `SWIGTYPE_p_Snazzy` must be used for all 3 parameters in the `spam` function. The Java function generated is:

    public static void spam(SWIGTYPE_p_Snazzy x, SWIGTYPE_p_Snazzy y, SWIGTYPE_p_Snazzy z) {
     ...
    }

Note that typedefs are tracked by SWIG and the typedef name is used to construct the type wrapper class name. For example, consider the case where `Snazzy` is a typedef to an `int` which SWIG does parse:

    typedef int Snazzy;
    void spam(Snazzy *x, Snazzy &y, Snazzy z);

Because the typedefs have been tracked the Java function generated is:

    public static void spam(SWIGTYPE_p_int x, SWIGTYPE_p_int y, int z) { ... }

### 27.4.6 Enum classes

SWIG can generate three types of enum classes. The Enumerations section discussed these but omitted all the details. The following sub-sections detail the various types of enum classes that can be generated.

#### 27.4.6.1 Typesafe enum classes

The following example demonstrates the typesafe enum classes which SWIG generates:

    %include "enumtypesafe.swg"
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

The following is the code that SWIG generates:

    public final class Beverage {
      public final static Beverage ALE = new Beverage("ALE");
      public final static Beverage LAGER = new Beverage("LAGER", 10);
      public final static Beverage STOUT = new Beverage("STOUT");
      public final static Beverage PILSNER = new Beverage("PILSNER");
      public final static Beverage PILZ = new Beverage("PILZ", PILSNER);

      public final int swigValue() {
        return swigValue;
      }

      public String toString() {
        return swigName;
      }

      public static Beverage swigToEnum(int swigValue) {
        if (swigValue < swigValues.length && swigValue >= 0 &&
            swigValues[swigValue].swigValue == swigValue)
          return swigValues[swigValue];
        for (int i = 0; i < swigValues.length; i++)
          if (swigValues[i].swigValue == swigValue)
            return swigValues[i];
        throw new IllegalArgumentException("No enum " + Beverage.class + " with value " +
                                                                             swigValue);
      }

      private Beverage(String swigName) {
        this.swigName = swigName;
        this.swigValue = swigNext++;
      }

      private Beverage(String swigName, int swigValue) {
        this.swigName = swigName;
        this.swigValue = swigValue;
        swigNext = swigValue+1;
      }

      private Beverage(String swigName, Beverage swigEnum) {
        this.swigName = swigName;
        this.swigValue = swigEnum.swigValue;
        swigNext = this.swigValue+1;
      }

      private static Beverage[] swigValues = { ALE, LAGER, STOUT, PILSNER, PILZ };
      private static int swigNext = 0;
      private final int swigValue;
      private final String swigName;
    }

As can be seen, there are a fair number of support methods for the typesafe enum pattern. The typesafe enum pattern involves creating a fixed number of static instances of the enum class. The constructors are private to enforce this. Three constructors are available - two for C/C++ enums with an initializer and one for those without an initializer. Note that the two enums with initializers, `LAGER` and `PILZ`, each call one the two different initializer constructors. In order to use one of these typesafe enums, the `swigToEnum` static method must be called to return a reference to one of the static instances. The JNI layer returns the enum value from the C/C++ world as an integer and this method is used to find the appropriate Java enum static instance. The `swigValue` method is used for marshalling in the other direction. The `toString` method is overridden so that the enum name is available.

#### 27.4.6.2 Proper Java enum classes

The following example demonstrates the Java enums approach:

    %include "enums.swg"
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

SWIG will generate the following Java enum:

    public enum Beverage {
      ALE,
      LAGER(10),
      STOUT,
      PILSNER,
      PILZ(PILSNER);

      public final int swigValue() {
        return swigValue;
      }

      public static Beverage swigToEnum(int swigValue) {
        Beverage[] swigValues = Beverage.class.getEnumConstants();
        if (swigValue < swigValues.length && swigValue >= 0 &&
            swigValues[swigValue].swigValue == swigValue)
          return swigValues[swigValue];
        for (Beverage swigEnum : swigValues)
          if (swigEnum.swigValue == swigValue)
            return swigEnum;
        throw new IllegalArgumentException("No enum " + Beverage.class +
                                           " with value " + swigValue);
      }

      private Beverage() {
        this.swigValue = SwigNext.next++;
      }

      private Beverage(int swigValue) {
        this.swigValue = swigValue;
        SwigNext.next = swigValue+1;
      }

      private Beverage(Beverage swigEnum) {
        this.swigValue = swigEnum.swigValue;
        SwigNext.next = this.swigValue+1;
      }

      private final int swigValue;

      private static class SwigNext {
        private static int next = 0;
      }
    }

The enum items appear first. Like the typesafe enum pattern, the constructors are private. The constructors are required to handle C/C++ enums with initializers. The `next` variable is in the `SwigNext` inner class rather than in the enum class as static primitive variables cannot be modified from within enum constructors. Marshalling between Java enums and the C/C++ enum integer value is handled via the `swigToEnum` and `swigValue` methods. All the constructors and methods in the Java enum are required just to handle C/C++ enums with initializers. These needn't be generated if the enum being wrapped does not have any initializers and the Simpler Java enums for enums without initializers section describes how typemaps can be used to achieve this.

#### 27.4.6.3 Type unsafe enum classes

The following example demonstrates type unsafe enums:

    %include "enumtypeunsafe.swg"
    %javaconst(1);
    enum Beverage { ALE, LAGER=10, STOUT, PILSNER, PILZ=PILSNER };

SWIG will generate the following simple class:

    public final class Beverage {
      public final static int ALE = 0;
      public final static int LAGER = 10;
      public final static int STOUT = LAGER + 1;
      public final static int PILSNER = STOUT + 1;
      public final static int PILZ = PILSNER;
    }

### 27.4.7 Interfaces

By default SWIG wraps all C++ classes as Java classes. As Java only supports derivation from a single base class, SWIG has to ignore all bases except the first when a C++ class inherits from more than one base class. However, there is a family of SWIG macros that change the default wrapping and allows a C++ class to be wrapped as a Java interface instead of a Java class. These macros provide a way to support some sort of multiple inheritance as there is no limit to the number of interfaces that a Java class can inherit from.

When a C++ class is wrapped as a Java interface, a Java proxy class is still needed. The `swiginterface.i` library file provides three macros for marking a C++ class to be wrapped as a Java interface. There is more than one macro in order to provide a choice for choosing the Java interface and Java proxy names.

Interface Macro Name | Description
`%interface(CTYPE)` | For C++ class `CTYPE`, proxy class name is unchanged without any suffix added, interface name has `SwigInterface` added as a suffix.
`%interface_impl(CTYPE)` | For C++ class `CTYPE`, proxy class name has `SwigImpl` added as a suffix, interface name has no added suffix.
`%interface_custom("PROXY", "INTERFACE", CTYPE)` | For C++ class `CTYPE`, proxy class name is given by the string `PROXY`, interface name is given by the string `INTERFACE`. The `PROXY` and `INTERFACE` names can use the string formatting functions used in `%rename`.
`%interface_additional("PROXY", "INTERFACE", "ADDITIONAL", CTYPE)` | For C++ class `CTYPE`, proxy class name is given by the string `PROXY`, interface name is given by the string `INTERFACE`. The `PROXY` and `INTERFACE` names can use the string formatting functions used in `%rename`. `ADDITIONAL` should contain a comma separated list of interfaces for the interface class to additionally extend. This is for adding interfaces not parsed by SWIG and is useful for adding in pure Java interfaces

The table below has a few examples showing the resulting proxy and interface names for a C++ class called `Base`.

Example Usage | Proxy Class Name | Interface Class Name
`%interface(Base)` | `Base` | `BaseSwigInterface`
`%interface_impl(Base)` | `BaseSwigImpl` | `Base`
`%interface_custom("BaseProxy", "IBase", Base)` | `BaseProxy` | `IBase`
`%interface_custom("%sProxy", "IBase", Base)` | `BaseProxy` | `IBase`
`%interface_custom("%sProxy", "%sInterface", Base)` | `BaseProxy` | `BaseProxyInterface`
`%interface_custom("%sProxy", "%(rstrip:[Proxy])sInterface", Base)` | `BaseProxy` | `BaseInterface`

The 2nd last example shows the names used in the string formatting functions. The input for `PROXY` that `"%s"` expands to is the proxy name, that is, Base. The input for `INTERFACE` that `"%s"` expands to is the proxy name, that is, `BaseProxy`.

The last example shows `rstrip` and in this case strips the `Proxy` suffix and then adds on `Interface`.

Consider the following C++ code:

    namespace Space {
      struct Base1 {
        virtual void Method1() const;
        virtual ~Base1();
      };
      struct Base2 {
        virtual void Method2() const;
        virtual ~Base2();
      };
      struct Derived : Base1, Base2 {
      };
      void UseBases(const Base1 &b1, const Base2 &b2);
    }

By default all classes are wrapped and are available in Java, but, `Derived` has all bases ignored except the first. SWIG generates a warning for the above code:

    example.i:12: Warning 813: Warning for Derived, base Base2 ignored.
    Multiple inheritance is not supported in Java.

If we decide to wrap the two base classes as interfaces and add the following before SWIG parses the above example code:

    %include <swiginterface.i>
    %interface_impl(Space::Base1);
    %interface_impl(Space::Base2);

then two interface files are generated, Base1.java and Base2.java in addition to proxy class files, Base1SwigImpl.java and Base2SwigImpl.java. The contents of interface file Base1.java for `Base1` is shown below:

    public interface Base1 {
      long Base1_GetInterfaceCPtr();
      void Method1();
    }

The proxy class in Base1SwigImpl.java for Base1 is as it would have been if `%interface` was not used, except the name has changed to `Base1SwigImpl` and it implements the appropriate base:

    public class Base1SwigImpl implements Base1 {
    ...
      public long Base1_GetInterfaceCPtr() {
        return exampleJNI.Base1SwigImpl_Base1_GetInterfaceCPtr(swigCPtr);
      }

      public void Method1() {
        exampleJNI.Base1SwigImpl_Method1(swigCPtr, this);
      }
    ...
    }

In fact any class using `Base` as an immediate base class will now implement the interface instead of deriving from it (or ignoring the base in the case of multiple base classes). Hence the `Derived` proxy class will now implement both bases:

    public class Derived implements Base1, Base2 {
    ...
      public long Base1_GetInterfaceCPtr() {
        return exampleJNI.Derived_Base1_GetInterfaceCPtr(swigCPtr);
      }

      public long Base2_GetInterfaceCPtr() {
        return exampleJNI.Derived_Base2_GetInterfaceCPtr(swigCPtr);
      }

      public void Method1() {
        exampleJNI.Derived_Method1(swigCPtr, this);
      }

      public void Method2() {
        exampleJNI.Derived_Method2(swigCPtr, this);
      }
    ...
    }

The proxy class has methods added to it, from the implemented bases, so that the underlying C++ implementation can be called. In the example above, `Method1` and `Method2` have been added from the implemented bases. If a method is ignored in the base, such as via `%ignore`, then that method will be excluded from the interface and there will not be an additional method added to the proxy class implementing that interface.

The Java interface only ever contains virtual and non-virtual instance methods from the wrapped C++ class. Any static methods, enums or variables in the wrapped C++ class are not supported and are not added to the interface. They are of course still available in the Java proxy class.

Wherever a class marked as an interface is used, such as the `UseBases` method in the example, the interface name is used as the type in the Java layer:

      public static void UseBases(Base1 b1, Base2 b2) {
        exampleJNI.UseBases(b1.Base1_GetInterfaceCPtr(), b1, b2.Base2_GetInterfaceCPtr(), b2);
      }

Note that each Java interface has a method added to obtain the correct C++ pointer for passing to the native function - `Base1_GetInterfaceCPtr` for `Base1`. This method is similar to the `getCPtr` method in the proxy classes. In fact, as shown above in the `Derived` class, the proxy classes implement this generated interface by calling a native method (`Derived_Base1_GetInterfaceCPtr`) which calls an appropriate C++ cast of the pointer up the inheritance chain.

The `%interface_additional` macro is useful for adding additional interfaces to the Java interface class. Consider a simple class hierarchy, where `Whizz` inherits from `Bang` and we want these two classes to be wrapped as Java interfaces and the `Whizz` interface to additionally extend from the pure Java interface `java.util.EventListener`.

    %interface_custom("Bang", "IBang", Space::Bang);
    %interface_additional("Whizz", "IWhizz", "java.util.EventListener", Space::Whizz)

    namespace Space {
      struct Bang {
        virtual void bang() const = 0;
      };
      struct Whizz : Bang {
        virtual void whizz() const = 0;
      };
    }

The classes and interfaces generated are shown below, noting that `IWhizz` extends the additional interface `java.util.EventListener`:

    public interface IBang { ... }

    public interface IWhizz extends java.util.EventListener, IBang { ... }

    public class Bang implements IBang { ... }

    public class Whizz implements IWhizz, IBang { ... }

Note the subtle difference to the `javainterfaces` typemap discussed elsewhere. The typemap applies to the generated Java proxy class only, not the Java interface. A slight change to our example above as follows:

    %interface_custom("Bang", "IBang", Space::Bang);
    %interface_custom("Whizz", "IWhizz", Space::Whizz)
    %typemap(javainterfaces) Space::Whizz "java.util.EventListener"

    namespace Space {
    ... as shown above ...
    }

will then the generate different code for the proxy class `Whizz` and the interface `IWhizz` as follows:

    public interface IBang { ... }

    public interface IWhizz extends IBang { ... }

    public class Bang implements IBang { ... }

    public class Whizz implements IWhizz, IBang, java.util.EventListener { ... }

Note that if a class uses one of the `%interface` family of macros on an abstract class (one with pure virtual methods such as `Bang` above) and another C++ class derives from it and is also abstract, say because it does not implement all pure virtual methods up the class hierarchy, then the wrapped Java proxy class will be abstract too. The generated Java proxy class for the derived class will not be compilable by the Java compiler as it is also abstract. The simple advice to avoid this problem is to use one of the `%interface` macros on all classes that are abstract in an inheritance chain as SWIG will then generate both a Java interface and a Java proxy class with an implementation for the interface methods for the wrapped abstract C++ class.

Finally, a note on the implementation of the interface macros. The interface macros are implemented using the `interface` feature and typemaps. A couple of examples:

    %define %interface(CTYPE...)
    %feature("interface", name="%sSwigInterface") CTYPE;
    INTERFACE_TYPEMAPS(CTYPE)
    %enddef

    %define %interface_additional(PROXY, INTERFACE, ADDITIONAL, CTYPE...)
    %rename(PROXY) CTYPE;
    %feature("interface", name=INTERFACE, additional=ADDITIONAL) CTYPE;
    INTERFACE_TYPEMAPS(CTYPE)
    %enddef

The feature accepts two attributes named `name` and `additional`. The `name` attribute should contain the name of the Java interface. The `additional` attribute should contain additional interfaces for the interface class, not parsed by SWIG. The `INTERFACE_TYPEMAPS` macro implements the typemaps and can be viewed in the `swiginterface.i` file and contains the usual Java typemaps for generating code plus the `javainterfacecode` typemap which is only used when a class is marked with the `interface` feature. See Java code typemaps for details.

Compatibility note: The `additional` attribute and `%interface_additional` macro was added in SWIG-4.3.0.

## 27.5 Cross language polymorphism using directors

Proxy classes provide a natural, object-oriented way to wrap C++ classes. as described earlier, each proxy instance has an associated C++ instance, and method calls from Java to the proxy are passed to the C++ instance transparently via C wrapper functions.

This arrangement is asymmetric in the sense that no corresponding mechanism exists to pass method calls down the inheritance chain from C++ to Java. In particular, if a C++ class has been extended in Java (by deriving from the proxy class), these classes will not be visible from C++ code. Virtual method calls from C++ are thus not able to access the lowest implementation in the inheritance chain.

SWIG can address this problem and make the relationship between C++ classes and proxy classes more symmetric. To achieve this goal, new classes called directors are introduced at the bottom of the C++ inheritance chain. The job of the directors is to route method calls correctly, either to C++ implementations higher in the inheritance chain or to Java implementations lower in the inheritance chain. The upshot is that C++ classes can be extended in Java and from C++ these extensions look exactly like native C++ classes. Neither C++ code nor Java code needs to know where a particular method is implemented: the combination of proxy classes, director classes, and C wrapper functions transparently takes care of all the cross-language method routing.

### 27.5.1 Enabling directors

The director feature is disabled by default. To use directors you must make two changes to the interface file. First, add the "directors" option to the %module directive, like this:

    %module(directors="1") modulename

Without this option no director code will be generated. Second, you must use the %feature("director") directive to tell SWIG which classes and methods should get directors. The %feature directive can be applied globally, to specific classes, and to specific methods, like this:

    // generate directors for all classes that have virtual methods
    %feature("director");

    // generate directors for the virtual methods in class Foo
    %feature("director") Foo;

You can use the %feature("nodirector") directive to turn off directors for specific classes or methods. So for example,

    %feature("director") Foo;
    %feature("nodirector") Foo::bar;

will generate directors for the virtual methods of class Foo except bar().

Directors can also be generated implicitly through inheritance. In the following, class Bar will get a director class that handles the methods one() and two() (but not three()):

    %feature("director") Foo;
    class Foo {
    public:
      virtual void one();
      virtual void two();
    };

    class Bar: public Foo {
    public:
      virtual void three();
    };

### 27.5.2 Director classes

For each class that has directors enabled, SWIG generates a new class that derives from both the class in question and a special `Swig::Director` class. These new classes, referred to as director classes, can be loosely thought of as the C++ equivalent of the Java proxy classes. The director classes store a pointer to their underlying Java proxy classes.

For simplicity let's ignore the `Swig::Director` class and refer to the original C++ class as the director's base class. By default, a director class extends all virtual methods in the inheritance chain of its base class (see the preceding section for how to modify this behavior). Virtual methods that have a final specifier are unsurprisingly excluded. Thus the virtual method calls, whether they originate in C++ or in Java via proxy classes, eventually end up in at the implementation in the director class. The job of the director methods is to route these method calls to the appropriate place in the inheritance chain. By "appropriate place" we mean the method that would have been called if the C++ base class and its Java derived classes were seamlessly integrated. That seamless integration is exactly what the director classes provide, transparently skipping over all the messy JNI glue code that binds the two languages together.

In reality, the "appropriate place" is one of only two possibilities: C++ or Java. Once this decision is made, the rest is fairly easy. If the correct implementation is in C++, then the lowest implementation of the method in the C++ inheritance chain is called explicitly. If the correct implementation is in Java, the Java API is used to call the method of the underlying Java object (after which the usual virtual method resolution in Java automatically finds the right implementation).

### 27.5.3 Overhead and code bloat

Enabling directors for a class will generate a new director method for every virtual method in the class' inheritance chain. This alone can generate a lot of code bloat for large hierarchies. Method arguments that require complex conversions to and from Java types can result in large director methods. For this reason it is recommended that directors are selectively enabled only for specific classes that are likely to be extended in Java and used in C++.

Although directors make it natural to mix native C++ objects with Java objects (as director objects), one should be aware of the obvious fact that method calls to Java objects from C++ will be much slower than calls to C++ objects. Additionally, compared to classes that do not use directors, the call routing in the director methods adds a small overhead. This situation can be optimized by selectively enabling director methods (using the %feature directive) for only those methods that are likely to be extended in Java.

### 27.5.4 Simple directors example

Consider the following SWIG interface file:

    %module(directors="1") example;

    %feature("director") DirectorBase;

    class DirectorBase {
    public:
      virtual ~DirectorBase() {}
      virtual void upcall_method() {}
    };

    void callup(DirectorBase *director) {
      director->upcall_method();
    }

The following `DirectorDerived` Java class is derived from the Java proxy class `DirectorBase` and overrides `upcall_method()`. When C++ code invokes `upcall_method()`, the SWIG-generated C++ code redirects the call via JNI to the Java `DirectorDerived` subclass. Naturally, the SWIG generated C++ code and the generated Java intermediary class marshal and convert arguments between C++ and Java when needed.

    class DirectorDerived extends DirectorBase {
      @Override
      public void upcall_method() {
        System.out.println("DirectorDerived.upcall_method() invoked.");
      }
    }

Running the following Java code

    DirectorDerived director = new DirectorDerived();
    example.callup(director);

will result in the following being output:

    DirectorDerived.upcall_method() invoked.

