Hello JNI

http://marakana.com/forums/android/examples/49.html
I was working on created java native interface for one of my Android app. Its old during Android 2.1 days. Today after couple of years, I tried to create interface again, in my Ubuntu host system, and got some issues. I though better to keep one hello world post for JNI.

Create a project with name of your choice and copy this minimal code in class, lets say in callnative.java


package com.example.ih;

public class callnative {

static {
System.loadLibrary("my_lib");  //to load library
}

public native int add(int x, int y); //calling native function

public native String hello();

}

Go to /workspace/ih/bin path and give below command,

$javah -jni -classpath /home/abhishekkd/workspace/ih/bin/classes com.example.ih.callnative

$cat com_example_ih_callnative.h

You will see a heard file generated something like,


/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_ih_callnative */

#ifndef _Included_com_example_ih_callnative
#define _Included_com_example_ih_callnative
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_ih_callnative
 * Method:    add
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_example_ih_callnative_add
  (JNIEnv *, jobject, jint, jint);

/*
 * Class:     com_example_ih_callnative
 * Method:    hello
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_ih_callnative_hello
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

Now, implement declared functions,

$gedit hello_lib.c

#include "com_example_ih_callnative.h"

JNIEXPORT jstring JNICALL Java_com_marakana_NativeLib_hello

  (JNIEnv * env, jobject obj) {

return (*env)->NewStringUTF(env, "Hello World!");

}

JNIEXPORT jint JNICALL Java_com_marakana_NativeLib_add

  (JNIEnv * env, jobject obj, jint value1, jint value2) {

return (value1 + value2);

}