Hello World ! Using Java Native Interface

// 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;
    2 
    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" {
   #endif
   /*
    * Class:     com_sangeethlabs_HelloJNI
    * Method:    greet
    * Signature: (Ljava/lang/String;)V
    */
   JNIEXPORT void JNICALL Java_com_sangeethlabs_HelloJNI_greet
         (JNIEnv *, jclass, jstring);
   
   #ifdef __cplusplus
   }
   #endif
   #endif
   

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

    1 #include "com_sangeethlabs_HelloJNI.h"
    2 
    3 #include <stdio.h>
    4 
    5 JNIEXPORT void JNICALL Java_com_sangeethlabs_HelloJNI_greet
    6       (JNIEnv *env, jclass jClazz, jstring jstrText) {
    7       
    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

   export LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH
   java -cp . com.sangeethlabs.HelloJNI
   

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