Using Java Native Interface on Ubuntu 10.04

從無到有: to achieve Java Native Interface(JNI) on Ubuntu 10.04

• How to install Java SDK 1.6 on Ubuntu (manual)
1. Download the Java JDK 1.6 from Sun's site.
http://java.sun.com/javase/downloads/index.jsp 
2. Select the Java SE Development Kit (JDK)
3. This leads to a page allowing us to select the Operating System, usually we just choose "Linux".
4. Then we came to a page which shows the available packages. I click on "jdk-6u21-linux-i586.bin" because this version is a package resembling to a tar.gz
5. Once the newly downloaded file is here, we need to extract it to a folder named "jdk1.6.0_21" (for example).
6. It's better then to move  Jdk1.6.0_21 folder to "/usr/local/" which is the standard folder for programs or GUI console.
7. We will proceed to add the JAVA_HOME variable to the system. To do this, open bash.bashrc file with the following command:

$sudo vi /etc/bash.bashrc

At the end of the file put the following instructions.

export JAVA_HOME=/usr/local/jdk1.6.0_21

export PATH=$JAVA_HOME/bin:$PATH

This makes sure that the JAVA_HOME variable is always available in the system.
8. Open a new console and run the command:

$java -version

To which we assume the following result:

java version "1.6.0_21"
Java(TM) SE Runtime Environment (build 1.6.0_21-b06)
Java HotSpot(TM) Server VM (build 17.0-b16, mixed mode)

• Achieve a simple JNI implementation on Ubuntu (Java call C/C++)
Example: To Pass a string from Java to native method (C++) and prints personalized Hello World.
1. HelloWorlo.java: main function to call native method

class HelloWorld {
    public native void sayHi(String name);
    static
    {
        System.loadLibrary("HelloWorld");
    }
    public static void main(String[] args)
    {
        HelloWorld h = new HelloWorld();
        h.sayHi("Ryan Cho");
    }
}

We announce our intention to use native method "sayHi" from "HelloWorld" library -- System.loadLibrary("HelloWorld").
2. Compile this class and generate class file "HelloWorld.class":

javac HelloWorld.java

3. Generate the header for native method:

javah -jni HelloWorld

The header file for native method "HelloWorld.h" is generated.
HelloWorld.h: The header file for native method (automatic generated)

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

#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloWorld
 * Method:    sayHi
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_HelloWorld_sayHi
  (JNIEnv *, jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

4. Write implementation C++ file (HelloWorld.cpp):
HelloWorld.cpp: The implementation file for native method.

#include <jni.h>
#include <iostream>
#include "HelloWorld.h"

using namespace std;

JNIEXPORT void JNICALL Java_HelloWorld_sayHi
  (JNIEnv *env, jobject o, jstring s)
{
    const char *str = env->GetStringUTFChars(s, 0);
    cout << "Hello! " << str << endl;
    env->ReleaseStringUTFChars(s, str);
}

5. There is no garbage collector in C++ world, so we need to do some cleanup. We save this as HelloWorld.cpp and compile it to generate shared library:

g++ -shared -I/usr/local/jdk1.6.0_21/include -I/usr/local/jdk1.6.0_21/include/linux HelloWorld.cpp -o libHelloWorld.so

!Attention: The path to your java include directory may be different.
6. Run this example as following:

java -Djava.library.path=. HelloWorld

The result like this:

Hello! Ryan Cho