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Trail: Java Native Interface

## Lesson: Writing a Java Program with Native Methods

> This lesson walks you through the steps necessary to integrate native code with programs written in Java.

> This lesson implements the canonical "Hello World!" program. The "Hello World!" program has one Java class, called `HelloWorld`. `HelloWorld.java` does two things: it declares a native method that displays "Hello World!" and it implements the `main` method for the overall program. The implementation for the native method is provided in C.

> > Note : This lesson assumes that you are starting with neither existing C functions nor Java classes. While "in the real world" you probably have existing C functions that you wish to integrate with Java programs, you will still need to modify the signatures for these C functions to work with the JNI. To be sure that you use the correct signatures, it is best to begin by writing and compiling the Java code, as described here.

### Background

Writing native methods for Java programs is a multi-step process.

1. Begin by writing the Java program. Create a Java class that declares the native method; this class contains the declaration or signature for the native method. It also includes a `main` method which calls the native method.
2. Compile the Java class that declares the native method and the `main` method.
3. Generate a header file for the native method using `javah` with the native interface flag `-jni`. Once you've generated the header file you have the formal signature for your native method.
4. Write the implementation of the native method in the programming language of your choice, such as C or C++.
5. Compile the header and implementation files into a shared library file.
6. Run the Java program.

The following figure illustrates these steps for the Hello World program:

### Step 1: Write the Java Code

> Create a Java class named `HelloWorld` that declares a native method. This class also includes a `main` method that creates a `HelloWorld` object and calls the native method.

### Step 2: Compile the Java Code

> Use `javac` to compile the Java code that you wrote in Step 1.

### Step 3: Create the .h File

> Use `javah` to create a JNI-style header file (a `.h` file) from the `HelloWorld` class. The header file provides a function signature for the implementation of the native method `displayHelloWorld`.

### Step 4: Write the Native Method Implementation

> Write the implementation for the native method in a native language (such as ANSI C) source file. The implementation will be a regular function that's integrated with your Java class.

### Step 5: Create a Shared Library

> Use the C compiler to compile the `.h` file and the `.c` file that you created in Steps 3 and 4 into a shared library. In Windows 95/NT terminology, a shared library is called a dynamically loadable library (DLL).

### Step 6: Run the Program

> And finally, use `java`, the Java interpreter, to run the program.

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