Compiling and Running a Java Program with a Native Method

Step 3: Create the .h File

In this step, you use the javah utility program to generate a header file (a .h file) from the HelloWorld class. The header file provides a C function signature for the implementation of the native method displayHelloWorld defined in that class.

Run javah now on the HelloWorld class that you created in the previous steps.

The name of the header file is the Java class name with a .h appended to the end. For example, the command shown above will generate a file named HelloWorld.h.

By default, javah places the new .h file in the same directory as the .class file. Use the -d option to instruct javah to place the header files in a different directory.

The Function Definition

Look at the header file HelloWorld.h.

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

#ifndef _Included_HelloWorld
/*
 * Class:     HelloWorld
 * Method:    displayHelloWorld
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld
  (JNIEnv *, jobject);

The Java_HelloWorld_displayHelloWorld function provides the implementation for the HelloWorld class's native method displayHelloWorld, which you will write in Step 4: Write the Native Method Implementation. You use the same function signature when you write the implementation for the native method.

If HelloWorld contained any other native methods, their function signatures would appear here as well.

The name of the native language function that implements the native method consists of the prefix Java_, the package name, the class name, and the name of the native method. Between each name component is an underscore "_" separator. Graphically, this looks as follows:

The native method displayHelloWorld within the HelloWorld class becomes Java_HelloWorld_displayHelloWorld. No package name appears in our example because HelloWorld is in the default package, which has no name.

Notice that the implementation of the native function, as it appears in the header file, accepts two parameters even though, in its definition on the Java side, it accepts no parameters. The JNI requires every native method to have these two parameters.

The first parameter for every native method is a JNIEnv interface pointer. It is through this pointer that your native code accesses parameters and objects passed to it from the Java application. The second parameter is jobject, which references the current object itself. In a sense, you can think of the jobject parameter as the "this" variable in Java. For a native instance method, such as the displayHelloWorld method in our example, the jobject argument is a reference to the current instance of the object. For native class methods, this argument would be a reference to the method's Java class. Our example ignores both parameters. The next lesson, Integrating Java and Native Programs , describes how to access data using the JNI interface pointer env parameter. The next lesson also provides more information about jobject.

Compiling and Running a Java Program with a Native Method