### 27.5.5 Director threading issues

Depending on your operating system and version of Java and how you are using threads, you might find the JVM hangs on exit. There are a couple of solutions to try out. The preferred solution requires jdk-1.4 and later and uses `AttachCurrentThreadAsDaemon` instead of `AttachCurrentThread` whenever a call into the JVM is required. This can be enabled by defining the SWIG_JAVA_ATTACH_CURRENT_THREAD_AS_DAEMON macro when compiling the C++ wrapper code. For older JVMs define SWIG_JAVA_NO_DETACH_CURRENT_THREAD instead, to avoid the `DetachCurrentThread` call but this will result in a memory leak instead. For further details inspect the source code in the java/director.swg library file.

Macros can be defined on the commandline when compiling your C++ code, or alternatively added to the C++ wrapper file as shown below:

    %insert("runtime") %{
    #define SWIG_JAVA_NO_DETACH_CURRENT_THREAD
    %}

### 27.5.6 Director performance tuning

When a new instance of a director (or subclass) is created in Java, the C++ side of the director performs a runtime check per director method to determine if that particular method is overridden in Java or if it should invoke the C++ base implementation directly. Although this makes initialization slightly more expensive, it is generally a good overall tradeoff.

However, if all director methods are expected to usually be overridden by Java subclasses, then initialization can be made faster by avoiding these checks via the `assumeoverride` attribute. For example:

    %feature("director", assumeoverride=1) Foo;

The disadvantage is that invocation of director methods from C++ when Java doesn't actually override the method will require an additional call up into Java and back to C++. As such, this option is only useful when overrides are extremely common and instantiation is frequent enough that its performance is critical.

### 27.5.7 Java exceptions from directors

With directors routing method calls to Java, and proxies routing them to C++, the handling of exceptions is an important concern. The default behavior for Java exceptions thrown in a director method overridden in Java is to store the thrown Java exception into a SWIG defined `Swig::DirectorException` C++ class exception in the C++ layer and then throw this C++ exception.

Of course, should this exception be thrown, your C++ code must catch it and handle it before returning back to Java. The default generated code does not attempt to handle the C++ exception, but there is a simple way to make this all work by catching the C++ exception and extracting the original Java exception by using `%catches` for `Swig::DirectorException`. Consider the example shown earlier with a modification to the `upcall_method` Java method to throw a Java exception:

    class DirectorDerived extends DirectorBase {
      @Override
      public void upcall_method() {
        System.out.println("DirectorDerived.upcall_method() invoked.");
        throw new RuntimeException("There was a problem!");
      }
    }

Now, by default, the JVM will abort when `example.callup(director)` is called as the C++ `Swig::DirectorException` (storing the Java exception) is thrown and not handled by the `callup` method. Needless to say this is not very user friendly and so the recommendation is to add the following simple `%catches` directive before SWIG parses the `callup` function:

    %catches(Swig::DirectorException) callup;

Or target all wrapped methods using:

    %catches(Swig::DirectorException);

This tells SWIG to generate a C++ catch handler using some code from the throws typemap for `Swig::DirectorException` that SWIG supplies by default, see Exception handling with %catches. This typemap code is written to simply catch the C++ `Swig::DirectorException` class and immediately return to Java throwing the original Java exception that it has stored. The net result is a stack trace containing the original Java exception including the location that the exception was thrown from.

    DirectorDerived.upcall_method() invoked.
    Exception in thread "main" java.lang.RuntimeException: There was a problem!
        at DirectorDerived.upcall_method(runme.java:4)
        at exampleJNI.SwigDirector_DirectorBase_upcall_method(exampleJNI.java:20)
        at exampleJNI.callup(Native Method)
        at example.callup(example.java:12)
        at runme.main(runme.java:21)

More on the `Swig::DirectorException` class can be found in the next section which details how to customize the handling of director exceptions.

#### 27.5.7.1 Customizing director exceptions

This section is for advanced customization of director exceptions. The recommendation for most users is to use the simple `%catches` directive described above as it should be sufficient for most users needs.

The conversion of Java exceptions into C++ exceptions can be customized in two different ways using the `director:except` feature. In the first approach, a code block is attached to each director method to handle the mapping of Java exceptions into C++ exceptions. The code block is generated just after the call up from the C++ director method into the overloaded method in Java. Its primary function is to check if a Java exception has been thrown and then handle it in C++. The example below converts a `java.lang.IndexOutOfBoundsException` into a C++ `std::out_of_range` exception and converts a user's Java `MyJavaException` into a C++ `MyCppException` exception. If the Java exception doesn't match either of these, a fallback `std::runtime_error` C++ exception is thrown.

    %feature("director:except") MyClass::dirmethod(int x) {
      jthrowable $error = jenv->ExceptionOccurred();
      if ($error) {
        if (Swig::ExceptionMatches(jenv, $error, "java/lang/IndexOutOfBoundsException"))
          throw std::out_of_range(Swig::JavaExceptionMessage(jenv, $error).message());
        if (Swig::ExceptionMatches(jenv, $error, "$packagepath/MyJavaException"))
          throw MyCppException(Swig::JavaExceptionMessage(jenv, $error).message());
        throw std::runtime_error("Unexpected exception thrown in MyClass::dirmethod");
      }
    }

    class MyClass {
    public:
      /** Throws either a std::out_of_range or MyCppException on error */
      virtual void dirmethod(int x);
      virtual ~MyClass();
    };

A few special variables are expanded within the `director:except` feature.

- The special variable `$error` is expanded into a unique variable name (swigerror) and should be used for the assignment of the jthrowable exception that occurred.
- The special variable `$packagepath` is replaced by the outer package provided for SWIG generation by the -package option.
- The special variable `$directorthrowshandlers` is not shown above, but is replaced by applicable "directorthrows" typemap contents (covered later in this section).
- The special variable `$null` is not shown above, but is replaced by a suitable default constructed object for returning from the director method (or nothing if the director method has a void return).

Utility functions/classes in director.swg are provided to aid the exception conversion as follows:

    namespace Swig {

      // Helper method to determine if a Java throwable matches a particular Java class type
      // Note side effect of clearing any pending exceptions
      bool ExceptionMatches(JNIEnv *jenv, jthrowable throwable, const char *classname);

      // Helper class to extract the exception message from a Java throwable
      class JavaExceptionMessage {
      public:
        JavaExceptionMessage(JNIEnv *jenv, jthrowable throwable);

        // Return a C string of the exception message in the jthrowable passed in the constructor
        // If no message is available, null_string is return instead
        const char *message(const char *null_string =
                            "Could not get exception message in JavaExceptionMessage") const;
      };

      // C++ Exception class for handling Java exceptions thrown during a director method Java upcall
      class DirectorException : public std::exception {
      public:

        // Construct exception from a Java throwable
        DirectorException(JNIEnv *jenv, jthrowable throwable);

        // More general constructor for handling as a java.lang.RuntimeException
        DirectorException(const char *msg);

        // Return exception message extracted from the Java throwable
        const char *what() const throw();

        // Reconstruct and raise/throw the Java Exception that caused the DirectorException
        // Note that any error in the JNI exception handling results in a Java RuntimeException
        void throwException(JNIEnv *jenv) const;

        // Create and throw the DirectorException
        static void raise(JNIEnv *jenv, jthrowable throwable) {
          throw DirectorException(jenv, throwable);
        }
      };

    }

The utility function `Swig::ExceptionMatches` and class `Swig::JavaExceptionMessage` are provided to simplify writing code for wrappers that use the `director:except` feature. The function `Swig::ExceptionMatches` matches the type of the `jthrowable` thrown against a fully qualified JNI style class name, such as `"java/lang/IOError"`. If the throwable class is the same type, or derives from the given type, `Swig::ExceptionMatches` will return true. Care must be taken to provide the correct fully qualified name, since for wrapped exceptions the generated proxy class will have an additional package qualification, depending on the '-package' argument and use of the nspace feature. The utility class `Swig::JavaExceptionMessage` is a holder providing access to the message from the thrown Java exception. The `message()` method returns the exception message as a `const char *`, which is only valid during the lifetime of the holder. Any code using this message needs to copy it, for example into a std::string or a newly constructed C++ exception.

Using the first approach above to write handlers for a large number of methods will require repetitive duplication of the `director:except` feature code for each director method. To mitigate this, a second approach is provided via typemaps in a fashion analogous to the "throws" typemap. The "throws" typemap provides a way to map all the C++ exceptions listed in a method's defined exceptions (either from a C++ exception specification or a `%catches` feature) into Java exceptions. The "directorthrows" typemap provides the inverse mapping and should contain code to convert a suitably matching Java exception into a C++ exception. Only use this typemap if you wish to write custom conversions of Java exceptions into C++ exceptions and apply them to many different methods. The default handling which uses the `Swig::DirectorException` class should otherwise meet your needs.

The example below converts a Java `java.lang.IndexOutOfBoundsException` exception to the typemap's type, that is a `std::out_of_range` C++ exception:

    %typemap(directorthrows) std::out_of_range %{
      if (Swig::ExceptionMatches(jenv, $error, "java/lang/IndexOutOfBoundsException")) {
        throw std::out_of_range(Swig::JavaExceptionMessage(jenv, $error).message());
      }
    %}

The "directorthrows" typemap is then used in conjunction with the `director:except` feature if the `$directorthrowshandlers` special variable is used in the code block. Consider the following, which also happens to be the default:

    %feature("director:except") %{
      jthrowable $error = jenv->ExceptionOccurred();
      if ($error) {
        $directorthrowshandlers
        Swig::DirectorException::raise(jenv, $error);
      }
    %}

where `Swig::DirectorException::raise` is the helper method to throw a C++ `Swig::DirectorException`, see above. The code generated from the `director:except` feature has the `$directorthrowshandlers` special variable replaced with the code in the relevant "directorthrows" typemaps, for each and every exception defined for the method. The relevant exceptions can be defined either with a C++ exception specification or `%catches` as described for the "throws" typemap.

Let's try and put all this together by considering the following director method:

    struct X {
      virtual void doSomething(int index) throw (std::out_of_range);
      ...
    };

    OR

    %catches(std::out_of_range) X::doSomething;
    struct X {
      virtual void doSomething(int index);
      ...
    };

When combined with the default `director:except` feature and the "directorthrows" typemap above, the resulting code generated in the director method after calling up to Java will be:

    jthrowable swigerror = jenv->ExceptionOccurred();
    if (swigerror) {
      if (Swig::ExceptionMatches(jenv, swigerror, "java/lang/IndexOutOfBoundsException")) {
        throw std::out_of_range(Swig::JavaExceptionMessage(jenv, swigerror).message());
      }
      Swig::DirectorException::raise(jenv, swigerror);
    }

Note: Beware of using exception specifications as the SWIG director methods will be generated with the same exception specifications and if the director method throws an exception that is not specified in the exception specifications list it is likely to terminate your program. See the C++ standard for more details. Using the %catches feature instead to define the handled exceptions does not suffer this potential fate.

Because the default code generation maps any unhandled Java exceptions to `Swig::DirectorException`, any director methods that have exception specifications may cause program termination as this exception class won't be in the exception specifications list. You can avoid throwing `Swig::DirectorException` by changing the default handling for all methods by adding a `director:except` feature without any method name. For example, you can just ignore them:

    %feature("director:except") %{
      jthrowable $error = jenv->ExceptionOccurred();
      if ($error) {
        $directorthrowshandlers
        jenv->ExceptionClear();
        return $null; // exception is ignored
      }
    %}

Alternatively an exception compatible with the existing director method exception specifications can be thrown. Assuming that all methods allow std::runtime_error to be thrown, the `return $null` line above could be changed to:

      throw std::runtime_error(Swig::JavaExceptionMessage(jenv, $error).message());

In more complex situations, a separate `director:except` feature may need to be attached to specific methods by providing a method name to the `director:except` feature.

This is all no doubt quite hard to follow without seeing a full example and some code. Below is a complete example demonstrating the use of most of the exception customizations one can use, that is, "directorthrows" and "throws" typemaps, %exception and %catches. See the Exception handling with %exception and %javaexception section for more on converting C++ exceptions to Java exceptions. The example also has a user defined C++ exception class called `MyNS::MyException` and this is wrapped as a Java exception. The director class being wrapped is `MyClass` and the director method is called `MyClass::dirmethod`. A number of `std::cout` calls have been added to help understand code flow. You can copy the code below into an interface file and run SWIG on it and examine the generated code.

    %module(directors="1") example

    %{
      #include <stdexcept>
      #include <iostream>
    %}

    // Generic catch handler for all wrapped methods
    %exception %{
      try {
        $action
      } catch (const std::exception &e) {
        std::cout << "Generic std::exception catch handler" << std::endl;
        jclass clazz = jenv->FindClass("java/lang/RuntimeException");
        jenv->ThrowNew(clazz, e.what());
        return $null;
      }
    %}

    // Expose C++ exception as a Java Exception by changing the Java base class and providing a getMessage()
    %typemap(javabase) MyNS::MyException "java.lang.RuntimeException"
    %rename(getMessage) MyNS::MyException::whatsup;

    %inline %{
    namespace MyNS {
      class MyException {
        std::string msg;
      public:
        MyException(const char *msg) : msg(msg) {}
        const char * whatsup() const { return msg.c_str(); }
      };
    }
    %}

    %typemap(directorthrows) MyNS::MyException %{
      if (Swig::ExceptionMatches(jenv, $error, "$packagepath/MyException")) {
        std::cout << "$1_type exception matched (directorthrows typemap)" << std::endl;
        throw $1_type(Swig::JavaExceptionMessage(jenv, $error).message());
      }
    %}

    %typemap(throws) MyNS::MyException %{
      std::cout << "$1_type caught (throws typemap)" << std::endl;
      jclass excep = jenv->FindClass("MyException");
      if (excep) {
        std::cout << "$1_type class found (throws typemap)" << std::endl;
        jenv->ThrowNew(excep, $1.whatsup());
      }
      return $null;
    %}

    // These are the exceptions that the director method MyClass::dirmethod will have catch handlers for.
    // Note that this is also a virtual method / director method and the C++ exceptions listed can be
    // thrown after converting them from Java exceptions.
    %catches(MyNS::MyException, Swig::DirectorException) MyClass::dirmethod;

    // These are the exceptions that call_dirmethod C++ wrapper will have catch handlers for.
    // Note that this is not a virtual method, hence not a director method.
    %catches(MyNS::MyException, Swig::DirectorException) call_dirmethod;

    %feature("director") MyClass;

    %feature("director:except") MyClass::dirmethod(int x) {
      jthrowable $error = jenv->ExceptionOccurred();
      if ($error) {
        std::cout << "Upcall finished, an exception was thrown in Java" << std::endl;
        $directorthrowshandlers
        std::cout << "Upcall finished, no exception conversion, throwing DirectorException" << std::endl;
        Swig::DirectorException::raise(jenv, $error);
      }
    }

    %inline %{
    class MyClass {
    public:
      /** Throws either a std::out_of_range or MyException on error */
      virtual void dirmethod(int x) {
        if (x <= 0)
          throw std::out_of_range("MyClass::dirmethod index is out of range");
        else if (x == 1)
          throw MyNS::MyException("MyClass::dirmethod some problem!");
      }
      virtual ~MyClass() {}
      static void call_dirmethod(MyClass& c, int x) {
        return c.dirmethod(x);
      }
    };
    %}

The generated code for the `call_dirmethod` wrapper contains the various exception handlers. The outer exception handler is from the `%exception` directive and the others are from the "throws" typemaps.

    SWIGEXPORT void JNICALL Java_exampleJNI_MyClass_1call_1dirmethod(JNIEnv *jenv, jclass jcls, jlong jarg1, jobject jarg1_, jint jarg2) {
      ...
      try {
        try {
          MyClass::call_dirmethod(*arg1,arg2);
        } catch(MyNS::MyException &_e) {
          std::cout << "MyNS::MyException caught (throws typemap)" << std::endl;
          jclass excep = jenv->FindClass("MyException");
          if (excep) {
            std::cout << "MyNS::MyException class found (throws typemap)" << std::endl;
            jenv->ThrowNew(excep, (&_e)->whatsup());
          }
          return ;

        } catch(Swig::DirectorException &_e) {
          (&_e)->throwException(jenv);
          return ;
        }
      } catch (const std::exception &e) {
        std::cout << "Generic std::exception catch handler" << std::endl;
        jclass clazz = jenv->FindClass("java/lang/RuntimeException");
        jenv->ThrowNew(clazz, e.what());
        return ;
      }

The director method calling up to Java contains the exception handling code from the "directorthrows" typemaps and `director:except` feature.

    void SwigDirector_MyClass::dirmethod(int x) {
          ... [call up to Java using CallStaticVoidMethod]
          jthrowable swigerror = jenv->ExceptionOccurred();
          if (swigerror) {
            std::cout << "Upcall finished, an exception was thrown in Java" << std::endl;

            if (Swig::ExceptionMatches(jenv, swigerror, "MyException")) {
              std::cout << "MyNS::MyException exception matched (directorthrows typemap)" << std::endl;
              throw MyNS::MyException(Swig::JavaExceptionMessage(jenv, swigerror).message());
            }

            std::cout << "Upcall finished, no exception conversion, throwing DirectorException" << std::endl;
            Swig::DirectorException::raise(jenv, swigerror);
          }

Let's use the following Java class to override the director method.

    class DerivedClass extends MyClass {
      @Override
      public void dirmethod(int x) {
        if (x < 0)
          throw new IndexOutOfBoundsException("Index is negative");
        else if (x == 0)
          throw new MyException("MyException: bad dirmethod");
      }
    }
    public class runme {
      public static void main(String argv[]) {
        System.loadLibrary("example");
        ... code snippets shown below ...
      }
    }

Consider the output using the Java code in the four slightly different scenarios below.

1. Non-director C++ class is used, thus, no upcall to a Java director method is made. A `std::out_of_range` exception is thrown, which is derived from `std::exception`, and hence caught by the generic exception handler in the `call_dirmethod` wrapper. The Java code snippet and resulting output is:

    MyClass.call_dirmethod(new MyClass(), 0);

    Generic std::exception catch handler
    Exception in thread "main" java.lang.RuntimeException: MyClass::dirmethod index is out of range
            at exampleJNI.MyClass_call_dirmethod(Native Method)
            at MyClass.call_dirmethod(MyClass.java:57)
            at runme.main(runme.java:14)

2. Non-director C++ class again but this time the `MyNS::MyException` class is thrown and caught:

    MyClass.call_dirmethod(new MyClass(), 1);

    MyNS::MyException caught (throws typemap)
    MyNS::MyException class found (throws typemap)
    Exception in thread "main" MyException: MyClass::dirmethod some problem!
            at exampleJNI.MyClass_call_dirmethod(Native Method)
            at MyClass.call_dirmethod(MyClass.java:57)
            at runme.main(runme.java:15)

3. The `DerivedClass` director class is used so the upcall to Java occurs, but it throws a Java `MyException`, which gets converted into a C++ `MyNS::MyException`, then caught and converted back into a Java `MyException`:

    MyClass.call_dirmethod(new DerivedClass(), 0);

    Upcall finished, an exception was thrown in Java
    MyNS::MyException exception matched (directorthrows typemap)
    MyNS::MyException caught (throws typemap)
    MyNS::MyException class found (throws typemap)
    Exception in thread "main" MyException: MyException: bad dirmethod
            at exampleJNI.MyClass_call_dirmethod(Native Method)
            at MyClass.call_dirmethod(MyClass.java:57)
            at runme.main(runme.java:16)

4. The director class is used again, but this time the director method throws a Java `IndexOutOfBoundsException` exception which is converted into a C++ `Swig::DirectorException`, thrown and caught again. This time the original Java exception is extracted from the `Swig::DirectorException` and rethrown. Note that this approach keeps the stack trace information of the original exception, so it has the exact location of where the `IndexOutOfBoundsException` exception was thrown. This is arguably an improvement over the approach above that converts from a Java excepton to C++ exception and then back to a new Java exception, losing the location of the original exception.

