Tuesday, November 06, 2007

Technical Post # 1 : Making a Basic JNI Call

For many reasons we need to make JNI calls. Here I give an example of making a simple JNI call. Follow the steps

Step 1: Create a Java File that declares the native method (HelloWorld.java)

class HelloWorld
private native void print();

public static void main(String[] args)
new HelloWorld().print();


Step 2: Compile the class using default java compiler javac, resulting a class file named HelloWorld.class

javac HelloWorld.java

Step 3: Use javah -jni to generate a C header file (HelloWorld.h). This header file will contain the prototype declaration for the native method to be implemented in C++.

javah -jni HelloWorld

Step 4: Write the C implementation (HelloWorld.c) of the native method.

#include "HelloWorld.h"

Java_HelloWorld_print(JNIEnv *env, jobject obj)
printf("Hello World!\n");

Step 5: Compile the C implementation into a native library, creating HelloWorld.dll
or libHelloWorld.so. Use the C compiler and linker available on the host

In Microsoft Visual Studio it can be easily done if we start a MFC dll project and then just include the header and implementation files.

Step 7: Put the dll into the Java project path and run the program.

Reference: java.sun.com/docs/books/jni/download/jni.pdf


Mahmudul Hasan said...

Well, Shwakkhar bhai, you missed one point. You need to add jni header files in your include directory from %JDK_HOME%\include and %JDK_HOME%\include\win32.

It must be configured in the project properties otherwise it does not build the solution. (I am using Microsoft Visual Studio 2003).

Mahmudul Hasan

Swakkhar Shatabda said...

Oops! vul hoye gese! ...