Native methods (native keyword)

A native method is a Java method whose implementation is written in another language, typically C or C++. The native keyword indicates this.

Why use native methods?

• Access platform‑specific features (OS, hardware).
• Reuse existing legacy code.
• Performance‑critical operations (e.g., cryptography, graphics).

Syntax:

public native returnType methodName(parameters);

Example:

public class NativeDemo {
    static {
        System.loadLibrary("nativeLib"); // load shared library
    }

    public native void sayHello();
}

Steps to create a native method:

1. Write Java code with native method declaration.
2. Compile Java code: javac NativeDemo.java
3. Generate C/C++ header: javac -h . NativeDemo.java
4. Implement the native method in C/C++ using the generated header.
5. Compile the native code into a shared library (.dll on Windows, .so on Linux, .dylib on macOS).
6. Load the library using System.loadLibrary().

C implementation example (simplified):

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

JNIEXPORT void JNICALL Java_NativeDemo_sayHello(JNIEnv *env, jobject obj) {
    printf("Hello from C!\n");
}

Important notes:

• Native methods bypass Java’s security and memory management.
• They are not portable; each platform needs its own library.
• Java Native Interface (JNI) is the standard API for native methods.
• Use sparingly; prefer pure Java when possible.