Step By Step

Step 4: Write the Native Method Implementation

Now, you can finally get down to the business of writing the implementation for the native method in another language.

The function that you write must have the same function signature as the one you generated with javah into the HelloWorld.h file in Step 3: Create the .h File. The function signature generated for the HelloWorld class's displayHelloWorld() native method looks like this:

JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld(JNIEnv *, jobject);

Here's the implementation for Java_HelloWorld_displayHelloWorld(), which is in the file named HelloWorldImp.c.

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

JNIEXPORT void JNICALL 
Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj) 
{
  printf("Hello world!\n");
  return;
}

The implementation for Java_HelloWorld_displayHelloWorld() is straightforward: the function uses the printf() function to display the string "Hello World!" and then returns.

This file includes three header files:

• jni.h, which provides information the native language code requires to interact with the Java runtime system. When writing native methods, you must always include this file in your native language source files.
• The .h file that you generated in Step 3: Create the .h File.
• The code snippet above also includes stdio.h because it uses the printf() function.

Step By Step