JNI Example

JNI stands for Java Native Interface, which allows you to use, inside your Java application, code written in other programming languages. I will walk you through an example (developed under Windows) of using JNI.

Here are the main steps we need to follow:
- Create a java application that declares the native method.
- Compile the program.
- Generate the header file using javah.
- Implement the native code inside a C application.
- Compile the C code and generate the native library (dll).
- Run the java program.

Let us go through each of these steps:

1. Create a java application that declares the native method.

/**
* Shows a simple example of using JNI
*/
public class JNIExample {
  //Declare native method
  private native void displayMessage();

  public static void main(String[] args) {
      //Load native library
      System.loadLibrary("JNIExample");
      //Call the native method
      new JNIExample().displayMessage();
  }
}

2. Compile the program.
javac JNIExample.java

3. Generate the header file using javah.
javah -jni JNIExample

4. Implement the native code inside a C application.

#include 
#include 
#include "JNIExample.h"

JNIEXPORT void JNICALL Java_JNIExample_displayMessage
(JNIEnv *lEnv, jobject lObj) {
  printf("This is a JNI Example!\n");
  return;
}

The method signature matches exactly the one from the JNIExample.h header file.

5. Compile the C code and generate the native library (dll).
Open a Visual Studio (or similar) command prompt. In my case, it was located under "start->Programs->Visual C++ 2005 Express Edition->Visual Studio Tools->Visual Studio 2005 Command Prompt".
Run the command from the directory where you have the JNIExample.c file:
"cl -I"c:\Program Files\Java\jdk1.5.0_08\include" -I"c:\Program Files\Java\jdk1.5.0_08\include\win32" -MD -LD JNIExample.c -FeJNIExample.dll". Your path to the include directory might be different, so make sure you adjust that according to your settings. The -LD option makes sure that the compiler generates a DLL file. The -MD option makes sure that the DLL generated is linked with the win32 multithreaded C library.

6. Run the Java program.
java JNIExample

Some problems I ran into it while testing the code:
- I was missing the msvcr80.dll file, so I downloaded from here and copied it to c:\windows\system32 directory.
- Even though a manifest file was created, I hade to embedd it inside the dll. I have used the following link to solve the problem: http://msdn.microsoft.com/en-us/library/ms235591(VS.80).aspx. Here is the command: "mt.exe -manifest JNIExample.dll.manifest -outputresource:JNIExample.dll;2"