    MyClass.call_dirmethod(new DerivedClass(), -1);

    Upcall finished, an exception was thrown in Java
    Upcall finished, no exception conversion, throwing DirectorException
    Exception in thread "main" java.lang.IndexOutOfBoundsException: Index is negative
            at DerivedClass.dirmethod(runme.java:5)
            at exampleJNI.SwigDirector_MyClass_dirmethod(exampleJNI.java:23)
            at exampleJNI.MyClass_call_dirmethod(Native Method)
            at MyClass.call_dirmethod(MyClass.java:57)
            at runme.main(runme.java:17)

### 27.5.8 Accessing virtual protected methods

By default, without enabling the director feature, protected methods are not wrapped and so cannot be accessed from Java. When using directors, the protected virtual methods are wrapped. These methods are wrapped with a protected Java proxy method, so the only way that Java code can access these is from within a Java class derived from the director class.

Unfortunately the rules for classes in an inheritance chain when changing the access specifiers in C++ are different to Java access modifiers. They are similar, but, unlike in C+++, one cannot assign weaker access modifiers in Java. Consider the following inheritance chain and using directors to turn on wrapping protected methods:

    %feature("director") BaseClass;
    %feature("director") DerivedClass;

    %inline %{
    class BaseClass {
    public:
      virtual ~BaseClass();
      virtual void baseMethod1();
      virtual void baseMethod2();
    protected:
      virtual void baseMethod3();
      virtual void baseMethod4();
    };

    class DerivedClass : public BaseClass {
    public:
      virtual ~DerivedClass();
      virtual void baseMethod1();
    protected:
      virtual void baseMethod2();
      virtual void baseMethod3();
    public:
      virtual void baseMethod4();
    };
    %}

SWIG will generate Java access modifiers that are a one-to-one mapping of the C++ access specifiers. So if a method is `public` in C++ it will also be `public` in Java, and similarly `protected` in C++ becomes `protected` in Java. This is fine for all of the methods in the above example, except for the `DerivedClass.baseMethod2`. The Java compiler issues an error:

    DerivedClass.java:69: error: baseMethod2() in DerivedClass cannot override baseMethod2() in BaseClass
      protected void baseMethod2() {
                     ^
      attempting to assign weaker access privileges; was public

This requires fixing with one of two solutions:

1. Change the access modifier to public for the derived class by adding the following before SWIG parses the `DerivedClass:`

       %feature("java:methodmodifiers") DerivedClass::baseMethod2 "public"

2. Simply ignore the method in the derived class altogether as it cannot be used as a `protected` method in Java as the C++ design intended:

       %ignore DerivedClass::baseMethod2;

### 27.5.9 Accessing non-virtual protected members

Members which are protected and non-virtual can also be accessed when using the 'allprotected' mode. The allprotected mode requires directors and is turned on by setting the `allprotected` option in addition to the `directors` option in the %module directive, like this:

    %module(directors="1", allprotected="1") modulename

Protected member variables and methods (both static and non-static) will then be wrapped with protected access in the Java proxy class.

Note: Neither the directors option nor the allprotected mode support types defined with protected scope. This includes any enums or typedefs declared in the protected section of the C++ class.

The following simple example is a class with numerous protected members, including the constructor and destructor:

    %module(directors="1", allprotected="1") example

    %feature("director") ProtectedBase;

    // Ignore use of unsupported types (those defined in the protected section)
    %ignore ProtectedBase::typedefs;

    %inline %{

    class ProtectedBase {
    protected:
      ProtectedBase() {}
      virtual ~ProtectedBase() {}
      virtual void virtualMethod() const {}
      void nonStaticMethod(double d) const {}
      static void staticMethod(int i) {}
      int instanceMemberVariable;
      static int staticMemberVariable;

      // unsupported: types defined with protected access and the methods/variables which use them
      typedef int IntegerType;
      IntegerType typedefs(IntegerType it) { return it; }
    };
    int ProtectedBase::staticMemberVariable = 10;

    %}

Note that the `IntegerType` has protected scope and the members which use this type must be ignored as they cannot be wrapped.

The proxy methods are protected, so the only way the protected members can be accessed is within a class that derives from the director class, such as the following:

    class MyProtectedBase extends ProtectedBase
    {
      public MyProtectedBase() {
      }

      public void accessProtected() {
        virtualMethod();
        nonStaticMethod(1.2);
        staticMethod(99);

        setInstanceMemberVariable(5);
        int i = getInstanceMemberVariable();

        setStaticMemberVariable(10);
        i = getStaticMemberVariable();
      }
    }

## 27.6 Common customization features

An earlier section presented the absolute basics of C/C++ wrapping. If you do nothing but feed SWIG a header file, you will get an interface that mimics the behavior described. However, sometimes this isn't enough to produce a nice module. Certain types of functionality might be missing or the interface to certain functions might be awkward. This section describes some common SWIG features that are used to improve the interface to existing C/C++ code.

### 27.6.1 C/C++ helper functions

Sometimes when you create a module, it is missing certain bits of functionality. For example, if you had a function like this

    typedef struct Image {...};
    void set_transform(Image *im, double m[4][4]);

it would be accessible from Java, but there may be no easy way to call it. The problem here is that a type wrapper class is generated for the two dimensional array parameter so there is no easy way to construct and manipulate a suitable `double [4][4]` value. To fix this, you can write some extra C helper functions. Just use the `%inline` directive. For example:

    %inline %{
    /* Note: double[4][4] is equivalent to a pointer to an array double (*)[4] */
    double (*new_mat44())[4] {
      return (double (*)[4]) malloc(16*sizeof(double));
    }
    void free_mat44(double (*x)[4]) {
      free(x);
    }
    void mat44_set(double x[4][4], int i, int j, double v) {
      x[i][j] = v;
    }
    double mat44_get(double x[4][4], int i, int j) {
      return x[i][j];
    }
    %}

From Java, you could then write code like this:

    Image im = new Image();
    SWIGTYPE_p_a_4__double a = example.new_mat44();
    example.mat44_set(a, 0, 0, 1.0);
    example.mat44_set(a, 1, 1, 1.0);
    example.mat44_set(a, 2, 2, 1.0);
    ...
    example.set_transform(im, a);
    example.free_mat44(a);

Admittedly, this is not the most elegant looking approach. However, it works and it wasn't too hard to implement. It is possible to improve on this using Java code, typemaps, and other customization features as covered in later sections, but sometimes helper functions are a quick and easy solution to difficult cases.

### 27.6.2 Class extension with %extend

One of the more interesting features of SWIG is that it can extend structures and classes with new methods or constructors. Here is a simple example:

    %module example
    %{
    #include "someheader.h"
    %}

    struct Vector {
      double x, y, z;
    };

    %extend Vector {
      char *toString() {
        static char tmp[1024];
        sprintf(tmp, "Vector(%g, %g, %g)", $self->x, $self->y, $self->z);
        return tmp;
      }
      Vector(double x, double y, double z) {
        Vector *v = (Vector *) malloc(sizeof(Vector));
        v->x = x;
        v->y = y;
        v->z = z;
        return v;
      }
    };

Now, in Java

    Vector v = new Vector(2, 3, 4);
    System.out.println(v);

will display

    Vector(2, 3, 4)

`%extend` works with both C and C++ code. It does not modify the underlying object in any way---the extensions only show up in the Java interface.

### 27.6.3 Class extension with %proxycode

The previous section described how to extend a wrapped class with C or C++ code. This section describes how to extend a wrapped class with Java code instead of C/C++ code. The `%proxycode` directive is used and is just a macro for `%insert("proxycode")`. The Code insertion block section describes the `%insert` directive. The section of code for insertion is "proxycode", that is, the Java proxy class. This directive must hence only be used within the scope of a class, otherwise it is silently ignored. There are two common ways to get the scope correct.

The first is to use `%proxycode` inside a class that SWIG parses, for example a `toString()` method can be added to a C++ class using pure Java code. A C++ header file can mix C++ and Java code inside the C++ class as follows:

    // flag.h header file
    class Flag {
      bool flag;
    public:
      Flag(bool flag) : flag(flag) {}
      bool FetchFlag() { return flag; }
    #if defined(SWIG)
    %proxycode %{
      public String toString() {
        boolean flag = FetchFlag();
        return Boolean.toString(flag);
      }
    %}
    #endif
    };

and wrapped using:

    %{
    #include "flag.h"
    %}
    %include "flag.h"

The second is to use `%proxycode` within `%extend` as everything within a `%extend` block is effectively within the scope of the class, for example:

    // flag.h header file
    class Flag {
      bool flag;
    public:
      Flag(bool flag) : flag(flag) {}
      bool FetchFlag() { return flag; }
    };

and wrapped using:

    %{
    #include "flag.h"
    %}
    %include "flag.h"

    %extend Flag {
    #if defined(SWIG)
    %proxycode %{
      public String toString() {
        boolean flag = FetchFlag();
        return Boolean.toString(flag);
      }
    %}
    #endif
    }

There is some very limited support of typemaps within a `%proxycode` block. A useful trick is to obtain the Java type for a given C/C++ type using the $typemap special macro. The following C++ template demonstrates this:

    %inline %{
    template<typename T> struct Value {
      T value;
      Value(const T& val) : value(val) {}
    };
    %}

    %extend Value {
    %proxycode %{
      public String toString() {
        // Note template type expansion is supported, so T is expanded to 'unsigned int' in this example
        // and $typemap(jstype, unsigned int) in turn is expanded to 'long'
        $typemap(jstype, T) val = getValue();
        return "$javaclassname value: " + val + " Java type: $typemap(jstype, T) JNI type: $typemap(jni, T)";
      }
    %}
    }
    %template(ValueUnsignedInt) Value<unsigned int>;

The generated Java contains the expanded special variable and macro resulting in Java proxy code:

    public class ValueUnsignedInt {
      ...
      public String toString() {
        long val = getValue();
        return "ValueUnsignedInt value: " + val + " Java type: long JNI type: jlong";
      }

    }

### 27.6.4 Exception handling with %exception and %javaexception

If a C or C++ function throws an error, you may want to convert that error into a Java exception. To do this, you can use the `%exception` directive. The `%exception` directive simply lets you rewrite part of the generated wrapper code to include an error check. It is detailed in full in the Exception handling with %exception section.

In C, a function often indicates an error by returning a status code (a negative number or a NULL pointer perhaps). Here is a simple example of how you might handle that:

    %exception malloc {
      $action
      if (!result) {
        jclass clazz = (*jenv)->FindClass(jenv, "java/lang/OutOfMemoryError");
        (*jenv)->ThrowNew(jenv, clazz, "Not enough memory");
        return $null;
      }
    }
    void *malloc(size_t nbytes);

In Java,

    SWIGTYPE_p_void a = example.malloc(2000000000);

will produce a familiar looking Java exception:

    Exception in thread "main" java.lang.OutOfMemoryError: Not enough memory
            at exampleJNI.malloc(Native Method)
            at example.malloc(example.java:16)
            at runme.main(runme.java:112)

If a library provides some kind of general error handling framework, you can also use that. For example:

    %exception malloc {
      $action
      if (err_occurred()) {
        jclass clazz = (*jenv)->FindClass(jenv, "java/lang/OutOfMemoryError");
        (*jenv)->ThrowNew(jenv, clazz, "Not enough memory");
        return $null;
      }
    }
    void *malloc(size_t nbytes);

If no declaration name is given to `%exception`, it is applied to all wrapper functions. The `$action` is a SWIG special variable and is replaced by the C/C++ function call being wrapped. The `return $null;` handles all native method return types, namely those that have a void return and those that do not. This is useful for typemaps that will be used in native method returning all return types. See the section on Java special variables for further explanation.

C++ exceptions are also easy to handle. We can catch the C++ exception and rethrow it as a Java exception like this:

    %exception getitem {
      try {
        $action
      } catch (std::out_of_range &e) {
        jclass clazz = jenv->FindClass("java/lang/Exception");
        jenv->ThrowNew(clazz, "Range error");
        return $null;
      }
    }

    class FooClass {
      public:
        FooClass *getitem(int index);      // Might throw std::out_of_range exception
        ...
    };

In the example above, `java.lang.Exception` is a checked exception class and so ought to be declared in the throws clause of `getitem`. Classes can be specified for adding to the throws clause using `%javaexception(classes)` instead of `%exception`, where `classes` is a string containing one or more comma separated Java classes. The `%clearjavaexception` feature is the equivalent to `%clearexception` and clears previously declared exception handlers. The `%nojavaexception` feature is the equivalent to `%noexception` and disables the exception handler. See Clearing features for the difference on disabling and clearing features.

    %javaexception("java.lang.Exception") getitem {
      try {
        $action
      } catch (std::out_of_range &e) {
        jclass clazz = jenv->FindClass("java/lang/Exception");
        jenv->ThrowNew(clazz, "Range error");
        return $null;
      }
    }

    class FooClass {
    public:
      FooClass *getitem(int index);      // Might throw std::out_of_range exception
      ...
    };

The generated proxy method now generates a throws clause containing `java.lang.Exception`:

    public class FooClass {
      ...
      public FooClass getitem(int index) throws java.lang.Exception { ... }
      ...
    }

The examples above first use the C JNI calling syntax then the C++ JNI calling syntax. The C++ calling syntax will not compile as C and also vice versa. It is however possible to write JNI calls which will compile under both C and C++ and is covered in the Typemaps for both C and C++ compilation section.

The language-independent `exception.i` library file can also be used to raise exceptions. See the SWIG Library chapter. The typemap example Handling C++ exception specifications as Java exceptions provides further exception handling capabilities.

### 27.6.5 Method access with %javamethodmodifiers

A Java feature called `%javamethodmodifiers` can be used to change the method modifiers from the default `public`. It applies to both module class methods and proxy class methods. For example:

    %javamethodmodifiers protect_me() "protected";
    void protect_me();

Will produce the method in the module class with protected access.

    protected static void protect_me() {
      exampleJNI.protect_me();
    }

### 27.6.6 Java begin

It is possible to add a common comment at the start of every generated Java file. The `%module` directive supports the `javabegin` option for this. The provided text is generated at the very beginning of each generated .java file. As it is generated before the package statement, is only really useful for adding in a common comment into all generated .java files. For example, copyright text for each file:

    %module(javabegin="/* Common comment. Copyright (C) 2000 Mr Nobody. */\n") nobodymodule

## 27.7 Tips and techniques

Although SWIG is largely automatic, there are certain types of wrapping problems that require additional user input. Examples include dealing with output parameters, strings and arrays. This chapter discusses the common techniques for solving these problems.

### 27.7.1 Input and output parameters using primitive pointers and references

A common problem in some C programs is handling parameters passed as simple pointers or references. For example:

    void add(int x, int y, int *result) {
      *result = x + y;
    }

or perhaps

    int sub(int *x, int *y) {
      return *x-*y;
    }

The `typemaps.i` library file will help in these situations. For example:

    %module example
    %include "typemaps.i"

    void add(int, int, int *OUTPUT);
    int  sub(int *INPUT, int *INPUT);

In Java, this allows you to pass simple values. For example:

    int result = example.sub(7, 4);
    System.out.println("7 - 4 = " + result);
    int[] sum = {0};
    example.add(3, 4, sum);
    System.out.println("3 + 4 = " + sum[0]);

Which will display:

    7 - 4 = 3
    3 + 4 = 7

Notice how the `INPUT` parameters allow integer values to be passed instead of pointers and how the `OUTPUT` parameter will return the result in the first element of the integer array.

If you don't want to use the names `INPUT` or `OUTPUT`, use the `%apply` directive. For example:

    %module example
    %include "typemaps.i"

    %apply int *OUTPUT { int *result };
    %apply int *INPUT  { int *x, int *y};

    void add(int x, int y, int *result);
    int  sub(int *x, int *y);

If a function mutates one of its parameters like this,

    void negate(int *x) {
      *x = -(*x);
    }

you can use `INOUT` like this:

    %include "typemaps.i"
    ...
    void negate(int *INOUT);

In Java, the input parameter is the first element in a 1 element array and is replaced by the output of the function. For example:

    int[] neg = {3};
    example.negate(neg);
    System.out.println("Negative of 3 = " + neg[0]);

And no prizes for guessing the output:

    Negative of 3 = -3

These typemaps can also be applied to C++ references. The above examples would work the same if they had been defined using references instead of pointers. For example, the Java code to use the `negate` function would be the same if it were defined either as it is above:

    void negate(int *INOUT);

or using a reference:

    void negate(int &INOUT);

Note: Since most Java primitive types are immutable and are passed by value, it is not possible to perform in-place modification of a type passed as a parameter.

Be aware that the primary purpose of the `typemaps.i` file is to support primitive datatypes. Writing a function like this

    void foo(Bar *OUTPUT);

will not have the intended effect since `typemaps.i` does not define an OUTPUT rule for `Bar`.

### 27.7.2 Simple pointers

If you must work with simple pointers such as `int *` or `double *` another approach to using `typemaps.i` is to use the `cpointer.i` pointer library file. For example:

    %module example
    %include "cpointer.i"

    %inline %{
    extern void add(int x, int y, int *result);
    %}

    %pointer_functions(int, intp);

The `%pointer_functions(type, name)` macro generates five helper functions that can be used to create, destroy, copy, assign, and dereference a pointer. In this case, the functions are as follows:

    int  *new_intp();
    int  *copy_intp(int *x);
    void  delete_intp(int *x);
    void  intp_assign(int *x, int value);
    int   intp_value(int *x);

In Java, you would use the functions like this:

    SWIGTYPE_p_int intPtr = example.new_intp();
    example.add(3, 4, intPtr);
    int result = example.intp_value(intPtr);
    System.out.println("3 + 4 = " + result);

If you replace `%pointer_functions(int, intp)` by `%pointer_class(int, intp)`, the interface is more class-like.

    intp intPtr = new intp();
    example.add(3, 4, intPtr.cast());
    int result = intPtr.value();
    System.out.println("3 + 4 = " + result);

See the SWIG Library chapter for further details.

### 27.7.3 Wrapping C arrays with Java arrays

SWIG can wrap arrays in a more natural Java manner than the default by using the `arrays_java.i` library file. Let's consider an example:

    %include "arrays_java.i";
    int array[4];
    void populate(int x[]) {
      int i;
      for (i=0; i<4; i++)
        x[i] = 100 + i;
    }

These one dimensional arrays can then be used as if they were Java arrays:

    int[] array = new int[4];
    example.populate(array);

    System.out.print("array: ");
    for (int i=0; i<array.length; i++)
      System.out.print(array[i] + " ");

    example.setArray(array);

    int[] global_array = example.getArray();

    System.out.print("\nglobal_array: ");
    for (int i=0; i<array.length; i++)
      System.out.print(global_array[i] + " ");

Java arrays are always passed by reference, so any changes a function makes to the array will be seen by the calling function. Here is the output after running this code:

    array: 100 101 102 103
    global_array: 100 101 102 103

Note that for assigning array variables the length of the C variable is used, so it is possible to use a Java array that is bigger than the C code will cope with. Only the number of elements in the C array will be used. However, if the Java array is not large enough then you are likely to get a segmentation fault or access violation, just like you would in C. When arrays are used in functions like `populate`, the size of the C array passed to the function is determined by the size of the Java array.

Please be aware that the typemaps in this library are not efficient as all the elements are copied from the Java array to a C array whenever the array is passed to and from JNI code. There is an alternative approach using the SWIG array library and this is covered in the next section.

