JNI Tutorial (2018)

A list of steps on how to run a C/C++ function from a Java Program on MacOS Sierra. The example files for HelloWorld.java, HelloWorld.h, and HelloWorld.cpp are provided.

Steps

There are several steps to this tutorial and are as follows:

1. Creating the Java Class
2. Compiling the Java Class
3. Creating the C++ code
4. Compiling the C++ code
5. Compiling the Java Program with the Native Library
6. Using the Shell Script

Java Program

Create a new file HelloWorld.java:

class HelloWorld {
    private native void print();
    public static void main(String[] args) {
        new HelloWorld().print();
    }
    static {
        System.loadLibrary("HelloWorld");
    }
}

Compiling the Java Class

To compile the Java Class, run the command:

$ javac HelloWorld.java

To generate the header file for the C++ code, run the command:

$ javah -jni HelloWorld

This will output the following file HelloWorld.h:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloWorld */

#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloWorld
 * Method:    print
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_HelloWorld_print
    (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

C++ Program

Create a new file HelloWorld.cpp:

#include <jni.h>
#include <iostream>
#include "HelloWorld.h"
using namespace std;

JNIEXPORT void JNICALL 
Java_HelloWorld_print(JNIEnv *, jobject){
    cout << "Hello World!\n";
    return;
}

Compiling the C++ Code

Compiling the C++ Code to include the jni.h library is going to be the most difficult part of this process.

To start, run the command:

$ export JAVA_HOME="$(/usr/libexec/java_home -v 1.8)"

and then:

$ g++ -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/darwin/" -o HelloWorld.o -shared HelloWorld.cpp

This will create the file HelloWorld.o. Finally run:

$ g++ -dynamiclib -o libhelloworld.jnilib HelloWorld.o

Compiling the Java Program with the Native Library

Finally, to run the Java Program, run:

$ java HelloWorld

The ouput should be:

Hello World!

Using the Shell Script

The shell script simply runs all of these commands:

#!/bin/bash

javac Helloworld.java
javah -jni HelloWorld
export JAVA_HOME="$(/usr/libexec/java_home -v 1.8)"
g++ -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/darwin/" -o HelloWorld.o -shared HelloWorld.cpp
g++ -dynamiclib -o libhelloworld.jnilib HelloWorld.o
java HelloWorld

Running the provided shell script with the command:

$ ./build.sh

will complete all of these steps and run the HelloWorld.java program. If permissions are denied, you may have to run:

$ chmod -x build.sh

Like running the java executable, running this script will output:

Hello World!

Credits

I did not come up with this tutorial by myself. I compiled it from several other pages to work on the current version of MacOS:

• http://mrjoelkemp.com/2012/01/getting-started-with-jni-and-c-on-osx-lion/
• https://gist.github.com/DmitrySoshnikov/8b1599a5197b5469c8cc07025f600fdb