SWIG is a software development tool that connects programs written in C and C++ with a variety of high-level programming languages, such as Java.

• http://www.swig.org/index.html

Let's see how to interface some C functions to Java. To install swig:

apt-get install swig

Let's say you have them in a file 'example.c':

#include <time.h>
 
double My_variable = 3.0;
 
int fact(int n) {
    if (n <= 1) return 1;
    else return n*fact(n-1);
}
 
int my_mod(int x, int y) {
    return (x%y);
}
 
char *get_time() {
    time_t ltime;
    time(&ltime);
    return ctime(&ltime);
}

Now, in order to add these files to your favorite language, you need to write an “interface file” which is the input to SWIG. An interface file for these C functions might look like this:

%module example
%{
/* Put header files here or function declarations like below */
extern double My_variable;
extern int fact(int n);
extern int my_mod(int x, int y);
extern char *get_time();
%}
 
extern double My_variable;
extern int fact(int n);
extern int my_mod(int x, int y);
extern char *get_time();

To install the java compiler and the headers needed for the binding:

apt-get install sun-java5-jdk

Generate the Java bindings:

swig -java example.i
javac exampleJNI.java

To build the native library:

gcc -fpic -c example.c example_wrap.c -I/usr/lib/jvm/java-1.5.0-sun/include -I/usr/lib/jvm/java-1.5.0-sun/include/linux
gcc -shared example.o example_wrap.o -o libexample.so

Let's see an use example of the Java interface:

public class Test {
    public static void main(String argv[]) {
        System.loadLibrary("example");
        System.out.println(exampleJNI.My_variable_get());
        System.out.println(exampleJNI.fact(5));
        System.out.println(exampleJNI.get_time());
    }
}

To compile it:

javac -classpath . Test.java

To run it:

java -Djava.library.path="." Test

An example output:

3.0
120
Fri Jun 22 20:32:37 2007