|Java| Say Hello to JNI

Although Java is easy to transfer platforms, we still need some C++/C libraries to control drivers or basic modules.

JNI can be an interface between Java and C++/C.
The following article will teach you step by step.

OS: Ubuntu 18.04
Java: Open JDK 8

Step 1. Implement Java: HelloWorldJNI.java

class HelloWorldJNI {
    private native void printByC();
    public static void main(String[] args) {
        new HelloWorldJNI().printByC();
    static {
        System.setProperty("java.library.path", "~/Desktop/JNI/");

Step 2. Generate Class and Header
(open your terminal)
$ javac HelloWorldJNI.java
$ javah -jni HelloWorldJNI

Then HelloWorldJNI.class and HelloWorldJNI.h are created.


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

#ifndef _Included_HelloWorldJNI
#define _Included_HelloWorldJNI
#ifdef __cplusplus
extern "C" {
 * Class:     HelloWorldJNI
 * Method:    printByC
 * Signature: ()V
JNIEXPORT void JNICALL Java_HelloWorldJNI_printByC
  (JNIEnv *, jobject);

#ifdef __cplusplus

Step 3. Implement your C++/C library: HelloWorldJNI.c

#include <jni.h>
#include <stdio.h>
#include "HelloWorldJNI.h"

JNIEXPORT void JNICALL Java_HelloWorldJNI_printByC (JNIEnv *env, jobject obj){

    printf("Hello from C\n");

Step 4. Build C++/C library as .so
(open your terminal)
$ gcc -shared -I/usr/lib/jvm/java-8-openjdk-amd64/include -I/usr/lib/jvm/java-8-openjdk-amd64/include/linux HelloWorldJNI.c -o libHelloWorldJNI.so

The name ,”lib“+name.so, is important because the definition of “System.loadLibrary” will recognize “lib”.

Step 5. Test
(open your terminal)
$ java HelloWorldJNI

$ Hello from C