### 27.7.4 Unbounded C Arrays

Sometimes a C function expects an array to be passed as a pointer. For example,

    int sumitems(int *first, int nitems) {
      int i, sum = 0;
      for (i = 0; i < nitems; i++) {
        sum += first[i];
      }
      return sum;
    }

One of the ways to wrap this is to apply the Java array typemaps that come in the `arrays_java.i` library file:

    %include "arrays_java.i"
    %apply int[] {int *};

The `ANY` size will ensure the typemap is applied to arrays of all sizes. You could narrow the typemap matching rules by specifying a particular array size. Now you can use a pure Java array and pass it to the C code:

    int[] array = new int[10000000];          // Array of 10-million integers
    for (int i=0; i<array.length; i++) {      // Set some values
      array[i] = i;
    }
    int sum = example.sumitems(array, 10000);
    System.out.println("Sum = " + sum);

and the sum would be displayed:

    Sum = 49995000

This approach is probably the most natural way to use arrays. However, it suffers from performance problems when using large arrays as a lot of copying of the elements occurs in transferring the array from the Java world to the C++ world. An alternative approach to using Java arrays for C arrays is to use an alternative SWIG library file `carrays.i`. This approach can be more efficient for large arrays as the array is accessed one element at a time. For example:

    %include "carrays.i"
    %array_functions(int, intArray);

The `%array_functions(type, name)` macro generates four helper functions that can be used to create and destroy arrays and operate on elements. In this case, the functions are as follows:

    int *new_intArray(size_t nelements);
    void delete_intArray(int *x);
    int intArray_getitem(int *x, size_t index);
    void intArray_setitem(int *x, size_t index, int value);

In Java, you would use the functions like this:

    SWIGTYPE_p_int array = example.new_intArray(10000000);  // Array of 10-million integers
    for (int i=0; i<10000; i++) {                           // Set some values
      example.intArray_setitem(array, i, i);
    }
    int sum = example.sumitems(array, 10000);
    System.out.println("Sum = " + sum);

If you replace `%array_functions(int, intp)` by `%array_class(int, intp)`, the interface is more class-like and a couple more helper functions are available for casting between the array and the type wrapper class.

    %include "carrays.i"
    %array_class(int, intArray);

The `%array_class(type, name)` macro creates wrappers for an unbounded array object that can be passed around as a simple pointer like `int *` or `double *`. For instance, you will be able to do this in Java:

    intArray array = new intArray(10000000);  // Array of 10-million integers
    for (int i=0; i<10000; i++) {             // Set some values
      array.setitem(i, i);
    }
    int sum = example.sumitems(array.cast(), 10000);
    System.out.println("Sum = " + sum);

The array "object" created by `%array_class()` does not encapsulate pointers inside a special array object. In fact, there is no bounds checking or safety of any kind (just like in C). Because of this, the arrays created by this library are extremely low-level indeed. You can't iterate over them nor can you even query their length. In fact, any valid memory address can be accessed if you want (negative indices, indices beyond the end of the array, etc.). Needless to say, this approach is not going to suit all applications. On the other hand, this low-level approach is extremely efficient and well suited for applications in which you need to create buffers, package binary data, etc.

### 27.7.5 Passing a string with length

SWIG provides multi-argument typemap available as mentioned in Passing a string with length. The following simple example demonstrates passing a string to the wrapped function.

    %apply (char *STRING, size_t LENGTH) { (const char data[], size_t len) }
    %inline %{
    void binaryChar1(const char data[], size_t len) {
      printf("len: %d data: ", len);
      for (size_t i=0; i<len; ++i)
        printf("%x ", data[i]);
      printf("\n");
    }
    %}

Calling from Java requires a string to be passed in as the multi-argument typemap being applied reduces the number of arguments in the target language to one, from the original two:

    String str = "hi\0jk";
    example.binaryChar1(str);

resulting in the output

    $ java runme
    len: 5 data: 68 69 0 6a 6b

The typemap uses Java `String::getBytes()` to convert the string to the default system encoding. If you wish to use another encoding, you can modify the typemap. For example to convert the string to UTF-8 use:

    %typemap(javain, throws="java.io.IllegalCharsetNameException")

      (const char *STRING, size_t LENGTH) %{($javainput == null) ? null : $javainput.getBytes("UTF-8")%}

### 27.7.6 Overriding new and delete to allocate from Java heap

Unlike some languages supported by SWIG, Java has a true garbage collection subsystem. Other languages will free SWIG wrapped objects when their reference count reaches zero. Java only schedules these objects for finalization, which may not occur for some time. Because SWIG objects are allocated on the C heap, Java users may find the JVM memory use quickly exceeds the assigned limits, as memory fills with unfinalized proxy objects. Forcing garbage collection is clearly an undesirable solution.

An elegant fix for C++ users is to override new and delete using the following code (here shown included in a SWIG interface file)

    /* File: java_heap.i */
    %module test
    %{
    #include <stdexcept>
    #include "jni.h"

    /**
     *  A stash area embedded in each allocation to hold java handles
     */
    struct Jalloc {
      jbyteArray jba;
      jobject ref;
    };

    static JavaVM *cached_jvm = 0;

    JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved) {
      cached_jvm = jvm;
      return JNI_VERSION_1_2;
    }

    static JNIEnv * JNU_GetEnv() {
      JNIEnv *env;
      jint rc = cached_jvm->GetEnv((void **)&env, JNI_VERSION_1_2);
      if (rc == JNI_EDETACHED)
        throw std::runtime_error("current thread not attached");
      if (rc == JNI_EVERSION)
        throw std::runtime_error("jni version not supported");
      return env;
    }

    void * operator new(size_t t) {
      if (cached_jvm != 0) {
        JNIEnv *env = JNU_GetEnv();
        jbyteArray jba = env->NewByteArray((int) t + sizeof(Jalloc));
        if (env->ExceptionOccurred())
          throw bad_alloc();
        void *jbuffer = static_cast<void *>(env->GetByteArrayElements(jba, 0));
        if (env->ExceptionOccurred())
          throw bad_alloc();
        Jalloc *pJalloc = static_cast<Jalloc *>(jbuffer);
        pJalloc->jba = jba;
        /* Assign a global reference so byte array will persist until delete'ed */
        pJalloc->ref = env->NewGlobalRef(jba);
        if (env->ExceptionOccurred())
          throw bad_alloc();
        return static_cast<void *>(static_cast<char *>(jbuffer) + sizeof(Jalloc));
      }
      else { /* JNI_OnLoad not called, use malloc and mark as special */
        Jalloc *pJalloc = static_cast<Jalloc *>(malloc((int) t + sizeof(Jalloc)));
        if (!pJalloc)
          throw bad_alloc();
        pJalloc->ref = 0;
        return static_cast<void *>(
          static_cast<char *>(static_cast<void *>(pJalloc)) + sizeof(Jalloc));
      }
    }

    void operator delete(void *v) {
      if (v != 0) {
        void *buffer = static_cast<void *>( static_cast<char *>(v) - sizeof(Jalloc));
        Jalloc *pJalloc = static_cast<Jalloc *>(buffer);
        if (pJalloc->ref) {
          JNIEnv *env = JNU_GetEnv();
          env->DeleteGlobalRef(pJalloc->ref);
          env->ReleaseByteArrayElements(pJalloc->jba, static_cast<jbyte *>(buffer), 0);
        }
        else {
          free(buffer);
        }
      }
    }
    %}
    ...

This code caches the Java environment during initialization, and when new is called, a Java ByteArray is allocated to provide the SWIG objects with space in the Java heap. This has the combined effect of re-asserting the Java virtual machine's limit on memory allocation, and puts additional pressure on the garbage collection system to run more frequently. This code is made slightly more complicated because allowances must be made if new is called before the JNI_OnLoad is executed. This can happen during static class initialization, for example.

Unfortunately, because most Java implementations call malloc and free, this solution will not work for C wrapped structures. However, you are free to make functions that allocate and free memory from the Java heap using this model and use these functions in place of malloc and free in your own code.

## 27.8 Java typemaps

This section describes how you can modify SWIG's default wrapping behavior for various C/C++ datatypes using the `%typemap` directive. You are advised to be familiar with the material in the "Typemaps" chapter. While not absolutely essential knowledge, this section assumes some familiarity with the Java Native Interface (JNI). JNI documentation can be consulted either online at the Java web site or from a good JNI book. The following two books are recommended:

- Title: 'Essential JNI: Java Native Interface.' Author: Rob Gordon. Publisher: Prentice Hall. ISBN: 0-13-679895-0.
- Title: 'The Java Native Interface: Programmer's Guide and Specification.' Author: Sheng Liang. Publisher: Addison-Wesley. ISBN: 0-201-32577-2. Also available online at the Sun Developer Network.

Before proceeding, it should be stressed that typemaps are not a required part of using SWIG---the default wrapping behavior is enough in most cases. Typemaps are only used if you want to change some aspect of the generated code.

### 27.8.1 Default primitive type mappings

The following table lists the default type mapping from Java to C/C++.

C/C++ type | Java type | JNI type
bool const bool & | boolean | jboolean
char const char & | char | jchar
signed char const signed char & | byte | jbyte
unsigned char const unsigned char & | short | jshort
short const short & | short | jshort
unsigned short const unsigned short & | int | jint
int const int & | int | jint
unsigned int const unsigned int & | long | jlong
long const long & | int | jint
unsigned long const unsigned long & | long | jlong
long long const long long & | long | jlong
unsigned long long const unsigned long long & | java.math.BigInteger | jobject
float const float & | float | jfloat
double const double & | double | jdouble
size_t const size_t & | long | jlong
char * char [] | String | jstring

Note that default mappings for the C long type is more suitable for 32-bit systems. If long is 64-bit, the full range can be obtained by defining `SWIGWORDSIZE64` when invoking SWIG. The long type will instead be mapped as follows:

C/C++ type | Java type | JNI type
long const long & | long | jlong
unsigned long const unsigned long & | java.math.BigInteger | jobject

Note that when size_t is 64-bit in C, the full unsigned range is not available. This can be fixed by applying the 64-bit unsigned long long typemaps as follows:

    %apply unsigned long long { size_t };
    %apply const unsigned long long & { const size_t & };

The net effect then changes from the default shown earlier to:

C/C++ type | Java type | JNI type
size_t const size_t & | java.math.BigInteger | jobject

Note that SWIG wraps the C `char` type as a character. Pointers and arrays of this type are wrapped as strings. The `signed char` type can be used if you want to treat `char` as a signed number rather than a character. Also note that all const references to primitive types are treated as if they are passed by value.

Given the following C function:

    void func(unsigned short a, char *b, const long &c, unsigned long long d);

The module class method would be:

    public static void func(int a, String b, int c, java.math.BigInteger d) {...}

The intermediary JNI class would use the same types:

    public final static native void func(int jarg1, String jarg2, int jarg3,
                                         java.math.BigInteger jarg4);

and the JNI function would look like this:

    SWIGEXPORT void JNICALL Java_exampleJNI_func(JNIEnv *jenv, jclass jcls,
                    jint jarg1, jstring jarg2, jint jarg3, jobject jarg4) {...}

The mappings for C `int` and C `long` are appropriate for 32 bit applications which are used in the 32 bit JVMs. There is no perfect mapping between Java and C as Java doesn't support all the unsigned C data types. However, the mappings allow the full range of values for each C type from Java.

### 27.8.2 Default typemaps for non-primitive types

The previous section covered the primitive type mappings. Non-primitive types such as classes and structs are mapped using pointers on the C/C++ side and storing the pointer into a Java `long` variable which is held by the proxy class or type wrapper class. This applies whether the type is marshalled as a pointer, by reference or by value. It also applies for any unknown/incomplete types which use type wrapper classes.

So in summary, the C/C++ pointer to non-primitive types is cast into the 64 bit Java `long` type and therefore the JNI type is a `jlong`. The Java type is either the proxy class or type wrapper class.

### 27.8.3 Sixty four bit JVMs

If you are using a 64 bit JVM you may have to override the C long, but probably not C int default mappings. Mappings will be system dependent, for example long will need remapping on Unix LP64 systems (long, pointer 64 bits, int 32 bits), but not on Microsoft 64 bit Windows which will be using a P64 IL32 (pointer 64 bits and int, long 32 bits) model. This may be automated in a future version of SWIG. Note that the Java write once run anywhere philosophy holds true for all pure Java code when moving to a 64 bit JVM. Unfortunately it won't of course hold true for JNI code.

### 27.8.4 What is a typemap?

A typemap is nothing more than a code generation rule that is attached to a specific C datatype. For example, to convert integers from Java to C, you might define a typemap like this:

    %module example

    %typemap(in) int {
      $1 = $input;
      printf("Received an integer : %d\n",  $1);
    }
    %inline %{
    extern int fact(int nonnegative);
    %}

Typemaps are always associated with some specific aspect of code generation. In this case, the "in" method refers to the conversion of input arguments to C/C++. The datatype `int` is the datatype to which the typemap will be applied. The supplied C code is used to convert values. In this code a number of special variables prefaced by a `$` are used. The `$1` variable is a placeholder for a local variable of type `int`. The `$input` variable contains the Java data, the JNI `jint` in this case.

When this example is compiled into a Java module, it can be used as follows:

    System.out.println(example.fact(6));

and the output will be:

    Received an integer : 6
    720

In this example, the typemap is applied to all occurrences of the `int` datatype. You can refine this by supplying an optional parameter name. For example:

    %module example

    %typemap(in) int nonnegative {
      $1 = $input;
      printf("Received an integer : %d\n",  $1);
    }

    %inline %{
    extern int fact(int nonnegative);
    %}

In this case, the typemap code is only attached to arguments that exactly match `int nonnegative`.

The application of a typemap to specific datatypes and argument names involves more than simple text-matching--typemaps are fully integrated into the SWIG C++ type-system. When you define a typemap for `int`, that typemap applies to `int` and qualified variations such as `const int`. In addition, the typemap system follows `typedef` declarations. For example:

    %typemap(in) int nonnegative {
      $1 = $input;
      printf("Received an integer : %d\n",  $1);
    }
    %inline %{
    typedef int Integer;
    extern int fact(Integer nonnegative);    // Above typemap is applied
    %}

However, the matching of `typedef` only occurs in one direction. If you defined a typemap for `Integer`, it is not applied to arguments of type `int`.

Typemaps can also be defined for groups of consecutive arguments. For example:

    %typemap(in) (char *str, int len) {
    ...
    };

    int count(char c, char *str, int len);

When a multi-argument typemap is defined, the arguments are always handled as a single Java parameter. This allows the function to be used like this (notice how the length parameter is omitted):

    int c = example.count('e', "Hello World");

### 27.8.5 Typemaps for mapping C/C++ types to Java types

The typemaps available to the Java module include the common typemaps listed in the main typemaps section. There are a number of additional typemaps which are necessary for using SWIG with Java. The most important of these implement the mapping of C/C++ types to Java types:

Typemap | Description
jni | JNI C types. These provide the default mapping of types from C/C++ to JNI for use in the JNI (C/C++) code.
jtype | Java intermediary types. These provide the default mapping of types from C/C++ to Java for use in the native functions in the intermediary JNI class. The type must be the equivalent Java type for the JNI C type specified in the "jni" typemap.
jstype | Java types. These provide the default mapping of types from C/C++ to Java for use in the Java module class, proxy classes and type wrapper classes.
javain | Conversion from jstype to jtype. These are Java code typemaps which transform the type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap) to the type used in the Java intermediary JNI class (as specified in the "jtype" typemap). In other words the typemap provides the conversion to the native method call parameter types.
javaout | Conversion from jtype to jstype. These are Java code typemaps which transform the type used in the Java intermediary JNI class (as specified in the "jtype" typemap) to the Java type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap). In other words the typemap provides the conversion from the native method call return type.
jboxtype | Java boxed type. These are Java code typemaps to provide the Java boxed type, such as, `Integer` for C type `int`. As autoboxing is only relevant to the Java primitive types, these are only provided for the C types that map to Java primitive types. This typemap is usually only used by C++ STL container wrappers that are wrapped by Java generic types as the boxed type must be used instead of the unboxed/primitive type when declaring a Java generic type.
javadirectorin | Conversion from jtype to jstype for director methods. These are Java code typemaps which transform the type used in the Java intermediary JNI class (as specified in the "jtype" typemap) to the Java type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap). This typemap provides the conversion for the parameters in the director methods when calling up from C++ to Java. See Director typemaps.
javadirectorout | Conversion from jstype to jtype for director methods. These are Java code typemaps which transform the type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap) to the type used in the Java intermediary JNI class (as specified in the "jtype" typemap). This typemap provides the conversion for the return type in the director methods when returning from the C++ to Java upcall. See Director typemaps.
directorin | Conversion from C++ type to jni type for director methods. These are C++ typemaps which convert the parameters used in the C++ director method to the appropriate JNI intermediary type. The conversion is done in JNI code prior to calling the Java function from the JNI code. See Director typemaps.
directorout | Conversion from jni type to C++ type for director methods. These are C++ typemaps which convert the JNI return type used in the C++ director method to the appropriate C++ return type. The conversion is done in JNI code after calling the Java function from the JNI code. See Director typemaps.

If you are writing your own typemaps to handle a particular type, you will normally have to write a collection of them. The default typemaps are in "`java.swg`" and so might be a good place for finding typemaps to base any new ones on.

The "jni", "jtype" and "jstype" typemaps are usually defined together to handle the Java to C/C++ type mapping. An "in" typemap should be accompanied by a "javain" typemap and likewise an "out" typemap by a "javaout" typemap. If an "in" typemap is written, a "freearg" and "argout" typemap may also need to be written as some types have a default "freearg" and/or "argout" typemap which may need overriding. The "freearg" typemap sometimes releases memory allocated by the "in" typemap. The "argout" typemap sometimes sets values in function parameters which are passed by reference in Java.

Note that the "in" typemap marshals the JNI type held in the "jni" typemap to the real C/C++ type and for the opposite direction, the "out" typemap marshals the real C/C++ type to the JNI type held in the "jni" typemap. For non-primitive types the "in" and "out" typemaps are responsible for casting between the C/C++ pointer and the 64 bit `jlong` type. There is no portable way to cast a pointer into a 64 bit integer type and the approach taken by SWIG is mostly portable, but breaks C/C++ aliasing rules. In summary, these rules state that a pointer to any type must never be dereferenced by a pointer to any other incompatible type. The following code snippet might aid in understand aliasing rules better:

    short a;
    short* pa = 0;
    int i = 0x1234;

    a = (short)i;    /* okay */
    a = *(short*)&i; /* breaks aliasing rules */

An email posting, Aliasing, pointer casts and gcc 3.3 elaborates further on the subject. In SWIG, the "in" and "out" typemaps for pointers are typically

    %typemap(in) struct Foo * %{
      $1 = *(struct Foo **)&$input; /* cast jlong into C ptr */
    %}
    %typemap(out) struct Bar * %{
      *(struct Bar **)&$result = $1; /* cast C ptr into jlong */
    %}
    struct Bar {...};
    struct Foo {...};
    struct Bar * FooBar(struct Foo *f);

resulting in the following code which breaks the aliasing rules:

    SWIGEXPORT jlong JNICALL Java_exampleJNI_FooBar(JNIEnv *jenv, jclass jcls,
                                                    jlong jarg1, jobject jarg1_) {
      jlong jresult = 0 ;
      struct Foo *arg1 = (struct Foo *) 0 ;
      struct Bar *result = 0 ;

      (void)jenv;
      (void)jcls;
      (void)jarg1_;
      arg1 = *(struct Foo **)&jarg1;
      result = (struct Bar *)FooBar(arg1);
      *(struct Bar **)&jresult = result;
      return jresult;
    }

If you are using gcc as your C compiler, you might get a "dereferencing type-punned pointer will break strict-aliasing rules" warning about this. Please see Compiling a dynamic module to avoid runtime problems with these strict aliasing rules.

The default code generated by SWIG for the Java module comes from the typemaps in the "`java.swg`" library file which implements the Default primitive type mappings and Default typemaps for non-primitive types covered earlier. There are other type mapping typemaps in the Java library. These are listed below:

C Type | Typemap | File | Kind | Java Type | Function
primitive pointers and references | INPUT | typemaps.i | input | Java basic types | Allows values to be used for C functions taking pointers for data input.
primitive pointers and references | OUTPUT | typemaps.i | output | Java basic type arrays | Allows values held within an array to be used for C functions taking pointers for data output.
primitive pointers and references | INOUT | typemaps.i | input output | Java basic type arrays | Allows values held within an array to be used for C functions taking pointers for data input and output.
string wstring | [unnamed] | std_string.i | input output | String | Use for std::string mapping to Java String.
arrays of primitive types | [unnamed] | arrays_java.i | input output | arrays of primitive Java types | Use for mapping C arrays to Java arrays.
arrays of classes/structs/unions | JAVA_ARRAYSOFCLASSES macro | arrays_java.i | input output | arrays of proxy classes | Use for mapping C arrays to Java arrays.
arrays of enums | ARRAYSOFENUMS | arrays_java.i | input output | int[] | Use for mapping C arrays to Java arrays (typeunsafe and simple enum wrapping approaches only).
char * | BYTE | various.i | input | byte[] | Java byte array is converted to char array
char ** | STRING_ARRAY | various.i | input output | String[] | Use for mapping NULL terminated arrays of C strings to Java String arrays
unsigned char * | NIOBUFFER | various.i | input output | java.nio.Buffer | Use for mapping directly allocated buffers to c/c++. useful with directors and long lived memory objects

### 27.8.6 Java typemap attributes

There are a few additional typemap attributes that the Java module supports.

