---
tags: Java,JNI
---
# JAVA JNI How To Use
## Introduce
JNI is an interface that allows java to interact with code wrriten in another language.
### JNI Componets
**javah** is a JDK tool that generate C-style header file from a given Java class that include native methods.
**jni.h** is a c/c++ header file included with the JDK that maps JAVA types to their native counterparts.

boolean    jboolean
byte    jbyte
char    jchar
double    jdouble
float    jfloat
int    jint
long    jlong
short    jshort
void    void

### JNI Development - Java Part
1. Create a Java class with native method
`public native void sayHi();`
2. Load the library which implements the method:
System.loadLibrary("Hello");
3. Invoke the native method from Java

Code.

public class Hello
{
	public native void sayHi();
	
	public static void main(String []args)
	{
		new Hello().sayHi();
	}
	
	static{
		System.loadLibrary("Hello");
	}
}

The method *sayHi* will be implemented in C/C++ in separate files, which will be compiled into a library.
The library filename will be called *libHello.so* (on Unix) or *Hello.dll* (on Windows), but when loaded in Java, the library has to be loaded as *Hello*

### JNI Development - C Part
1. Use the JDK javah utility to generate the header file *Hello.h* with a function prototype for *sayHi* method.

$ javac Hello.java
$ javah -jni Hello
$ ls
Hello.class  Hello.h  Hello.java

2. Create *Hello.c* to implement the *sayHi* function

The file *Hello.h* looks like
Code.

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

#ifndef _Included_Hello
#define _Included_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Hello
 * Method:    sayHi
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_Hello_sayHi
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

The file *Hello.c* look like:
Code.

#include <stdio.h>
#include "Hello.h"

JNIEXPORT void JNICALL Java_Hello_sayHi
  (JNIEnv *env, jobject object)
{
	printf("Hi\n");
}

### Compiling
Compiling *Hello.c* to generate libHello.so

$ gcc -o libHello.so -fPIC -lc -shared \
-I/usr/lib/jvm/java-8-openjdk-amd64/include \
-I/usr/lib/jvm/java-8-openjdk-amd64/include/linux \
Hello.c

Set *LD_LIBRARY_PATH* to point to directory where the compiled library is stored.
Run your Java application.
Code.

$ export LD_LIBRARY_PATH=.
$ java Hello
Hi