Category: Hacking

I am currently playing around with JNI and Java due the colleagues question to make the connect features of jack-audio (http://jackaudio.org) accessible to java.
There is already a javalib (http://jjack.berlios.de) with some features, there seems still some needes ones missing.
So i started today to have a look into SWIG (http://swig.org).
“SWIG is a software development tool that connects programs written in C and C++ with a variety of high-level programming languages.”
After some hours of research i ended up with some facts:
To created yourself a Java binding to a given c/c++ Program or Library you need one or more Interface files (*.I) and swig file with all the necessary swig module descriptions.
There is an example on the swig homepage ( http://www.swig.org/Doc1.3/SWIGDocumentation.html#Introduction) to explain the workflow of SWIG.
There is a c file exmple.c:

/* File : example.c */
double  My_variable  = 3.0;

/* Compute factorial of n */
int  fact(int n) {
    if (n <= 1) 
        return 1;
    else 
        return n*fact(n-1);
}

/* Compute n mod m */
int my_mod(int n, int m) {
    return(n % m);
}

The mapping example.i files looks as the following:

/* File : example.i */
%module example
%{
/* Put headers and other declarations here */
    extern double My_variable;
    extern int    fact(int);
    extern int    my_mod(int n, int m);
%}
extern double My_variable;
extern int    fact(int);
extern int    my_mod(int n, int m);

As you can see, the Interface file has a similar syntax with some additional meta information.
You can now create your JNI bindings:

swig -java example.i
There are also flags for different other languages:
-allegrocl - Generate ALLEGROCL wrappers
-chicken - Generate CHICKEN wrappers
-clisp - Generate CLISP wrappers
-cffi - Generate CFFI wrappers
-csharp - Generate C# wrappers
-guile - Generate Guile wrappers
-java - Generate Java wrappers
-lua - Generate Lua wrappers
-modula3 - Generate Modula 3 wrappers
-mzscheme - Generate Mzscheme wrappers
-ocaml - Generate Ocaml wrappers
-octave - Generate Octave wrappers
-perl - Generate Perl wrappers
-php - Generate PHP wrappers
-pike - Generate Pike wrappers
-python - Generate Python wrappers
-r - Generate R (aka GNU S) wrappers
-ruby - Generate Ruby wrappers
-sexp - Generate Lisp S-Expressions wrappers
-tcl - Generate Tcl wrappers
-uffi - Generate Common Lisp / UFFI wrappers
-xml - Generate XML wrappers

As a result you get three new files:

• example.java
• exampleJNI.java
• example_wrap.c

The example_wrap.c can be used to compile the needed library file for your JNI access.
The two java Files are the basic JNI implementation:

    class exampleJNI {
        public final static native void My_variable_set(double jarg1);
        public final static native double My_variable_get();
        public final static native int fact(int jarg1);
        public final static native int my_mod(int jarg1, int jarg2);
    }

And a basic java example how to access these functions:

public class example {
    public static void setMy_variable(double value) {
        exampleJNI.My_variable_set(value);
    }
    public static double getMy_variable() {
        return exampleJNI.My_variable_get();
    }
    public static int fact(int arg0) {
        return exampleJNI.fact(arg0);
    }
    public static int my_mod(int n, int m) {
        return exampleJNI.my_mod(n, m);
    }
}

To get into working with SWIG i can advise the sources of the G4Java Project.
There is also a maven plugin to use SWIG from within your maven build: http://java.freehep.org/freehep-swig-plugin.
I am currently trying to create the necessary Interface files from the jack-audio sources to use them for a first run of SWIG. For python and tck you can use cmake to create these files.