The first of these is the 'throws' attribute. The throws attribute is optional and specified after the typemap name and contains one or more comma separated classes for adding to the throws clause for any methods that use that typemap. It is analogous to the %javaexception feature's throws attribute.

    %typemap(typemapname, throws="ExceptionClass1, ExceptionClass2") type { ... }

The attribute is necessary for supporting Java checked exceptions and can be added to just about any typemap. The list of typemaps include all the C/C++ (JNI) typemaps in the "Typemaps" chapter and the Java specific typemaps listed in the previous section, barring the "jni", "jtype" and "jstype" typemaps as they could never contain code to throw an exception.

The throws clause is generated for the proxy method as well as the JNI method in the JNI intermediary class. If a method uses more than one typemap and each of those typemaps have classes specified in the throws clause, the union of the exception classes is added to the throws clause ensuring there are no duplicate classes. See the NaN exception example for further usage.

The "jtype" typemap has the optional 'nopgcpp' attribute which can be used to suppress the generation of the premature garbage collection prevention parameter.

The "javain" typemap has the optional 'pre', 'post' and 'pgcppname' attributes. These are used for generating code before and after the JNI call in the proxy class or module class. The 'pre' attribute contains code that is generated before the JNI call and the 'post' attribute contains code generated after the JNI call. The 'pgcppname' attribute is used to change the premature garbage collection prevention parameter name passed to the JNI function. This is sometimes needed when the 'pre' typemap creates a temporary variable which is then passed to the JNI function.

Note that when the 'pre' or 'post' attributes are specified and the associated type is used in a constructor, a constructor helper function is generated. This is necessary as the Java proxy constructor wrapper makes a call to a support constructor using a this call. In Java the this call must be the first statement in the constructor body. The constructor body thus calls the helper function and the helper function instead makes the JNI call, ensuring the 'pre' code is called before the JNI call is made. There is a Date marshalling example showing 'pre', 'post' and 'pgcppname' attributes in action.

### 27.8.7 Java special variables

The standard SWIG special variables are available for use within typemaps as described in the Typemaps documentation, for example `$1`, `$input`, `$result` etc.

The Java module uses a few additional special variables:

`$javaclassname`
This special variable works like the other special variables and `$javaclassname` is similar to `$1_type`. It expands to the class name for use in Java given a pointer. SWIG wraps unions, structs and classes using pointers and in this case it expands to the Java proxy class name. For example, `$javaclassname` is replaced by the proxy classname `Foo` when wrapping a `Foo *` and `$&javaclassname` expands to the proxy classname when wrapping the C/C++ type `Foo` and `$*javaclassname` expands to the proxy classname when wrapping `Foo *&`. If the type does not have an associated proxy class, it expands to the type wrapper class name, for example, `SWIGTYPE_p_unsigned_short` is generated when wrapping `unsigned short *`. The class name is fully qualified with the package name when using the nspace feature.

`$javaclazzname`
This special variable works like `$javaclassname`, but expands the fully qualified C++ class into the package name, if used by the nspace feature, and the proxy class name, mangled for use as a function name. For example, `Namespace1::Namespace2::Klass` is expanded into `Namespace1_Namespace2_Klass_`. This special variable is usually used for making calls to a function in the intermediary JNI class, as they are mangled with this prefix.

`$null`
Used in input typemaps to return early from JNI functions that have either void or a non-void return type. Example:

    %typemap(check) int * %{
      if (error) {
        SWIG_JavaThrowException(jenv, SWIG_JavaIndexOutOfBoundsException, "Array element error");
        return $null;
      }
    %}

If the typemap gets put into a function with void as return, $null will expand to nothing:

    SWIGEXPORT void JNICALL Java_jnifn(...) {
      if (error) {
        SWIG_JavaThrowException(jenv, SWIG_JavaIndexOutOfBoundsException, "Array element error");
        return ;
      }
      ...
    }

otherwise $null expands to NULL

    SWIGEXPORT jobject JNICALL Java_jnifn(...) {
      if (error) {
        SWIG_JavaThrowException(jenv, SWIG_JavaIndexOutOfBoundsException, "Array element error");
        return NULL;
      }
      ...
    }

`$javainput, $jnicall and $owner`
The $javainput special variable is used in "javain" typemaps and $jnicall and $owner are used in "javaout" typemaps. $jnicall is analogous to $action in %exception. It is replaced by the call to the native method in the intermediary JNI class. $owner is replaced by either `true` if %newobject has been used, otherwise `false`. $javainput is analogous to the $input special variable. It is replaced by the parameter name.

Here is an example:

    %typemap(javain) Class "Class.getCPtr($javainput)"
    %typemap(javain) unsigned short "$javainput"
    %typemap(javaout) Class * {
        return new Class($jnicall, $owner);
      }

    %inline %{
      class Class {...};
      Class * bar(Class cls, unsigned short ush) { return new Class(); };
    %}

The generated proxy code is then:

    public static Class bar(Class cls, int ush) {
      return new Class(exampleJNI.bar(Class.getCPtr(cls), cls, ush), false);
    }

Here $javainput has been replaced by `cls` and `ush`. $jnicall has been replaced by the native method call, `exampleJNI.bar(...)` and $owner has been replaced by `false`. If %newobject is used by adding the following at the beginning of our example:

    %newobject bar(Class cls, unsigned short ush);

The generated code constructs the return type using `true` indicating the proxy class `Class` is responsible for destroying the C++ memory allocated for it in `bar`:

    public static Class bar(Class cls, int ush) {
      return new Class(exampleJNI.bar(Class.getCPtr(cls), cls, ush), true);
    }

`$static`
This special variable expands to either static or nothing depending on whether the class is an inner Java class or not. It is used in the "javaclassmodifiers" typemap so that global classes can be wrapped as Java proxy classes and nested C++ classes/enums can be wrapped with the Java equivalent, that is, static inner proxy classes.

`$error, $jniinput, $javacall and $packagepath`
These special variables are used in the directors typemaps. See Director specific typemaps for details.

`$module`
This special variable expands to the module name, as specified by `%module` or the `-module` commandline option.

`$imclassname`
This special variable expands to the intermediary class name. Usually this is the same as '$moduleJNI', unless the jniclassname attribute is specified in the %module directive.

`$imfuncname`
This special variable expands to the name of the function in the intermediary class that will be used in $jnicall. Like, $jnicall, this special variable is only expanded in the "javaout" typemap.

`$javainterfacename`
This special variable is only expanded when the `interface` feature is applied to a class. It works much like `$javaclassname`, but instead of expanding to the proxy classname, it expands to the value in the `name` attribute in the `interface` feature. For example:

    %feature("interface", name="MyInterface") MyClass;
    %typemap(jstype) MyClass  "$&javainterfacename"
    %typemap(jstype) MyClass * "$javainterfacename"

will result in the `jstype` typemap expanding to `MyInterface` for both `MyClass` and `MyClass *`. The interface name is fully qualified with the package name when using the nspace feature.

`$interfacename`
This special variable is only expanded when the `interface` feature is applied to a class. It expands to just the interface name and is thus different to `$javainterfacename` in that it is not fully qualified with the package name when using the nspace feature.

### 27.8.8 Typemaps for both C and C++ compilation

JNI calls must be written differently depending on whether the code is being compiled as C or C++. For example C compilation requires the pointer to a function pointer struct member syntax like

    const jclass clazz = (*jenv)->FindClass(jenv, "java/lang/String");

whereas C++ code compilation of the same function call is a member function call using a class pointer like

    const jclass clazz = jenv->FindClass("java/lang/String");

To enable typemaps to be used for either C or C++ compilation, a set of JCALLx macros have been defined in Lib/java/javahead.swg, where x is the number of arguments in the C++ version of the JNI call. The above JNI calls would be written in a typemap like this

    const jclass clazz = JCALL1(FindClass, jenv, "java/lang/String");

Note that the SWIG preprocessor expands these into the appropriate C or C++ JNI calling convention. The C calling convention is emitted by default and the C++ calling convention is emitted when using the -c++ SWIG commandline option. If you do not intend your code to be targeting both C and C++ then your typemaps can use the appropriate JNI calling convention and need not use the JCALLx macros.

### 27.8.9 Java code typemaps

Most of SWIG's typemaps are used for the generation of C/C++ code. The typemaps in this section are used solely for the generation of Java code. Elements of proxy classes and type wrapper classes come from the following typemaps (the defaults).

`%typemap(javabase)`

base (extends) for Java class: empty default
Note that this typemap accepts a `replace` attribute as an optional flag. When set to "1", it will replace/override any C++ base classes that might have been parsed. If this flag is not specified and there are C++ base classes, then a multiple inheritance warning is issued and the code in the typemap is ignored. The typemap also accepts a `notderived` attribute as an optional flag. When set to "1", it will not apply to classes that are derived from a C++ base. When used with the SWIGTYPE type, it is useful for giving a common base for all proxy classes, that is, providing a base class that sits in between all proxy classes and the Java base class `Object` for example: `%typemap(javabase, notderived="1") SWIGTYPE "CommonBase"`.

`%typemap(javabody)`

the essential support body for proxy classes (proxy base classes only), typewrapper classes and enum classes. Default contains extra constructors, memory ownership control member variables (`swigCMemOwn`, `swigCPtr`), the `getCPtr` method etc.

`%typemap(javabody_derived)`

the essential support body for proxy classes (derived classes only). Same as "javabody" typemap, but only used for proxy derived classes.

`%typemap(javaclassmodifiers)`

class modifiers for the Java class: default is "public class"

`%typemap(javacode)`

Java code is copied verbatim to the Java class: empty default As there can only be one "javacode" typemap per class, also consider using the %proxycode directive which can be used multiple times per class and offers nearly identical functionality.

`%typemap(javadestruct, methodname="delete", methodmodifiers="public synchronized", parameters="")`

destructor wrapper - the `delete()` method (proxy classes only), used for all proxy classes except those which have a base class : default calls C++ destructor (or frees C memory) and resets `swigCPtr` and `swigCMemOwn` flags
Note that the `delete()` method name is configurable and is specified by the `methodname` attribute. The method modifiers are also configurable via the `methodmodifiers` attribute. If a `%javamethodmodifiers` is attached to the class' destructor, it will be used in preference to the `methodmodifiers` typemap attribute for the class. The `delete` method's parameters declaration can be provided in the optional `parameters` typemap attribute.

`%typemap(javadestruct_derived, methodname="delete", methodmodifiers="public synchronized", parameters="")`

destructor wrapper - the `delete()` method (proxy classes only), same as "javadestruct" but only used for derived proxy classes : default calls C++ destructor (or frees C memory) and resets `swigCPtr` and `swigCMemOwn` flags
Note that the `delete()` method name is configurable and is specified by the `methodname` attribute. The method modifiers are also configurable via the `methodmodifiers` attribute. If a `%javamethodmodifiers` is attached to the class' destructor, it will be used in preference to the `methodmodifiers` typemap attribute for the class. The `delete` method's parameters declaration can be provided in the optional `parameters` typemap attribute.

`%typemap(javaimports)`

import statements for Java class: empty default

`%typemap(javainterfaces)`

interfaces (implements) for Java class: empty default

`%typemap(javafinalize)`

the `finalize()` method (proxy classes only): default calls the `delete()` method

Note that the default javafinalize typemap must contain the full implementation of the finalize method. Any customization to this typemap must still declare a java finalize method with the correct signature. Note also that the name of the generated "delete" method may be affected by `javadestruct` and `javadestruct_derived` typemaps. Below shows an example modifying the finalizer, assuming the `delete` method has been renamed to `swig_delete`.

    %typemap(javafinalize) SWIGTYPE %{
      protected void finalize() {
        swig_delete();  // renamed to prevent conflict with existing delete method
      }
    ]%

`%typemap(javainterfacemodifiers)`

Interface modifiers for the Java interface generated when using the `interface` feature, see Java interfaces section. The default is "public interface".

Compatibility note: This typemap was added in SWIG-4.1.0.

`%typemap(javainterfacecode, declaration="...", cptrmethod="...")`

The code in this typemap is added to the body of a Java proxy class but only when a class is marked with the `interface` feature. The typemap is used in the proxy class marked with the interface feature as well as all proxy classes derived from the marked C++ class, as they are all generated as implementing the Java interface. The default typemap used in the `%interface` family of macros mentioned in the Java interfaces section, where `CTYPE` is the C++ class macro argument, is as follows:

    %typemap(javainterfacecode,
             declaration="  long $interfacename_GetInterfaceCPtr();\n",
             cptrmethod="$interfacename_GetInterfaceCPtr") CTYPE %{
      public long $interfacename_GetInterfaceCPtr() {
        return $imclassname.$javaclazzname$interfacename_GetInterfaceCPtr(swigCPtr);
      }
    %}

The special variable `$interfacename` is expanded into the name specified in the `interface` feature.

Compatibility Note: In SWIG-1.3.21 and earlier releases, typemaps called "javagetcptr" and "javaptrconstructormodifiers" were available. These were removed and the "javabody" typemap can be used instead. The `javainterfacecode` typemap and interface feature was introduced in SWIG-3.0.9.

In summary the contents of the typemaps make up a proxy class like this:

    [ javaimports typemap ]
    [ javaclassmodifiers typemap ] javaclassname extends [ javabase typemap ]
                                                 implements [ javainterfaces typemap ] {
    [ javabody or javabody_derived typemap ]
    [ javafinalize typemap ]
    public synchronized void delete() [ javadestruct OR javadestruct_derived typemap ]
    [ javacode typemap ]
    [ javainterfacecode typemap]
    ... proxy functions ...
    }

Note the `delete()` methodname and method modifiers are configurable, see "javadestruct" and "javadestruct_derived" typemaps above.

The `javainterfacecode` typemap is only used when bases are marked by the `interface` feature and the `implements` list will also then be expanded to include these Java interfaces.

The type wrapper class is similar in construction:

    [ javaimports typemap ]
    [ javaclassmodifiers typemap ] javaclassname extends [ javabase typemap ]
                                                 implements [ javainterfaces typemap ] {
    [ javabody typemap ]
    [ javacode typemap ]
    }

The enum class is also similar in construction:

    [ javaimports typemap ]
    [ javaclassmodifiers typemap ] javaclassname extends [ javabase typemap ]
                                                 implements [ javainterfaces typemap ] {
    ... Enum values ...
    [ javabody typemap ]
    [ javacode typemap ]
    }

The "javaimports" typemap is ignored if the enum class is wrapped by an inner Java class, that is when wrapping an enum declared within a C++ class.

The Java interface turned on by the `interface` feature is fairly simple:

    [ javaimports typemap ]
    [ javainterfacemodifiers typemap ] [ name ] [extends additional [, ...] ] {
    [ javainterfacecode:cptrmethod typemap attribute ]
    ... interface declarations ...
    }

where `name` is the `name` attribute and `additional` is the `additional` attribute in the interface feature.

The defaults can be overridden to tailor the generated classes. Here is an example which will change the `getCPtr` method and constructor from the default public access to protected access. If the classes in one package are not using the classes in another package, then these methods need not be public and removing access to these low level implementation details, is a good thing. If you are invoking SWIG more than once and generating the wrapped classes into different packages in each invocation, then you cannot do this as you will then have different packages.

    %typemap(javabody) SWIGTYPE %{
      private transient long swigCPtr;
      protected transient boolean swigCMemOwn;

      protected $javaclassname(long cPtr, boolean cMemoryOwn) {
        swigCMemOwn = cMemoryOwn;
        swigCPtr = cPtr;
      }

      protected static long getCPtr($javaclassname obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }
    %}

The typemap code is the same that is in "`java.swg`", barring the last two method modifiers. Note that `SWIGTYPE` will target all proxy classes, but not the type wrapper classes. Also the above typemap is only used for proxy classes that are potential base classes. To target proxy classes that are derived from a wrapped class as well, the "javabody_derived" typemap should also be overridden.

For the typemap to be used in all type wrapper classes, all the different types that type wrapper classes could be used for should be targeted:

    %typemap(javabody) SWIGTYPE *, SWIGTYPE &, SWIGTYPE [], SWIGTYPE (CLASS::*) %{
      private transient long swigCPtr;

      protected $javaclassname(long cPtr, boolean bFutureUse) {
        swigCPtr = cPtr;
      }

      protected $javaclassname() {
        swigCPtr = 0;
      }

      protected static long getCPtr($javaclassname obj) {
        return (obj == null) ? 0 : obj.swigCPtr;
      }
    %}

Again this is the same that is in "`java.swg`", barring the method modifier for `getCPtr`.

When using multiple modules or the nspace feature it is common to invoke SWIG with a different `-package` command line option for each module. However, by default the generated code may not compile if generated classes in one package use generated classes in another package. The visibility of the `getCPtr()` and pointer constructor generated from the `javabody` typemaps needs changing. The default visibility is `protected` but it needs to be `public` for access from a different package. Just changing 'protected' to 'public' in the typemap achieves this. Two macros are available in `java.swg` to make this easier and using them is the preferred approach over simply copying the typemaps and modifying as this is forward compatible with any changes in the `javabody` typemap in future versions of SWIG. The macros are for the proxy and typewrapper classes and can respectively be used to to make the method and constructor public:

      SWIG_JAVABODY_PROXY(public, public, SWIGTYPE)
      SWIG_JAVABODY_TYPEWRAPPER(public, public, public, SWIGTYPE)

### 27.8.10 Director specific typemaps

The Java directors feature requires the "javadirectorin", "javadirectorout", "directorin" and the "directorout" typemaps in order to work properly. The "javapackage" typemap is an optional typemap used to identify the Java package path for individual SWIG generated proxy classes used in director methods.

`%typemap(directorin)`

The "directorin" typemap is used for converting arguments in the C++ director class to the appropriate JNI type before the upcall to Java. This typemap also specifies the JNI field descriptor for the type in the "descriptor" attribute. For example, integers are converted as follows:

    %typemap(directorin, descriptor="I") int "$input = (jint) $1;"

`$input` is the SWIG name of the JNI temporary variable passed to Java in the upcall. The `descriptor="I"` will put an `I` into the JNI field descriptor that identifies the Java method that will be called from C++. For more about JNI field descriptors and their importance, refer to the JNI documentation mentioned earlier. A typemap for C character strings is:

    %typemap(directorin, descriptor="Ljava/lang/String;", noblock=1) char * {
      $input = 0;
      if ($1) {
        $input = JCALL1(NewStringUTF, jenv, (const char *)$1);
        if (!$input) return $null;
      }
      Swig::LocalRefGuard $1_refguard(jenv, $input);
    }

The `Swig::LocalRefGuard` class should be used in directorin typemaps for newly allocated objects. It is used to control local reference counts ensuring the count is decremented after the call up into Java has completed. Its destructor simply calls `jenv->DeleteLocalRef(obj)` on the `obj` passed in during construction.

User-defined types have the default "descriptor" attribute "`L$packagepath/$javaclassname;`" where `$packagepath` is the package name passed from the SWIG command line and `$javaclassname` is the Java proxy class' name. If the `-package` commandline option is not used to specify the package, then '$packagepath/' will be removed from the resulting output JNI field descriptor. Do not forget the terminating ';' for JNI field descriptors starting with 'L'. If the ';' is left out, Java will generate a "method not found" runtime error. Note that the `$packagepath` substitution always uses the path separator '/' when expanded. The `$javaclassname` expansion can be confusing as it is normally expanded using the '.' separator. However, `$javaclassname` is expanded using the path separator '/' in typemap's "descriptor" attribute as well as in the "directorthrows" typemap.

`%typemap(directorout)`

The "directorout" typemap is used for converting the JNI return type in the C++ director class to the appropriate C++ type after the upcall to Java. For example, integers are converted as follows:

    %typemap(directorout) int %{ $result = (int)$input; %}

`$input` is the SWIG name of the JNI temporary variable returned from Java after the upcall. `$result` is the resulting output. A typemap for C character strings is:

    %typemap(directorout) char * {
      $1 = 0;
      if ($input) {
        $result = (char *)jenv->GetStringUTFChars($input, 0);
        if (!$1) return $null;
      }
    }

`%typemap(javadirectorin)`

Conversion from jtype to jstype for director methods. These are Java code typemaps which transform the type used in the Java intermediary JNI class (as specified in the "jtype" typemap) to the Java type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap). This typemap provides the conversion for the parameters in the director methods when calling up from C++ to Java.

