// Assumption: I assume that you have already installed Java SDK on your machine and you are using Linux.

  1. Open a Terminal

  2. Open your favourite text editor, create a new file and write the following code

     1 package com.sangeethlabs;
     3 public class HelloJNI {
     4     static {
     5         System.loadLibrary("HelloJNI");
     6     }
     7     public static native void greet(String message);
     8     public static void main(String [] args) {
     9         greet("Hello World !");
    10     }
    11 }

    Save the file as HelloJNI.java under com/sangeethlabs directory.
    // NOTE: Ensure that the class name and the file name are same.

  3. Go back to your Terminal

  4. run the following command to compile the source code:

    javac com/sangeethlabs/HelloJNI.java

    This generates a class file named HelloJNI.class in the same directory.

  5. Now run the following command to generate header file for native code.

    javah com.sangeethlabs.HelloJNI

    This results in a C/C++ header file named com_sangeethlabs_HelloJNI.h in the same directory with the following contents

    /* DO NOT EDIT THIS FILE - it is machine generated */
    #include <jni.h>
    /* Header for class com_sangeethlabs_HelloJNI */
    #ifndef _Included_com_sangeethlabs_HelloJNI
    #define _Included_com_sangeethlabs_HelloJNI
    #ifdef __cplusplus
    extern "C" {
     * Class:     com_sangeethlabs_HelloJNI
     * Method:    greet
     * Signature: (Ljava/lang/String;)V
    JNIEXPORT void JNICALL Java_com_sangeethlabs_HelloJNI_greet
          (JNIEnv *, jclass, jstring);
    #ifdef __cplusplus
  6. Open your favourite text editor, create a new file and write the following code

     1 #include "com_sangeethlabs_HelloJNI.h"
     3 #include <stdio.h>
     5 JNIEXPORT void JNICALL Java_com_sangeethlabs_HelloJNI_greet
     6       (JNIEnv *env, jclass jClazz, jstring jstrText) {
     8     const char *str;
     9     str = env->GetStringUTFChars(jstrText, NULL);
    10     printf("%s\n", str);
    11     env->ReleaseStringUTFChars(jstrText, str);
    12 }
    Save the file as HelloJNI.cpp.
  7. Now compile the C++ code and create a shared object named libHelloJNI.so using the following commands

    g++ -shared -I$JAVA_HOME/include/ -I$JAVA_HOME/include/linux -fPIC -g -c HelloJNI.cpp
    g++ -shared -Wl,-soname,libHelloJNI.so -o libHelloJNI.so -lc HelloJNI.o
  8. Now run the following command to execute the Java class.

    java -cp . -Djava.library.path=. com.sangeethlabs.HelloJNI

    The output of the program will be as shown below,

    Hello World !

    // NOTE: “-Djava.library.path=.” option indicates to JVM that “libHelloJNI.so” can be located under the current directory. Another alternative is to set the environment variable LD_LIBRARY_PATH to include the path under which “libHelloJNI.so” can be located. By doing so, we can execute the Java code as shown below

    java -cp . com.sangeethlabs.HelloJNI

Now you have successfully executed your first Java program using Java Native Interface on Linux !