[    Java语言本身是通过Java的虚拟机解释执行的，因此对于Java中调用本地动态链接库的问题便提上了日程，为何会存在这样的需求呢？因为Java本身的机制导致一些要求高效率

在上上篇中已经介绍了如何编译native code 生成 so动态库，这里再介绍下Java中如何通过JNI接口来调用动态库中的native code方法。

通过此篇文章，我们可以认识和了解如何完整地在Android的java code中调用native code。

1 创建一个 android project, 名字叫Why

2 在工程Why中添加一个Java类，class名为Jni。这个类是一个JNI接口的Java类，文件名为Jni.java。

package com.yarin.android.Why;

public class Jni {

public native int getCInt();

public native String getCString();

}

3 将工程Why下的 "src\com\yarin\android\Why" 目录下的Jni.java文件copy到“Why\bin\classes”下.

4  Generate Jni.class file via the command below:

javac jni.java

Then copy the Jni.class file generated to cover and replace the original Jni.class file in directory “Why\bin\classes\com\yarin\android\Why”.

------The original Jni.class file, you must build the java project first to generate it.

5  Generate c style header file

javah –classpath C:\android-ndk-r6b\myproject\Why\bin\classes  com.yarin.android.Why.Jni

com.yarin.android.Why is my package name appeared in Jni.java file.

com_yarin_android_Why_Jni.h is like below:

/* DO NOT EDIT THIS FILE - it is machine generated */

#include

/* Header for class com_yarin_android_Why_Jni */

#ifndef _Included_com_yarin_android_Why_Jni

#define _Included_com_yarin_android_Why_Jni

#ifdef __cplusplus

extern "C" {

#endif

/*

* Class:     com_yarin_android_Why_Jni

* Method:    getCInt

* Signature: ()I

*/

JNIEXPORT jint JNICALL Java_com_yarin_android_Why_Jni_getCInt

(JNIEnv *env, jobject object); ---you need to supplement the parameter name yourself

/*

* Class:     com_yarin_android_Why_Jni

* Method:    getCString

* Signature: ()Ljava/lang/String;

*/

JNIEXPORT jstring JNICALL Java_com_yarin_android_Why_Jni_getCString

(JNIEnv *env, jobject object);

#ifdef __cplusplus

}

#endif

#endif

6     Add com_yarin_android_Why_Jni.c file corresponding to the above c style header file, then add implemented code.

#include

#include

#include "com_yarin_android_Why_Jni.h"

int add()

{

int x,y;

x = 111;

y = 22;

x += y;

return x;

}

JNIEXPORT jint JNICALL Java_com_yarin_android_Why_Jni_getCInt

(JNIEnv *env, jobject object)

{

return add();

}

JNIEXPORT jstring JNICALL Java_com_yarin_android_Why_Jni_getCString

(JNIEnv *env, jobject object)

{

(*env)->NewStringUTF(env, " Why is ok ^_^ ----->> ");

}

7 然后实现在工程Why下创建目录jni，并且copy com_yarin_android_Why_Jni.c /h 到jni目录下。

8 在"Why\jni"目录下编写Android.mk ，在"android-ndk-r6b\jni"下编写Application.mk , 然后在NDK环境下编译native code，生成动态库libWhy.so。这个你已经从上上篇中知道的……。

9 在java工程中加入调用native 动态库的代码：[在Android项目中 编写c++动态链接库, 通过jni调用]

package com.yarin.android.Why;

import android.app.Activity;

import android.os.Bundle;

import android.widget.TextView;

public class WhyActivity extends Activity {

static

{

System.loadLibrary("Why");

}

/** Called when the activity is first created. */

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

Jni jni = new Jni();

TextView view = new TextView(this);

view.setText(jni.getCString() + Integer.toString(jni.getCInt()));

setContentView(view);

}

}

10  编译Why工程，然后 run as Android Application, 你就能看到native code的调用结果了。

Tip:

Usage of the command javah.

javah -d -classpath

where:

'outputdir' is the directory where to put the generated header file

'classpath' contains an absolute path to the directory containing your root package (as mentionned by Glen)

'fully_qualified_class' is the name of the class containing native methods without .class extension

-jni option is not required (set by default)

[1. JNIEnv对象   对于本地函数   JNIEXPORT void JNICALL Java_video1_TestNative_sayHello(JNIEnv * env, jobject obj)   {        cout   }           JNIEnv类型代表Ja