For primitive types, this typemap is usually specified as:

    %typemap(javadirectorin) int "$jniinput"

The `$jniinput` special variable is analogous to `$javainput` special variable. It is replaced by the input parameter name.

`%typemap(javadirectorout)`

Conversion from jstype to jtype for director methods. These are Java code typemaps which transform the type used in the Java module class, proxy classes and type wrapper classes (as specified in the "jstype" typemap) to the type used in the Java intermediary JNI class (as specified in the "jtype" typemap). This typemap provides the conversion for the return type in the director methods when returning from the C++ to Java upcall.

For primitive types, this typemap is usually specified as:

    %typemap(javadirectorout) int "$javacall"

The `$javacall` special variable is analogous to the `$jnicall` special variable. It is replaced by the call to the target Java method. The target method is the method in the Java proxy class which overrides the virtual C++ method in the C++ base class.

`%typemap(directorthrows)`

Conversion of Java exceptions to C++ exceptions in director method's exception handling. This typemap is expected to test the `$error` special variable for a matching Java exception and if successful convert and throw it into a C++ exception given by the typemap's type. The `$error` special variable is of type `jthrowable` and is substituted with a unique variable name in the generated code.

The example below converts a Java `java.lang.IndexOutOfBoundsException` exception to the typemap's type, that is `std::out_of_range`:

    %typemap(directorthrows) std::out_of_range %{
      if (Swig::ExceptionMatches(jenv, $error, "java/lang/IndexOutOfBoundsException")) {
        throw std::out_of_range(Swig::JavaExceptionMessage(jenv, $error).message());
      }
    %}

The utility function `Swig::ExceptionMatches` and class `Swig::JavaExceptionMessage` are helpers available when using directors and are described in the Java Exceptions from Directors section.

`%typemap(javapackage)`

The "javapackage" typemap is optional; it serves to identify a class's Java package. This typemap should be used in conjunction with classes that are defined outside of the current SWIG interface file. The typemap is only used if the type is used in a director method, that is, in a virtual method in a director class. For example:

    // class Foo is handled in a different interface file:
    %import "Foo.i"

    %feature("director") Example;

    %inline {
      class Bar { };

      class Example {
      public:
        virtual ~Example();
        virtual void ping(Foo *arg1, Bar *arg2);
      };
    }

Assume that the Foo class is part of the Java package com.wombat.foo but the above interface file is part of the Java package com.wombat.example. Without the "javapackage" typemap, SWIG will assume that the Foo class belongs to com.wombat.example class. The corrected interface file looks like:

    // class Foo is handled in a different interface file:
    %import "Foo.i"
    %typemap("javapackage") Foo, Foo *, Foo & "com.wombat.foo"
    %feature("director") Example;

    %inline {
      class Bar { };

      class Example {
      public:
        virtual ~Example();
        virtual void ping(Foo *arg1, Bar *arg2);
      };
    }

SWIG looks up the package based on the actual type (plain Foo, Foo pointer and Foo reference), so it is important to associate all three types with the desired package. Practically speaking, you should create a separate SWIG interface file, which is %import-ed into each SWIG interface file, when you have multiple Java packages. Note the helper macros below, `OTHER_PACKAGE_SPEC` and `ANOTHER_PACKAGE_SPEC`, which reduce the amount of extra typing. "`TYPE...`" is useful when passing templated types to the macro, since multiargument template types appear to the SWIG preprocessor as multiple macro arguments.

    %typemap("javapackage") SWIGTYPE, SWIGTYPE *, SWIGTYPE &
                                                "package.for.most.classes";

    %define OTHER_PACKAGE_SPEC(TYPE...)
    %typemap("javapackage") TYPE, TYPE *, TYPE & "package.for.other.classes"
    %enddef

    %define ANOTHER_PACKAGE_SPEC(TYPE...)
    %typemap("javapackage") TYPE, TYPE *, TYPE & "package.for.another.set"
    %enddef

    OTHER_PACKAGE_SPEC(Package_2_class_one)
    ANOTHER_PACKAGE_SPEC(Package_3_class_two)
    /* etc */

The basic strategy here is to provide a default package typemap for the majority of the classes, only providing "javapackage" typemaps for the exceptions.

## 27.9 Typemap Examples

This section includes a few examples of typemaps. For more examples, you might look at the files "`java.swg`" and "`typemaps.i`" in the SWIG library.

### 27.9.1 Simpler Java enums for enums without initializers

The default Proper Java enums approach to wrapping enums is somewhat verbose. This is to handle all possible C/C++ enums, in particular enums with initializers. The generated code can be simplified if the enum being wrapped does not have any initializers.

The following shows how to remove the support methods that are generated by default and instead use the methods in the Java enum base class `java.lang.Enum` and `java.lang.Class` for marshalling enums between C/C++ and Java. The type used for the typemaps below is `enum SWIGTYPE` which is the default type used for all enums. The "enums.swg" file should be examined in order to see the original overridden versions of the typemaps.

    %include "enums.swg"

    %typemap(javain) enum SWIGTYPE "$javainput.ordinal()"
    %typemap(javaout) enum SWIGTYPE {
        return $javaclassname.class.getEnumConstants()[$jnicall];
      }
    %typemap(javabody) enum SWIGTYPE ""

    %inline %{
      enum HairType { blonde, ginger, brunette };
      void setHair(HairType h);
      HairType getHair();
    %}

SWIG will generate the following Java enum, which is somewhat simpler than the default:

    public enum HairType {
      blonde,
      ginger,
      brunette;
    }

and the two Java proxy methods will be:

    public static void setHair(HairType h) {
      exampleJNI.setHair(h.ordinal());
    }

    public static HairType getHair() {
      return HairType.class.getEnumConstants()[exampleJNI.getHair()];
    }

For marshalling Java enums to C/C++ enums, the `ordinal` method is used to convert the Java enum into an integer value for passing to the JNI layer, see the "javain" typemap. For marshalling C/C++ enums to Java enums, the C/C++ enum value is cast to an integer in the C/C++ typemaps (not shown). This integer value is then used to index into the array of enum constants that the Java language provides. See the `getEnumConstants` method in the "javaout" typemap.

These typemaps can often be used as the default for wrapping enums as in many cases there won't be any enum initializers. In fact a good strategy is to always use these typemaps and to specifically handle enums with initializers using %apply. This would be done by using the original versions of these typemaps in "enums.swg" under another typemap name for applying using %apply.

### 27.9.2 Handling C++ exception specifications as Java exceptions

This example demonstrates various ways in which C++ exceptions can be tailored and converted into Java exceptions. Let's consider a simple file class `SimpleFile` and an exception class `FileException` which it may throw on error:

    %include "std_string.i" // for std::string typemaps
    #include <string>

    class FileException {
      std::string message;
    public:
      FileException(const std::string& msg) : message(msg) {}
      std::string what() {
        return message;
      }
    };

    class SimpleFile {
      std::string filename;
    public:
      SimpleFile(const std::string& filename) : filename(filename) {}
      void open() throw(FileException) {
      ...
      }
    };

As the `open` method has a C++ exception specification, SWIG will parse this and know that the method can throw an exception. The "throws" typemap is then used when SWIG encounters an exception specification. The default generic "throws" typemap looks like this:

    %typemap(throws) SWIGTYPE, SWIGTYPE &, SWIGTYPE *, SWIGTYPE [ANY] %{
      SWIG_JavaThrowException(jenv, SWIG_JavaRuntimeException,
                              "C++ $1_type exception thrown");
      return $null;
    %}

Basically SWIG will generate a C++ try catch block and the body of the "throws" typemap constitutes the catch block. The above typemap calls a SWIG supplied method which throws a `java.lang.RuntimeException`. This exception class is a runtime exception and therefore not a checked exception. If, however, we wanted to throw a checked exception, say `java.io.IOException`, then we could use the following typemap:

    %typemap(throws, throws="java.io.IOException") FileException {
      jclass excep = jenv->FindClass("java/io/IOException");
      if (excep)
        jenv->ThrowNew(excep, $1.what().c_str());
      return $null;
    }

Note that this typemap uses the 'throws' typemap attribute to ensure a throws clause is generated. The generated proxy method then specifies the checked exception by containing `java.io.IOException` in the throws clause:

    public class SimpleFile {
      ...
      public void open() throws java.io.IOException { ... }
    }

Lastly, if you don't want to map your C++ exception into one of the standard Java exceptions, the C++ class can be wrapped and turned into a custom Java exception class. If we go back to our example, the first thing we must do is get SWIG to wrap `FileException` and ensure that it derives from `java.lang.Exception`. Additionally, we might want to override the `java.lang.Exception.getMessage()` method. The typemaps to use then are as follows:

    %typemap(javabase) FileException "java.lang.Exception"
    %typemap(javacode) FileException %{
      public String getMessage() {
        return what();
      }
    %}

This generates:

    public class FileException extends java.lang.Exception {
      ...
      public String getMessage() {
        return what();
      }

      public FileException(String msg) { ... }

      public String what() {
        return exampleJNI.FileException_what(swigCPtr, this);
      }
    }

We could alternatively have used `%rename` to rename `what()` into `getMessage()`.

### 27.9.3 NaN Exception - exception handling for a particular type

A Java exception can be thrown from any Java or JNI code. Therefore, as most typemaps contain either Java or JNI code, just about any typemap could throw an exception. The following example demonstrates exception handling on a type by type basis by checking for 'Not a number' (NaN) whenever a parameter of type `float` is wrapped.

Consider the following C++ code:

    bool calculate(float first, float second);

To validate every `float` being passed to C++, we could precede the code being wrapped by the following typemap which throws a runtime exception whenever the `float` is 'Not a Number':

    %module example
    %typemap(javain) float "$module.CheckForNaN($javainput)"
    %pragma(java) modulecode=%{
      /** Simply returns the input value unless it is not a number,
          whereupon an exception is thrown. */
      static protected float CheckForNaN(float num) {
        if (Float.isNaN(num))
          throw new RuntimeException("Not a number");
        return num;
      }
    %}

Note that the `CheckForNaN` support method has been added to the module class using the `modulecode` pragma. The following shows the generated code of interest:

    public class example {
      ...

      /** Simply returns the input value unless it is not a number,
          whereupon an exception is thrown. */
      static protected float CheckForNaN(float num) {
        if (Float.isNaN(num))
          throw new RuntimeException("Not a number");
        return num;
      }

      public static boolean calculate(float first, float second) {
        return exampleJNI.calculate(example.CheckForNaN(first), example.CheckForNaN(second));
      }
    }

Note that the "javain" typemap is used for every occurrence of a `float` being used as an input. Of course, we could have targeted the typemap at a particular parameter by using `float first`, say, instead of just `float`.

The exception checking could alternatively have been placed into the 'pre' attribute that the "javain" typemap supports. The "javain" typemap above could be replaced with the following:

    %typemap(javain, pre="    $module.CheckForNaN($javainput);") float "$javainput"

which would modify the `calculate` function to instead be generated as:

    public class example {
      ...
      public static boolean calculate(float first, float second) {
        example.CheckForNaN(first);
        example.CheckForNaN(second);
        {
          return exampleJNI.calculate(first, second);
        }
      }
    }

See the Date marshalling example for an example using further "javain" typemap attributes.

If we decide that what we actually want is a checked exception instead of a runtime exception, we can change this easily enough. The proxy method that uses `float` as an input, must then add the exception class to the throws clause. SWIG can handle this as it supports the 'throws' typemap attribute for specifying classes for the throws clause. Thus we can modify the pragma and the typemap for the throws clause:

    %typemap(javain, throws="java.lang.Exception") float "$module.CheckForNaN($javainput)"
    %pragma(java) modulecode=%{
      /** Simply returns the input value unless it is not a number,
          whereupon an exception is thrown. */
      static protected float CheckForNaN(float num) throws java.lang.Exception {
        if (Float.isNaN(num))
          throw new RuntimeException("Not a number");
        return num;
      }
    %}

The `calculate` method now has a throws clause and even though the typemap is used twice for both `float first` and `float second`, the throws clause contains a single instance of `java.lang.Exception`:

    public class example {
      ...

      /** Simply returns the input value unless it is not a number,
          whereupon an exception is thrown. */
      static protected float CheckForNaN(float num) throws java.lang.Exception {
        if (Float.isNaN(num))
          throw new RuntimeException("Not a number");
        return num;
      }

      public static boolean calculate(float first, float second) throws java.lang.Exception {
        return exampleJNI.calculate(example.CheckForNaN(first), example.CheckForNaN(second));
      }
    }

If we were a martyr to the JNI cause, we could replace the succinct code within the "javain" typemap with a few pages of JNI code. If we had, we would have put it in the "in" typemap which, like all JNI and Java typemaps, also supports the 'throws' attribute.

### 27.9.4 Converting Java String arrays to char **

A common problem in many C programs is the processing of command line arguments, which are usually passed in an array of NULL terminated strings. The following SWIG interface file allows a Java String array to be used as a `char **` object.

    %module example

    /* This tells SWIG to treat char ** as a special case when used as a parameter
       in a function call */
    %typemap(in) char ** (jint size) {
      int i = 0;
      size = (*jenv)->GetArrayLength(jenv, $input);
      $1 = (char **) malloc((size+1)*sizeof(char *));
      /* make a copy of each string */
      for (i = 0; i<size; i++) {
        jstring j_string = (jstring)(*jenv)->GetObjectArrayElement(jenv, $input, i);
        const char * c_string = (*jenv)->GetStringUTFChars(jenv, j_string, 0);
        $1[i] = malloc((strlen(c_string)+1)*sizeof(char));
        strcpy($1[i], c_string);
        (*jenv)->ReleaseStringUTFChars(jenv, j_string, c_string);
        (*jenv)->DeleteLocalRef(jenv, j_string);
      }
      $1[i] = 0;
    }

    /* This cleans up the memory we malloc'd before the function call */
    %typemap(freearg) char ** {
      int i;
      for (i=0; i<size$argnum-1; i++)
        free($1[i]);
      free($1);
    }

    /* This allows a C function to return a char ** as a Java String array */
    %typemap(out) char ** {
      int i;
      int len=0;
      jstring temp_string;
      const jclass clazz = (*jenv)->FindClass(jenv, "java/lang/String");

      while ($1[len]) len++;
      jresult = (*jenv)->NewObjectArray(jenv, len, clazz, NULL);
      /* exception checking omitted */

      for (i=0; i<len; i++) {
        temp_string = (*jenv)->NewStringUTF(jenv, *result++);
        (*jenv)->SetObjectArrayElement(jenv, jresult, i, temp_string);
        (*jenv)->DeleteLocalRef(jenv, temp_string);
      }
    }

    /* These 3 typemaps tell SWIG what JNI and Java types to use */
    %typemap(jni) char ** "jobjectArray"
    %typemap(jtype) char ** "String[]"
    %typemap(jstype) char ** "String[]"

    /* These 2 typemaps handle the conversion of the jtype to jstype typemap type
       and vice versa */
    %typemap(javain) char ** "$javainput"
    %typemap(javaout) char ** {
        return $jnicall;
      }

    /* Now a few test functions */
    %inline %{

    int print_args(char **argv) {
      int i = 0;
      while (argv[i]) {
        printf("argv[%d] = %s\n", i, argv[i]);
        i++;
      }
      return i;
    }

    char **get_args() {
      static char *values[] = { "Dave", "Mike", "Susan", "John", "Michelle", 0};
      return &values[0];
    }

    %}

Note that the 'C' JNI calling convention is used. Checking for any thrown exceptions after JNI function calls has been omitted. When this module is compiled, our wrapped C functions can be used by the following Java program:

    // File runme.java

    public class runme {

      static {
        try {
          System.loadLibrary("example");
        } catch (UnsatisfiedLinkError e) {
          System.err.println("Native code library failed to load. " + e);
          System.exit(1);
        }
      }

      public static void main(String argv[]) {
        String animals[] = {"Cat", "Dog", "Cow", "Goat"};
        example.print_args(animals);
        String args[] = example.get_args();
        for (int i=0; i<args.length; i++)
          System.out.println(i + ":" + args[i]);
      }
    }

When compiled and run we get:

    $ java runme
    argv[0] = Cat
    argv[1] = Dog
    argv[2] = Cow
    argv[3] = Goat
    0:Dave
    1:Mike
    2:Susan
    3:John
    4:Michelle

In the example, a few different typemaps are used. The "in" typemap is used to receive an input argument and convert it to a C array. Since dynamic memory allocation is used to allocate memory for the array, the "freearg" typemap is used to later release this memory after the execution of the C function. The "out" typemap is used for function return values. Lastly the "jni", "jtype" and "jstype" typemaps are also required to specify what Java types to use.

### 27.9.5 Expanding a Java object to multiple arguments

Suppose that you had a collection of C functions with arguments such as the following:

    int foo(int argc, char **argv);

In the previous example, a typemap was written to pass a Java String array as the `char **argv`. This allows the function to be used from Java as follows:

    example.foo(4, new String[]{"red", "green", "blue", "white"});

Although this works, it's a little awkward to specify the argument count. To fix this, a multi-argument typemap can be defined. This is not very difficult--you only have to make slight modifications to the previous example's typemaps:

    %typemap(in) (int argc, char **argv) {
      int i = 0;
      $1 = (*jenv)->GetArrayLength(jenv, $input);
      $2 = (char **) malloc(($1+1)*sizeof(char *));
      /* make a copy of each string */
      for (i = 0; i<$1; i++) {
        jstring j_string = (jstring)(*jenv)->GetObjectArrayElement(jenv, $input, i);
        const char * c_string = (*jenv)->GetStringUTFChars(jenv, j_string, 0);
        $2[i] = malloc((strlen(c_string)+1)*sizeof(char));
        strcpy($2[i], c_string);
        (*jenv)->ReleaseStringUTFChars(jenv, j_string, c_string);
        (*jenv)->DeleteLocalRef(jenv, j_string);
      }
      $2[i] = 0;
    }

    %typemap(freearg) (int argc, char **argv) {
      int i;
      for (i=0; i<$1-1; i++)
        free($2[i]);
      free($2);
    }

    %typemap(jni) (int argc, char **argv) "jobjectArray"
    %typemap(jtype) (int argc, char **argv) "String[]"
    %typemap(jstype) (int argc, char **argv) "String[]"

    %typemap(javain) (int argc, char **argv) "$javainput"

When writing a multiple-argument typemap, each of the types is referenced by a variable such as `$1` or `$2`. The typemap code simply fills in the appropriate values from the supplied Java parameter.

With the above typemap in place, you will find it no longer necessary to supply the argument count. This is automatically set by the typemap code. For example:

    example.foo(new String[]{"red", "green", "blue", "white"});

### 27.9.6 Using typemaps to return arguments

A common problem in some C programs is that values may be returned in function parameters rather than in the return value of a function. The `typemaps.i` file defines INPUT, OUTPUT and INOUT typemaps which can be used to solve some instances of this problem. This library file uses an array as a means of moving data to and from Java when wrapping a C function that takes non const pointers or non const references as parameters.

Now we are going to outline an alternative approach to using arrays for C pointers. The INOUT typemap uses a `double[]` array for receiving and returning the `double*` parameters. In this approach we are able to use a Java class `myDouble` instead of `double[]` arrays where the C pointer `double*` is required.

