native Java Keyword with Examples

The native keyword is applied to a method to indicate that the method is implemented in native code using JNI (Java Native Interface).

Key points about native Java Keyword

  • The native keyword may be applied to a method to indicate that the method is implemented in a language other than Java.
  • The native keyword is used to declare a method which is implemented in platform-dependent code such as C or C++. When a method is marked as native, it cannot have a body and must ends with a semicolon instead.
  • The Java Native Interface (JNI) specification governs rules and guidelines for implementing native methods, such as data type conversion between Java and the native application.

native Java Keyword Example

  1. Main.java
    Let’s create a Java Main.java class: 
public class Main {
    public native int square(int i);
    public static void main(String[] args) {
        System.loadLibrary("Main");
        System.out.println(new Main().square(2));
    }
}
  1. Main.c:
    Let’s create sample code in C language. 
#include <jni.h>
#include "Main.h"

JNIEXPORT jint JNICALL Java_Main_square(
    JNIEnv *env, jobject obj, jint i) {
  return i * i;
}
  1. Compile and run: 
sudo apt-get install build-essential openjdk-7-jdk
export JAVA_HOME='/usr/lib/jvm/java-7-openjdk-amd64'
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