What are native methods in Java and where should they be used?

Question

A native method has the same syntax as an abstract method, but where is it implemented?

Answer 1

What are native methods in Java and where should they be used?

Once you see a small example, it becomes clear:

Main.java:

public class Main {
    public native int intMethod(int i);
    public static void main(String[] args) {
        System.loadLibrary("Main");
        System.out.println(new Main().intMethod(2));
    }
}

Main.c:

#include <jni.h>
#include "Main.h"

JNIEXPORT jint JNICALL Java_Main_intMethod(
    JNIEnv *env, jobject obj, jint i) {
  return i * i;
}

Compile and run:

javac Main.java
javah -jni Main
gcc -shared -fpic -o libMain.so -I${JAVA_HOME}/include \
  -I${JAVA_HOME}/include/linux Main.c
java -Djava.library.path=. Main

Output:

4

Tested on Ubuntu 14.04 with Oracle JDK 1.8.0_45.

So it is clear that it allows you to:

• call a compiled dynamically loaded library (here written in C) with arbitrary assembly code from Java
• and get results back into Java

This could be used to:

• write faster code on a critical section with better CPU assembly instructions (not CPU portable)
• make direct system calls (not OS portable)

with the tradeoff of lower portability.

It is also possible for you to call Java from C, but you must first create a JVM in C: How to call Java functions from C++?

Example on GitHub for you to play with.