Here is our example function:

    /* Returns a status value and two values in out1 and out2 */
    int spam(double a, double b, double *out1, double *out2);

If we define a structure `MyDouble` containing a `double` member variable and use some typemaps we can solve this problem. For example we could put the following through SWIG:

    %module example

    /* Define a new structure to use instead of double * */
    %inline %{
    typedef struct {
      double value;
    } MyDouble;
    %}

    %{
    /* Returns a status value and two values in out1 and out2 */
    int spam(double a, double b, double *out1, double *out2) {
      int status = 1;
      *out1 = a*10.0;
      *out2 = b*100.0;
      return status;
    }
    %}

    /*
    This typemap will make any double * function parameters with name OUTVALUE take an
    argument of MyDouble instead of double *. This will
    allow the calling function to read the double * value after returning from the function.
    */
    %typemap(in) double *OUTVALUE {
      jclass clazz = jenv->FindClass("MyDouble");
      jfieldID fid = jenv->GetFieldID(clazz, "swigCPtr", "J");
      jlong cPtr = jenv->GetLongField($input, fid);
      MyDouble *pMyDouble = NULL;
      *(MyDouble **)&pMyDouble = *(MyDouble **)&cPtr;
      $1 = &pMyDouble->value;
    }

    %typemap(jtype) double *OUTVALUE "MyDouble"
    %typemap(jstype) double *OUTVALUE "MyDouble"
    %typemap(jni) double *OUTVALUE "jobject"

    %typemap(javain) double *OUTVALUE "$javainput"

    /* Now we apply the typemap to the named variables */
    %apply double *OUTVALUE { double *out1, double *out2 };
    int spam(double a, double b, double *out1, double *out2);

Note that the C++ JNI calling convention has been used this time and so must be compiled as C++ and the -c++ commandline must be passed to SWIG. JNI error checking has been omitted for clarity.

What the typemaps do are make the named `double*` function parameters use our new `MyDouble` wrapper structure. The "in" typemap takes this structure, gets the C++ pointer to it, takes the `double value` member variable and passes it to the C++ `spam` function. In Java, when the function returns, we use the SWIG created `getValue()` function to get the output value. The following Java program demonstrates this:

    // File: runme.java

    public class runme {

      static {
        try {
          System.loadLibrary("example");
        } catch (UnsatisfiedLinkError e) {
          System.err.println("Native code library failed to load. " + e);
          System.exit(1);
        }
      }

      public static void main(String argv[]) {
        MyDouble out1 = new MyDouble();
        MyDouble out2 = new MyDouble();
        int ret = example.spam(1.2, 3.4, out1, out2);
        System.out.println(ret + "  " + out1.getValue() + "  " + out2.getValue());
      }
    }

When compiled and run we get:

    $ java runme
    1 12.0  340.0

### 27.9.7 Adding Java downcasts to polymorphic return types

SWIG support for polymorphism works in that the appropriate virtual function is called. However, the default generated code does not allow for downcasting. Let's examine this with the following code:

    %include "std_string.i"

    #include <iostream>
    using namespace std;
    class Vehicle {
    public:
      virtual void start() = 0;
    ...
    };

    class Ambulance : public Vehicle {
      string vol;
    public:
      Ambulance(string volume) : vol(volume) {}
      virtual void start() {
        cout << "Ambulance started" << endl;
      }
      void sound_siren() {
        cout << vol << " siren sounded!" << endl;
      }
    ...
    };

    Vehicle *vehicle_factory() {
      return new Ambulance("Very loud");
    }

If we execute the following Java code:

    Vehicle vehicle = example.vehicle_factory();
    vehicle.start();

    Ambulance ambulance = (Ambulance)vehicle;
    ambulance.sound_siren();

We get:

    Ambulance started
    java.lang.ClassCastException
            at runme.main(runme.java:16)

Even though we know from examination of the C++ code that `vehicle_factory` returns an object of type `Ambulance`, we are not able to use this knowledge to perform the downcast in Java. This occurs because the runtime type information is not completely passed from C++ to Java when returning the type from `vehicle_factory()`. Usually this is not a problem as virtual functions do work by default, such as in the case of `start()`. There are a few solutions to getting downcasts to work.

The first is not to use a Java cast but a call to C++ to make the cast. Add this to your code:

    %exception Ambulance::dynamic_cast(Vehicle *vehicle) {
      $action
        if (!result) {
          jclass excep = jenv->FindClass("java/lang/ClassCastException");
          if (excep) {
            jenv->ThrowNew(excep, "dynamic_cast exception");
          }
        }
    }
    %extend Ambulance {
      static Ambulance *dynamic_cast(Vehicle *vehicle) {
        return dynamic_cast<Ambulance *>(vehicle);
      }
    };

It would then be used from Java like this

    Ambulance ambulance = Ambulance.dynamic_cast(vehicle);
    ambulance.sound_siren();

Should `vehicle` not be of type `ambulance` then a Java `ClassCastException` is thrown. The next solution is a purer solution in that Java downcasts can be performed on the types. Add the following before the definition of `vehicle_factory`:

    %typemap(out) Vehicle * {
      Ambulance *downcast = dynamic_cast<Ambulance *>($1);
      *(Ambulance **)&$result = downcast;
    }

    %typemap(javaout) Vehicle * {
        return new Ambulance($jnicall, $owner);
      }

Here we are using our knowledge that `vehicle_factory` always returns type `Ambulance` so that the Java proxy is created as a type `Ambulance`. If `vehicle_factory` can manufacture any type of `Vehicle` and we want to be able to downcast using Java casts for any of these types, then a different approach is needed. Consider expanding our example with a new Vehicle type and a more flexible factory function:

    class FireEngine : public Vehicle {
    public:
      FireEngine() {}
      virtual void start() {
        cout << "FireEngine started" << endl;
      }
      void roll_out_hose() {
        cout << "Hose rolled out" << endl;
      }
      ...
    };
    Vehicle *vehicle_factory(int vehicle_number) {
      if (vehicle_number == 0)
        return new Ambulance("Very loud");
      else
        return new FireEngine();
    }

To be able to downcast with this sort of Java code:

    FireEngine fireengine = (FireEngine)example.vehicle_factory(1);
    fireengine.roll_out_hose();
    Ambulance ambulance = (Ambulance)example.vehicle_factory(0);
    ambulance.sound_siren();

the following typemaps targeted at the `vehicle_factory` function will achieve this. Note that in this case, the Java class is constructed using JNI code rather than passing a pointer across the JNI boundary in a Java long for construction in Java code.

    %typemap(jni) Vehicle *vehicle_factory "jobject"
    %typemap(jtype) Vehicle *vehicle_factory "Vehicle"
    %typemap(jstype) Vehicle *vehicle_factory "Vehicle"
    %typemap(javaout) Vehicle *vehicle_factory {
        return $jnicall;
      }

    %typemap(out) Vehicle *vehicle_factory {
      Ambulance *ambulance = dynamic_cast<Ambulance *>($1);
      FireEngine *fireengine = dynamic_cast<FireEngine *>($1);
      if (ambulance) {
        // call the Ambulance(long cPtr, boolean cMemoryOwn) constructor
        jclass clazz = jenv->FindClass("Ambulance");
        if (clazz) {
          jmethodID mid = jenv->GetMethodID(clazz, "<init>", "(JZ)V");
          if (mid) {
            jlong cptr = 0;
            *(Ambulance **)&cptr = ambulance;
            $result = jenv->NewObject(clazz, mid, cptr, false);
          }
        }
      } else if (fireengine) {
        // call the FireEngine(long cPtr, boolean cMemoryOwn) constructor
        jclass clazz = jenv->FindClass("FireEngine");
        if (clazz) {
          jmethodID mid = jenv->GetMethodID(clazz, "<init>", "(JZ)V");
          if (mid) {
            jlong cptr = 0;
            *(FireEngine **)&cptr = fireengine;
            $result = jenv->NewObject(clazz, mid, cptr, false);
          }
        }
      }
      else {
        cout << "Unexpected type " << endl;
      }

      if (!$result)
        cout << "Failed to create new java object" << endl;
    }

Better error handling would need to be added into this code. There are other solutions to this problem, but this last example demonstrates some more involved JNI code. SWIG usually generates code which constructs the proxy classes using Java code as it is easier to handle error conditions and is faster. Note that the JNI code above uses a number of string lookups to call a constructor, whereas this would not occur using byte compiled Java code.

### 27.9.8 Adding an equals method to the Java classes

When a pointer is returned from a JNI function, it is wrapped using a new Java proxy class or type wrapper class. Even when the pointers are the same, it will not be possible to know that the two Java classes containing those pointers are actually the same object. It is common in Java to use the `equals()` method to check whether two objects are equivalent. The `equals()` method is usually accompanied by a `hashCode()` method in order to fulfill the requirement that the hash code is equal for equal objects. Pure Java code methods like these can be easily added:

    %typemap(javacode) SWIGTYPE %{
      public boolean equals(Object obj) {
        boolean equal = false;
        if (obj instanceof $javaclassname)
          equal = ((($javaclassname)obj).swigCPtr == this.swigCPtr);
        return equal;
      }
      public int hashCode() {
        return (int)getPointer();
      }
    %}

    class Foo { };
    Foo* returnFoo(Foo *foo) { return foo; }

The following would display `false` without the `javacode` typemap above. With the typemap defining the `equals` method the result is `true`.

    Foo foo1 = new Foo();
    Foo foo2 = example.returnFoo(foo1);
    System.out.println("foo1? " + foo1.equals(foo2));

### 27.9.9 Void pointers and a common Java base class

One might wonder why the common code that SWIG emits for the proxy and type wrapper classes is not pushed into a base class. The reason is that although `swigCPtr` could be put into a common base class for all classes wrapping C structures, it would not work for C++ classes involved in an inheritance chain. Each class derived from a base needs a separate `swigCPtr` because C++ compilers sometimes use a different pointer value when casting a derived class to a base. Additionally as Java only supports single inheritance, it would not be possible to derive wrapped classes from your own pure Java classes if the base class has been 'used up' by SWIG. However, you may want to move some of the common code into a base class. Here is an example which uses a common base class for all proxy classes and type wrapper classes:

    %typemap(javabase) SWIGTYPE, SWIGTYPE *, SWIGTYPE &, SWIGTYPE [],
                                                             SWIGTYPE (CLASS::*) "SWIG"

    %typemap(javacode) SWIGTYPE, SWIGTYPE *, SWIGTYPE &, SWIGTYPE [],
                                                             SWIGTYPE (CLASS::*) %{
      protected long getPointer() {
        return swigCPtr;
      }
    %}

Define new base class called SWIG:

    public abstract class SWIG {
      protected abstract long getPointer();

      public boolean equals(Object obj) {
        boolean equal = false;
        if (obj instanceof SWIG)
          equal = (((SWIG)obj).getPointer() == this.getPointer());
        return equal;
      }

      SWIGTYPE_p_void getVoidPointer() {
        return new SWIGTYPE_p_void(getPointer(), false);
      }
    }

This example contains some useful functionality which you may want in your code.

- It has an `equals()` method. Unlike the previous example, the method code isn't replicated in all classes.
- It also has a function which effectively implements a cast from the type of the proxy/type wrapper class to a void pointer. This is necessary for passing a proxy class or a type wrapper class to a function that takes a void pointer.

### 27.9.10 Struct pointer to pointer

Pointers to pointers are often used as output parameters in C factory type functions. These are a bit more tricky to handle. Consider the following situation where a `Butler` can be hired and fired:

    typedef struct {
      int hoursAvailable;
      char *greeting;
    } Butler;

    // Note: HireButler will allocate the memory
    // The caller must free the memory by calling FireButler()!!
    extern int HireButler(Butler **ppButler);
    extern void FireButler(Butler *pButler);

C code implementation:

    int HireButler(Butler **ppButler) {
      Butler *pButler = (Butler *)malloc(sizeof(Butler));
      pButler->hoursAvailable = 24;
      pButler->greeting = (char *)malloc(32);
      strcpy(pButler->greeting, "At your service Sir");
      *ppButler = pButler;
      return 1;
    }
    void FireButler(Butler *pButler) {
      free(pButler->greeting);
      free(pButler);
    }

Let's take two approaches to wrapping this code. The first is to provide a functional interface, much like the original C interface. The following Java code shows how we intend the code to be used:

    Butler jeeves = new Butler();
    example.HireButler(jeeves);
    System.out.println("Greeting:     " + jeeves.getGreeting());
    System.out.println("Availability: " + jeeves.getHoursAvailable() + " hours per day");
    example.FireButler(jeeves);

Resulting in the following output when run:

    Greeting:     At your service Sir
    Availability: 24 hours per day

Note the usage is very much like it would be used if we were writing C code, that is, explicit memory management is needed. No C memory is allocated in the construction of the `Butler` proxy class and the proxy class will not destroy the underlying C memory when it is collected. A number of typemaps and features are needed to implement this approach. The following interface file code should be placed before SWIG parses the above C code.

    %module example

    // Do not generate the default proxy constructor or destructor
    %nodefaultctor Butler;
    %nodefaultdtor Butler;

    // Add in pure Java code proxy constructor
    %typemap(javacode) Butler %{
      /** This constructor creates the proxy which initially does not create nor own any C memory */
      public Butler() {
        this(0, false);
      }
    %}

    // Type typemaps for marshalling Butler **
    %typemap(jni) Butler ** "jobject"
    %typemap(jtype) Butler ** "Butler"
    %typemap(jstype) Butler ** "Butler"

    // Typemaps for Butler ** as a parameter output type
    %typemap(in) Butler ** (Butler *ppButler = 0) %{
      $1 = &ppButler;
    %}
    %typemap(argout) Butler ** {
      // Give Java proxy the C pointer (of newly created object)
      jclass clazz = (*jenv)->FindClass(jenv, "Butler");
      jfieldID fid = (*jenv)->GetFieldID(jenv, clazz, "swigCPtr", "J");
      jlong cPtr = 0;
      *(Butler **)&cPtr = *$1;
      (*jenv)->SetLongField(jenv, $input, fid, cPtr);
    }
    %typemap(javain) Butler ** "$javainput"

Note that the JNI code sets the proxy's `swigCPtr` member variable to point to the newly created object. The `swigCMemOwn` remains unchanged (at false), so that the proxy does not own the memory.

Note: The old %nodefault directive disabled the default constructor and destructor at the same time. This is unsafe in most of the cases, and you can use the explicit %nodefaultctor and %nodefaultdtor directives to achieve the same result if needed.

The second approach offers a more object oriented interface to the Java user. We do this by making the Java proxy class's constructor call the `HireButler()` method to create the underlying C object. Additionally we get the proxy to take ownership of the memory so that the finalizer will call the `FireButler()` function. The proxy class will thus take ownership of the memory and clean it up when no longer needed. We will also prevent the user from being able to explicitly call the `HireButler()` and `FireButler()` functions. Usage from Java will simply be:

    Butler jeeves = new Butler();
    System.out.println("Greeting:     " + jeeves.getGreeting());
    System.out.println("Availability: " + jeeves.getHoursAvailable() + " hours per day");

Note that the Butler class is used just like any other Java class and no extra coding by the user needs to be written to clear up the underlying C memory as the finalizer will be called by the garbage collector which in turn will call the `FireButler()` function. To implement this, we use the above interface file code but remove the `javacode` typemap and add the following:

    // Don't expose the memory allocation/de-allocation functions
    %ignore FireButler(Butler *pButler);
    %ignore HireButler(Butler **ppButler);

    // Add in a custom proxy constructor and destructor
    %extend Butler {
      Butler() {
        Butler *pButler = 0;
        HireButler(&pButler);
        return pButler;
      }
      ~Butler() {
        FireButler($self);
      }
    }

Note that the code in `%extend` is using a C++ type constructor and destructor, yet the generated code will still compile as C code, see Adding member functions to C structures. The C functional interface has been completely morphed into an object-oriented interface and the Butler class would behave much like any pure Java class and feel more natural to Java users.

### 27.9.11 Memory management for accessing member variables

There are lifetime issues that need to be considered when accessing member variables directly or via an accessor method that returns by pointer or reference. Accessor methods that return by value are fine as a copy is made and the lifetime of the copied object is managed entirely by the JVM without any additional underlying C/C++ pointers to worry about. The examples here show how to prevent premature garbage collection of objects when the underlying C++ class returns a pointer or reference to a member variable.

#### 27.9.11.1 Member variables via accessor methods

Consider the following C++ code which provides the `getWheel()` accessor method to return a reference to a member variable:

    struct Wheel {
      int size;
      Wheel(int sz = 0) : size(sz) {}
    };

    class Bike {
      Wheel wheel;
    public:
      Bike(int val) : wheel(val) {}
      Wheel& getWheel() { return wheel; }
    };

and the following usage from Java after running the code through SWIG:

    Wheel wheel = new Bike(10).getWheel();
    System.out.println("wheel size: " + wheel.getSize());
    // Simulate a garbage collection
    System.gc();
    System.runFinalization();
    System.out.println("wheel size: " + wheel.getSize());

Don't be surprised that if the resulting output gives strange results after the garbage collector has run such as...

    wheel size: 10
    wheel size: 135019664

The garbage collector is non-deterministic; sometimes it does not collect any objects and other times it does. What has happened this time is the garbage collector has collected the `Bike` instance as it doesn't think it is needed any more. The proxy instance, `wheel`, contains a reference to memory that was deleted when the `Bike` instance was collected. In order to prevent the garbage collector from collecting the `Bike` instance a reference to the `Bike` must be added to the `wheel` instance. You can do this by adding the reference when the `getWheel()` method is called using the following typemaps.

    %typemap(javacode) Wheel %{
      // Ensure that the GC doesn't collect any Bike instance set from Java
      private Bike bikeReference;
      protected void addReference(Bike bike) {
        bikeReference = bike;
      }
    %}

    // Add a Java reference to prevent premature garbage collection and resulting use
    // of dangling C++ pointer. Intended for methods that return pointers or
    // references to a member variable.
    %typemap(javaout) Wheel &getWheel {
      long cPtr = $jnicall;
      $javaclassname ret = null;
      if (cPtr != 0) {
        ret = new $javaclassname(cPtr, $owner);
        ret.addReference(this);
      }
      return ret;
    }

The code in the first typemap gets added to the `Wheel` proxy class. The code in the second typemap constitutes the bulk of the code in the generated `getWheel()` function:

    public class Wheel {
      ...
      // Ensure that the GC doesn't collect any bike set from Java
      private Bike bikeReference;
      protected void addReference(Bike bike) {
        bikeReference = bike;
      }
    }

    public class Bike {
      ...
      public Wheel getWheel() {
        long cPtr = exampleJNI.Bike_getWheel(swigCPtr, this);
        Wheel ret = null;
        if (cPtr != 0) {
          ret = new Wheel(cPtr, false);
          ret.addReference(this);
        }
        return ret;
      }
    }

Note the `addReference` call. The second display of the wheel size from Java will now be reliable without inadvertent garbage collection.

#### 27.9.11.2 Direct access to member variables

Accessing public member variables directly unfortunately has the same problem. Consider a simple change to the `Bike` class above so that the `getWheel()` method is replaced by direct public access to the `wheel` member variable:

    class Bike {
    public:
      Wheel wheel;
      Bike(int val) : wheel(val) {}
    };

As described in Member data and for reasons described in Structure data members, public member variables are wrapped as if the class had two public accessor methods using pointers where the getter returns a pointer to the member:

    class Bike {
      ...
    public:
      Wheel *wheel();
      void wheel(Wheel *w);
      ...
    };

The early garbage collection problem described for the `getWheel()` accessor method is identical when using the member variable getter as references are treated like pointers. The solution is the same with the same typemap adjusted for returning by pointer instead of by reference. In fact we can just use the earlier typemap via `%apply` as reference and pointer typemaps are largely interchangeable:

    %apply Wheel &getWheel { Wheel *wheel }

Or alternatively the typemap can be provided in full as shown below (it's the same typemap shown earlier, except for the typemap pattern matching). Note that the name of the type in the typemap is `wheel` (the member variable name). Being familiar with Debugging typemap pattern matching is highly recommended to see these details.

    %typemap(javaout) Wheel *wheel {
      long cPtr = $jnicall;
      $javaclassname ret = null;
      if (cPtr != 0) {
        ret = new $javaclassname(cPtr, $owner);
        ret.addReference(this);
      }
      return ret;
    }

Also, confusingly, the Java getter name in the Java proxy class for the `wheel` member variable is the JavaBean name `getWheel()`. There are no premature garbage collection issues with member variable setters as there are no reference or pointer lifetime issues given the member variable is copied by value.

