Compiling and Running a Java Program with a Native Method

Thanks :http://www.iam.ubc.ca/guides/javatut99/native1.1/stepbystep/step1.html

Step 1: 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();

}

}

Declare a Native Method

You must declare all methods, whether Java methods or native methods,
within a class on the Java side. When you write a method implementation
in a language other than Java, you must include the keyword

native

as part of the method's definition within the Java class. The

native

keyword signals to the Java compiler that the function is a native
language function. It is easy to tell that the implementation for the

HelloWorld

class's

displayHelloWorld

method is written in another programming language
because the

native

keyword appears as part of its method
definition:

public native

void displayHelloWorld();

This native method declaration in your Java class provides only the method signature for

displayHelloWorld

. It provides no implementation for the method. You must provide the implementation for

displayHelloWorld

in a separate native language source file.

The method declaration for

displayHelloWorld

also indicates that the method
is a public instance method, accepts no arguments, and returns no value. For more information
about arguments to and return values from native methods see
Interacting with Java from the Native Side





.

Load the Library

You compile the native language code that implements

displayHelloWorld

into a
shared library (you will do this in Step 5: Create a Shared Library

).
The runtime system later loads the shared library into the Java class that requires it. Loading the
library into the Java class maps the implementation of the native method to its declaration.

The

HelloWorld

class uses the

System.loadLibrary

method. The

System.loadLibrary

method loads the shared library that will be created when you compile
the implementation code. Place this method within a static initializer.
The argument to

System.loadLibrary

is the shared library
name. This can be any name that you choose. The system uses a standard,
but platform-specific, approach to convert the library name to a native
library name. For example, the Solaris system converts the library name
"hello" to

libhello.so

, while a Win32 system converts the same name to

hello.dll

.

The following static initializer from the

HelloWorld

class loads the
appropriate library, named

hello

. The runtime system executes a class's static initializer when it loads the class.

static {

System.loadLibrary("hello");

}

Write the Main Method

The

HelloWorld

class, because it is an application, also includes a

main

method to instantiate the class and call the native method. The

main

method instantiates

HelloWorld

and
calls the

displayHelloWorld

native method.

public static void main(String[] args) {

new HelloWorld().displayHelloWorld();

}

You can see from the code sample that you call a native method in
the same manner as you call a regular method: just append the name
of the method to the end of the object name, separated with a period ('.'). A matched
set of parentheses, (), follow the method name and enclose any arguments
to pass into the method. The

displayHelloWorld

method doesn't
take any arguments.