JNI Example « zhao-sun.com

April 9, 2004

JNI Example

Filed under: Computer — blogadmin @ 10:45 pm

Write the Java Code

The following Javacode segment defines a class named HelloWorld. This class declares one native method, implements a main method, and has a static code segment.

class HelloWorld {

public native void displayHelloWorld();

static {

System.loadLibrary(“hello”);

}

public static void main(String[] args) {

new HelloWorld().displayHelloWorld();

}

}
Compile the Java Code

Use the Java compiler to compile the class that you created
in the previous step. Here’s the command to use:

javac HelloWorld.java
Create the .h File

Running javah :

UNIX & LINUX

% javah -jni HelloWorld

DOS shell (Windows 95/NT)

C:\> javah -jni HelloWorld

MacOS

Drag the HelloWorld.class file onto the JavaH icon. This creates a file called HelloWorld.h in the same folder as JavaH. Move the file into your working folder.
Write the Native Method Implementation

This implementation is in the file named HelloWorldImp.c.

#include

#include “HelloWorld.h”

#include

JNIEXPORT void JNICALL

Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj)

{

printf(“Hello world!\n”);

return;

}
Create a Shared Library

UNIX

cc -G -I/usr/local/java/include -I/usr/local/java/include/solaris HelloWorldImp.c -o libhello.so

LINUX

cc -shared -I/usr/local/java/include -I/usr/local/java/include/solaris HelloWorldImp.c -o libhello.so

DOS shell (Windows 95/NT)

cl -Ic:\java\include -Ic:\java\include\win32 -LD HelloWorldImp.c -Fehello.dll

Of course, you need to specify the include path that corresponds to the setup on your own machine.
Run the Program

Now run the Java application (the HelloWorld class) with the Java interpreter, as follows:

java HelloWorld

You should see the following output:

Hello World!
PS: Set Your Library Path

UNIX

% setenv LD_LIBRARY_PATH mylibrarypath

where mylibrarypath is the name of the directory that contains libhello.so.

DOS shell (Windows 95/NT)

On Windows 95/NT, the loadLibrary()
method searches for DLLs in the same manner as other language environments do.

In c:\autoexec.bat file:

SET INCLUDE=path to find .h files

SET LIB=path to find .lib files

MacOS

The Java runtime searches the JavaSoft Folder in the Extensions folder in the System Folder for shared libraries. Create an alias to your shared library in the JavaSoft Folder.

zhao-sun.com

April 9, 2004

JNI Example

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