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

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

Solution 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:

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.

Solution 2:

The method is implemented in "native" code. That is, code that does not run in the JVM. It's typically written in C or C++.

Native methods are usually used to interface with system calls or libraries written in other programming languages.

Solution 3:

I like to know where does we use Native Methods

Ideally, not at all. In reality some functionality is not available in Java and you have to call some C code.

The methods are implemented in C code.

Solution 4:

Java native code necessities:

h/w access and control.
use of commercial s/w and system services[h/w related].
use of legacy s/w that hasn't or cannot be ported to Java.
Using native code to perform time-critical tasks.

hope these points answers your question :)

Solution 5:

Native methods allow you to use code from other languages such as C or C++ in your java code. You use them when java doesn't provide the functionality that you need. For example, if I were writing a program to calculate some equation and create a line graph of it, I would use java, because it is the language I am best in. However, I am also proficient in C. Say in part of my program I need to calculate a really complex equation. I would use a native method for this, because I know some C++ and I know that C++ is much faster than java, so if I wrote my method in C++ it would be quicker. Also, say I want to interact with another program or device. This would also use a native method, because C++ has something called pointers, which would let me do that.