### 27.9.12 Memory management for objects passed to the C++ layer

Managing memory can be tricky when using C++ and Java proxy classes. The previous example shows one such case and this example looks at memory management for a class passed to a C++ method which expects the object to remain in scope after the function has returned. Consider the following two C++ classes:

    struct Element {
      int value;
      Element(int val) : value(val) {}
    };
    class Container {
      Element* element;
    public:
      Container() : element(0) {}
      void setElement(Element* e) { element = e; }
      Element* getElement() { return element; }
    };

and usage from C++

    Container container;
    Element element(20);
    container.setElement(&element);
    cout << "element.value: " << container.getElement()->value << endl;

and more or less equivalent usage from Java

    Container container = new Container();
    container.setElement(new Element(20));
    System.out.println("element value: " + container.getElement().getValue());

The C++ code will always print out 20, but the value printed out may not be this in the Java equivalent code. In order to understand why, consider a garbage collection occurring...

    Container container = new Container();
    container.setElement(new Element(20));
    // Simulate a garbage collection
    System.gc();
    System.runFinalization();
    System.out.println("element value: " + container.getElement().getValue());

The temporary element created with `new Element(20)` could get garbage collected which ultimately means the `container` variable is holding a dangling pointer, thereby printing out any old random value instead of the expected value of 20. One solution is to add in the appropriate references in the Java layer...

    public class Container {

      ...

      // Ensure that the GC doesn't collect any Element set from Java
      // as the underlying C++ class stores a shallow copy
      private Element elementReference;

      public void setElement(Element e) {
        exampleJNI.Container_setElement(swigCPtr, this, Element.getCPtr(e), e);
        elementReference = e;
      }
    }

The following typemaps can be used to generate this code:

    %typemap(javacode) Container %{
      // Ensure that the GC doesn't collect any element set from Java
      // as the underlying C++ class stores a shallow copy
      private Element elementReference;
    %}

    %typemap(javain,
             post="      elementReference = $javainput;"
             ) Element *e "Element.getCPtr($javainput)"

The 'javacode' typemap simply adds in the specified code into the Java proxy class. The 'javain' typemap matches the input parameter type and name for the `setElement` method and the 'post' typemap attribute allows adding code after the JNI call. The 'post' code is generated into a finally block after the JNI call so the resulting code isn't quite as mentioned earlier, `setElement` is actually:

      public void setElement(Element e) {
        try {
          exampleJNI.Container_setElement(swigCPtr, this, Element.getCPtr(e), e);
        } finally {
          elementReference = e;
        }
      }

### 27.9.13 Date marshalling using the javain typemap and associated attributes

The NaN Exception example is a simple example of the "javain" typemap and its 'pre' attribute. This example demonstrates how a C++ date class, say `CDate`, can be mapped onto the standard Java date class, `java.util.GregorianCalendar` by using the 'pre', 'post' and 'pgcppname' attributes of the "javain" typemap. The idea is that the `GregorianCalendar` is used wherever the C++ API uses a `CDate`. Let's assume the code being wrapped is as follows:

    class CDate {
    public:
      CDate(int year, int month, int day);
      int getYear();
      int getMonth();
      int getDay();
      ...
    };
    struct Action {
      static int doSomething(const CDate &dateIn, CDate &dateOut);
      Action(const CDate &date, CDate &dateOut);
    };

Note that `dateIn` is const and therefore read only and `dateOut` is a non-const output type.

First let's look at the code that is generated by default, where the Java proxy class `CDate` is used in the proxy interface:

    public class Action {
      ...
      public static int doSomething(CDate dateIn, CDate dateOut) {
        return exampleJNI.Action_doSomething(CDate.getCPtr(dateIn), dateIn,
                                             CDate.getCPtr(dateOut), dateOut);
      }

      public Action(CDate date, CDate dateOut) {
        this(exampleJNI.new_Action(CDate.getCPtr(date), date,
                                   CDate.getCPtr(dateOut), dateOut), true);
      }
    }

The `CDate &` and `const CDate &` Java code is generated from the following two default typemaps:

    %typemap(jstype) SWIGTYPE & "$javaclassname"
    %typemap(javain) SWIGTYPE & "$javaclassname.getCPtr($javainput)"

where '$javaclassname' is translated into the proxy class name, `CDate` and '$javainput' is translated into the name of the parameter, eg `dateIn`. From Java, the intention is then to call into a modified API with something like:

    java.util.GregorianCalendar calendarIn =
      new java.util.GregorianCalendar(2011, java.util.Calendar.APRIL, 13, 0, 0, 0);
    java.util.GregorianCalendar calendarOut = new java.util.GregorianCalendar();

    // Note in calls below, calendarIn remains unchanged and calendarOut
    // is set to a new value by the C++ call
    Action.doSomething(calendarIn, calendarOut);
    Action action = new Action(calendarIn, calendarOut);

To achieve this mapping, we need to alter the default code generation slightly so that at the Java layer, a `GregorianCalendar` is converted into a `CDate`. The JNI intermediary layer will still take a pointer to the underlying `CDate` class. The typemaps to achieve this are shown below.

    %typemap(jstype) const CDate& "java.util.GregorianCalendar"
    %typemap(javain,
             pre="    CDate temp$javainput = new CDate($javainput.get(java.util.Calendar.YEAR), "
                 "$javainput.get(java.util.Calendar.MONTH), $javainput.get(java.util.Calendar.DATE));",
             pgcppname="temp$javainput") const CDate &
             "$javaclassname.getCPtr(temp$javainput)"

    %typemap(jstype) CDate& "java.util.Calendar"
    %typemap(javain,
             pre="    CDate temp$javainput = new CDate($javainput.get(java.util.Calendar.YEAR), "
                 "$javainput.get(java.util.Calendar.MONTH), $javainput.get(java.util.Calendar.DATE));",
             post="      $javainput.set(temp$javainput.getYear(), temp$javainput.getMonth(), "
                  "temp$javainput.getDay(), 0, 0, 0);",
             pgcppname="temp$javainput") CDate &
             "$javaclassname.getCPtr(temp$javainput)"

The resulting generated proxy code in the `Action` class follows:

    public class Action {
      ...
      public static int doSomething(java.util.GregorianCalendar dateIn,
                                    java.util.Calendar dateOut) {
        CDate tempdateIn = new CDate(dateIn.get(java.util.Calendar.YEAR),
                                     dateIn.get(java.util.Calendar.MONTH),
                                     dateIn.get(java.util.Calendar.DATE));
        CDate tempdateOut = new CDate(dateOut.get(java.util.Calendar.YEAR),
                                      dateOut.get(java.util.Calendar.MONTH),
                                      dateOut.get(java.util.Calendar.DATE));
        try {
          return exampleJNI.Action_doSomething(CDate.getCPtr(tempdateIn), tempdateIn,
                                               CDate.getCPtr(tempdateOut), tempdateOut);
        } finally {
          dateOut.set(tempdateOut.getYear(), tempdateOut.getMonth(), tempdateOut.getDay(), 0, 0, 0);
        }
      }

      static private long SwigConstructAction(java.util.GregorianCalendar date,
                                              java.util.Calendar dateOut) {
        CDate tempdate = new CDate(date.get(java.util.Calendar.YEAR),
                                   date.get(java.util.Calendar.MONTH),
                                   date.get(java.util.Calendar.DATE));
        CDate tempdateOut = new CDate(dateOut.get(java.util.Calendar.YEAR),
                                      dateOut.get(java.util.Calendar.MONTH),
                                      dateOut.get(java.util.Calendar.DATE));
        try {
          return exampleJNI.new_Action(CDate.getCPtr(tempdate), tempdate,
                                       CDate.getCPtr(tempdateOut), tempdateOut);
        } finally {
          dateOut.set(tempdateOut.getYear(), tempdateOut.getMonth(), tempdateOut.getDay(), 0, 0, 0);
        }
      }

      public Action(java.util.GregorianCalendar date, java.util.Calendar dateOut) {
        this(Action.SwigConstructAction(date, dateOut), true);
      }
    }

A few things to note:

- The "javatype" typemap has changed the parameter type to `java.util.GregorianCalendar` or `java.util.Calendar` instead of the default generated `CDate` proxy.
- The code in the 'pre' attribute appears before the JNI call (`exampleJNI.new_Action` / `exampleJNI.Action_doSomething`).
- The code in the 'post' attribute appears after the JNI call.
- A try .. finally block is generated with the JNI call in the try block and 'post' code in the finally block. The alternative of just using a temporary variable for the return value from the JNI call and the 'post' code being generated before the return statement is not possible given that the JNI call is in one line and comes from the "javaout" typemap.
- The temporary variables in the "javain" typemaps are called `temp$javain`, where "$javain" is replaced with the parameter name. "$javain" is used to mangle the variable name so that more than one `CDate &` type can be used as a parameter in a method, otherwise two or more local variables with the same name would be generated.
- The use of the "javain" typemap causes a constructor helper function (`SwigConstructAction`) to be generated. This allows Java code to be called before the JNI call and is required as the Java compiler won't compile code inserted before the 'this' call.
- The 'pgcppname' attribute is used to modify the object being passed as the premature garbage collection prevention parameter (the 2nd and 4th parameters in the JNI calls).

## 27.10 Living with Java Directors

This section is intended to address frequently asked questions and frequently encountered problems when using Java directors.

1. When my program starts up, it complains that method_foo cannot be found in a Java method called swig_module_init. How do I fix this?

   Open up the C++ wrapper source code file and look for `"method_foo"` (include the double quotes, they are important!) Look at the JNI field descriptor and make sure that each class that occurs in the descriptor has the correct package name in front of it. If the package name is incorrect, put a "javapackage" typemap in your SWIG interface file.

2. I'm compiling my code and I'm using templates. I provided a javapackage typemap, but SWIG doesn't generate the right JNI field descriptor.

   Provide the package name in a "javapackage" typemap for the type:

       %typemap(javapackage)  std::vector<int>  "com.funky"
       %template(VectorOfInt) std::vector<int>;

       %feature("director") MyDirector;
       struct MyDirector {
         virtual ~MyDirector();
         virtual void takeVector(std::vector<int> vec);
       };

   The descriptor `com/funky/VectorOfInt` for `VectorOfInt` in the generated code will then be derived from the provided package "com.funky". Below shows the relevant snippet of generated code with the full descriptor:

           SwigDirectorMethod(jenv, baseclass, "takeVector", "(Lcom/funky/VectorOfInt;)V")

3. When I pass class pointers or references through a C++ upcall and I try to type cast them, Java complains with a ClassCastException. What am I doing wrong?

   Normally, a non-director generated Java proxy class creates temporary Java objects as follows:

       public static void MyClass_method_upcall(MyClass self, long jarg1)
       {
         Foo darg1 = new Foo(jarg1, false);

         self.method_upcall(darg1);
       }

   Unfortunately, this loses the Java type information that is part of the underlying Foo director proxy class's Java object pointer causing the type cast to fail. The SWIG Java module's director code attempts to correct the problem, but only for director-enabled classes, since the director class retains a global reference to its Java object. Thus, for director-enabled classes and only for director-enabled classes, the generated proxy Java code looks something like:

       public static void MyClass_method_upcall(MyClass self, long jarg1,
                                                Foo jarg1_object)
       {
         Foo darg1 = (jarg1_object != null ? jarg1_object : new Foo(jarg1, false));

         self.method_upcall(darg1);
       }

   When you import a SWIG interface file containing class definitions, the classes you want to be director-enabled must be have the `feature("director")` enabled for type symmetry to work. This applies even when the class being wrapped isn't a director-enabled class but takes parameters that are director-enabled classes.

   The current "type symmetry" design will work for simple C++ inheritance, but will most likely fail for anything more complicated such as tree or diamond C++ inheritance hierarchies. Those who are interested in challenging problems are more than welcome to hack the `Java::Java_director_declaration` method in `Source/Modules/java.cxx`.

   If all else fails, you can use the downcastXXXXX() method to attempt to recover the director class's Java object pointer. For the Java Foo proxy class, the Foo director class's java object pointer can be accessed through the javaObjectFoo() method. The generated method's signature is:

         public static Foo javaObjectFoo(Foo obj);

   From your code, this method is invoked as follows:

       public class MyClassDerived {
         public void method_upcall(Foo foo_object)
         {
           FooDerived    derived = (foo_object != null ?
                        (FooDerived) Foo.downcastFoo(foo_object) : null);
           /* rest of your code here */
         }
       }

   An good approach for managing downcasting is placing a static method in each derived class that performs the downcast from the superclass, e.g.,

       public class FooDerived extends Foo {
         /* ... */
         public static FooDerived downcastFooDerived(Foo foo_object)
         {
           try {
             return foo_object != null ? (FooDerived) Foo.downcastFoo(foo_object);
           }

           catch (ClassCastException exc) {
             // Wasn't a FooDerived object, some other subclass of Foo
             return null;
           }
         }
       }

   Then change the code in MyClassDerived as follows:

       public class MyClassDerived extends MyClass {
         /* ... */
         public void method_upcall(Foo foo_object)
         {
           FooDerived derived = FooDerived.downcastFooDerived(foo_object);
           /* rest of your code here */
         }
       }

4. Why isn't the proxy class declared abstract? Why aren't the director upcall methods in the proxy class declared abstract?

   Declaring the proxy class and its methods abstract would break the JNI argument marshalling and SWIG's downcall functionality (going from Java to C++.) Create an abstract Java subclass that inherits from the director-enabled class instead. Using the previous Foo class example:

       public abstract class UserVisibleFoo extends Foo {
         /** Make sure user overrides this method, it's where the upcall
          * happens.
          */
         public abstract void method_upcall(Foo foo_object);

         /// Downcast from Foo to UserVisibleFoo
         public static UserVisibleFoo downcastUserVisibleFoo(Foo foo_object)
         {
           try {
             return foo_object != null ? (FooDerived) Foo.downcastFoo(foo_object) : null;
           } catch (ClassCastException exc) {
             // Wasn't a FooDerived object, some other subclass of Foo
             return null;
           }
         }
       }

   This doesn't prevent the user from creating subclasses derived from Foo, however, UserVisibleFoo provides the safety net that reminds the user to override the `method_upcall()` method.

## 27.11 Odds and ends

### 27.11.1 JavaDoc comments

SWIG can translate Doxygen comments in the C/C++ headers being wrapped to JavaDoc. For details of this, see the Doxygen to Javadoc section.

If you don't have Doxygen comments to translate then it's possible to add JavaDoc comments from your interface file. The `%javamethodmodifiers` feature can be used for adding proxy class method comments and module class method comments. The "javaimports" typemap can be hijacked for adding in proxy class JavaDoc comments. The `jniclassimports` or `jniclassclassmodifiers` pragmas can also be used for adding intermediary JNI class comments and likewise the `moduleimports` or `moduleclassmodifiers` pragmas for the module class. Here is an example adding in a proxy class and method comment:

    %javamethodmodifiers Barmy::lose_marbles() "
      /**
        * Calling this method will make you mad.
        * Use with <b>utmost</b> caution.
        */
      public";

    %typemap(javaimports) Barmy "
    /** The crazy class. Use as a last resort. */"

    class Barmy {
    public:
      void lose_marbles() {}
    };

Note the "public" added at the end of the `%javamethodmodifiers` as this is the default for this feature. The generated proxy class with JavaDoc comments is then as follows:

    /** The crazy class. Use as a last resort. */
    public class Barmy {
    ...
      /**
        * Calling this method will make you mad.
        * Use with <b>utmost</b> caution.
        */
      public void lose_marbles() {
        ...
      }
    ...
    }

### 27.11.2 Functional interface without proxy classes

It is possible to run SWIG in a mode that does not produce proxy classes by using the -noproxy commandline option. The interface is rather primitive when wrapping structures or classes and is accessed through function calls to the module class. All the functions in the module class are wrapped by functions with identical names as those in the intermediary JNI class.

Consider the example we looked at when examining proxy classes:

    class Foo {
    public:
      int x;
      int spam(int num, Foo* foo);
    };

When using `-noproxy`, type wrapper classes are generated instead of proxy classes. Access to all the functions and variables is through a C like set of functions where the first parameter passed is the pointer to the class, that is an instance of a type wrapper class. Here is what the module class looks like:

    public class example {
      public static void Foo_x_get(SWIGTYPE_p_Foo self, int x) {...}
      public static int Foo_x_get(SWIGTYPE_p_Foo self) {...}
      public static int Foo_spam(SWIGTYPE_p_Foo self, int num, SWIGTYPE_p_Foo foo) {...}
      public static SWIGTYPE_p_Foo new_Foo() {...}
      public static void delete_Foo(SWIGTYPE_p_Foo self) {...}
    }

This approach is not nearly as natural as using proxy classes as the functions need to be used like this:

    SWIGTYPE_p_Foo foo = example.new_Foo();
    example.Foo_x_set(foo, 10);
    int var = example.Foo_x_get(foo);
    example.Foo_spam(foo, 20, foo);
    example.delete_Foo(foo);

Unlike proxy classes, there is no attempt at tracking memory. All destructors have to be called manually for example the `delete_Foo(foo)` call above.

### 27.11.3 Using your own JNI functions

You may have some hand written JNI functions that you want to use in addition to the SWIG generated JNI functions. Adding these to your SWIG generated package is possible using the `%native` directive. If you don't want SWIG to wrap your JNI function then of course you can simply use the `%ignore` directive. However, if you want SWIG to generate just the Java code for a JNI function then use the `%native` directive. The C types for the parameters and return type must be specified in place of the JNI types and the function name must be the native method name. For example:

    %native (HandRolled) void HandRolled(int, char *);
    %{
    JNIEXPORT void JNICALL Java_packageName_moduleName_HandRolled(JNIEnv *, jclass,
                                                                  jlong, jstring);
    %}

No C JNI function will be generated and the `Java_packageName_moduleName_HandRolled` function will be accessible using the SWIG generated Java native method call in the intermediary JNI class which will look like this:

      public final static native void HandRolled(int jarg1, String jarg2);

and as usual this function is wrapped by another which for a global C function would appear in the module class:

      public static void HandRolled(int arg0, String arg1) {
        exampleJNI.HandRolled(arg0, arg1);
      }

The `packageName` and `moduleName` must of course be correct else you will get linker errors when the JVM dynamically loads the JNI function. You may have to add in some "jtype", "jstype", "javain" and "javaout" typemaps when wrapping some JNI types. Here the default typemaps work for `int` and `char *`.

Note that if you're wanting to effectively replace the JNI code generated for a C/C++ function then you'll need to use `%ignore` as well to tell SWIG not to automatically generate a JNI wrapper for it.

In summary the `%native` directive is telling SWIG to generate the Java code to access the JNI C code, but not the JNI C function itself. This directive is only really useful if you want to mix your own hand crafted JNI code and the SWIG generated code into one Java class or package.

### 27.11.4 Performance concerns and hints

If you're directly manipulating huge arrays of complex objects from Java, performance may suffer greatly when using the array functions in `arrays_java.i`. Try and minimise the expensive JNI calls to C/C++ functions, perhaps by using temporary Java variables instead of accessing the information directly from the C/C++ object.

Java classes without any finalizers generally speed up code execution as there is less for the garbage collector to do. Finalizer generation can be stopped by using an empty `javafinalize` typemap:

    %typemap(javafinalize) SWIGTYPE ""

However, you will have to be careful about memory management and make sure that you code in a call to the `delete()` member function. This method normally calls the C++ destructor or `free()` for C code.

### 27.11.5 Debugging

The generated code can be debugged using both a Java debugger and a C++ debugger using the usual debugging techniques. Breakpoints can be set in either Java or C++ code and so both can be debugged simultaneously. Most debuggers do not understand both Java and C++, with one notable exception of Sun Studio, where it is possible to step from Java code into a JNI method within one environment.

Alternatively, debugging can involve placing debug printout statements in the JNI layer using the `%exception` directive. See the special variables for %exception section. Many of the default typemaps can also be overridden and modified for adding in extra logging/debug display information.

The `-Xcheck:jni` and `-Xcheck:nabounds` Java executable options are useful for debugging to make sure the JNI code is behaving. The -verbose:jni and -verbose:gc are also useful options for monitoring code behaviour.

## 27.12 Java Examples

The directory Examples/java has a number of further examples. Take a look at these if you want to see some of the techniques described in action. The Examples/index.html file in the parent directory contains the SWIG Examples Documentation and is a useful starting point. If your SWIG installation went well Unix users should be able to type `make` in each example directory, then `java main` to see them running. For the benefit of Windows users, there are also Visual C++ project files in a couple of the Windows Examples. There are also many regression tests in the Examples/test-suite directory. Many of these have runtime tests in the java subdirectory.