JNI WITH UBUNTU

Most of the native methods are written in "C". The mechanism to integrate "C" code with a Java program is called the Java Native Interface(JNI).

Java Client to call native method :

package org.satish.chapter_13;

public class NativeDemo {
    int i;
    public static void main(String[] args) {
        NativeDemo ob = new NativeDemo();
       
        ob.i = 10;
        System.out.println("This is ob.i before this native method : " + ob.i);
        ob.test();
        System.out.println("This is ob.i after this native method : " + ob.i);
    }
   
    public native void test();

   
    static{
        System.load("/home/satish/workspace/CompleteReference2010/bin/NativeDemo.so");
    }
}

Create headed file out of the Class :

Before to this compile the java to ".class" file. Now it is ready to create the ".h "file.

javah -jni org.satish.chapter_13.NativeDemo

Content of the header file :

/* DO NOT EDIT THIS FILE - it is machine generated */
#include
/* Header for class org_satish_chapter_13_NativeDemo */

#ifndef _Included_org_satish_chapter_13_NativeDemo
#define _Included_org_satish_chapter_13_NativeDemo
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_satish_chapter_13_NativeDemo
 * Method:    test
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_org_satish_chapter_113_NativeDemo_test
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

The C program : 

/*
    This file contains the C version of the test()
*/

#include
#include "org_satish_chapter_13_NativeDemo.h"
#include
JNIEXPORT void JNICALL Java_org_satish_chapter_113_NativeDemo_test(JNIEnv *env, jobject obj)
{
    jclass cls;
    jfieldID fid;
    jint i;
   
    printf("Starting the native method. \n");
    cls = (*env) ->GetObjectClass(env, obj);
    fid = (*env) ->GetFieldID(env, cls, "i", "I");

    if(fid == 0){
        printf("could not get field id. \n");
        return;
    }
    i = (*env)->GetIntField(env, obj, fid);
    printf("i = %d\n",i);
    (*env)->SetIntField(env,obj,fid,2*i);
    printf("Ending the native method.\n");
}

Create .SO file : 

Before to create the .so file, you need to install the gcc in your system.

sudo apt-get install gcc

Once gcc is installed, you can create the .so file using the following command.

sudo gcc -I /usr/lib/jvm/java-6-sun-1.6.0.20/include/linux/ -o NativeDemo.so -shared NativeDemo.c

Now your java class is ready to run and following is the output.

This is ob.i before this native method : 10
This is ob.i after this native method : 20
Starting the native method.
i = 10
Ending the native method.