This is a simple JNI example on Windows.
Tested with: MingW, Windows XP, Java 6
Create a java file Hello.java
public class Hello {
static {
System.loadLibrary("hello");
}
public native void sayHello();
public static void main(String args[]) {
Hello hello=new Hello();
hello.sayHello();
}
}
Compile the file and get Hello.class file
Execute the command. This will find native methods in the class and create appropriate declaration in C language in Header file. This will generate Hello.h file
javah -jni Hello
Hello.h file looks like this
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Hello */
#ifndef _Included_Hello
#define _Included_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: Hello
* Method: sayHello
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_Hello_sayHello(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
Now implement the header file's declared method. For that we have to create a C file.
Hello.c
#include <jni.h>
#include <stdio.h>
#include <conio.h>
#include "Hello.h"
JNIEXPORT void JNICALL Java_Hello_sayHello(JNIEnv * env, jobject jobj)
{
printf("Hello World");
}
We have implemented method in C which is declared as native in Java.
We will compile the C file and generate dll which will be used by Hello.class, see the line System.loadLibrary("hello");
Running the command will generate hello.dll file
gcc -Wl,--add-stdcall-alias -I <JAVA_HOME>/include -I <JAVA_HOME>/include/win32 Hello.c -shared -o hello.dll
"-Wl" is an option with which we can pass linker options to gcc separating by commas.
"--add-stdcall-alias" is a linker option.
The option prevents linker error like: "java.lang.UnsatisfiedLinkError: Hello.sayHello()V".
"-I" will search for included files in specified directories.
"-shared" will generate shared library.
"-o" specifies name of output file.
Now simply run the class file.
java Hello
You might get "Hello World" as output.
