Android JNI知识简介

public class MediaPlayer{
	static {
		System.loadLibrary("media_jni");
	}
}

这要求VM去载入Android的/system/lib/libmedia_jni.so档案。载入*.so之后，Java类与*.so档案就汇合起来，一起执行了。

if (register_android_media_MediaPlayer(env) <0) {
	LOGE("ERROR: MediaPlayer native registration failed\n");
	goto bail;
}

就将此组件提供的各个本地函数(Native Function)登记到VM里，以便能加快后续呼叫本地函数的效率。

jint JNI_OnLoad(JavaVM* vm, void* reserved){
	JNIEnv* env = NULL;
	jint result =-1;
	if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
		LOGE("ERROR: GetEnv failed\n");
		goto bail;
	}
}

由 于VM通常是多执行绪(Multi-threading)的执行环境。每一个执行绪在呼叫JNI_OnLoad()时，所传递进来的JNIEnv指标值都 是不同的。为了配合这种多执行绪的环境，C组件开发者在撰写本地函数时，可藉由JNIEnv指标值之不同而避免执行绪的资料冲突问题，才能确保所写的本地 函数能安全地在Android的多执行绪VM里安全地执行。基于这个理由，当在呼叫C组件的函数时，都会将JNIEnv指标值传递给它，如下：

jint JNI_OnLoad(JavaVM* vm, void* reserved){
	JNIEnv* env = NULL;
	if (register_android_media_MediaPlayer(env) <0) {
	}
}

这 JNI_OnLoad()呼叫register_android_media_MediaPlayer(env)函数时，就将env指标值传递过去。如 此，在register_android_media_MediaPlayer()函数就能藉由该指标值而区别不同的执行绪，以便化解资料冲突的问题。

//#define LOG_NDEBUG 0
#define LOG_TAG "MediaPlayer-JNI"

static JNINativeMethod gMethods[] = {
		{"setDataSource","(Ljava/lang/String;)V",
		(void*)android_media_MediaPlayer_setDataSource},
		{"setDataSource","(Ljava/io/FileDescriptor;JJ)V",
		(void*)android_media_MediaPlayer_setDataSourceFD},
		{"prepare","()V",(void*)android_media_MediaPlayer_prepare},
		{"prepareAsync","()V",(void*)android_media_MediaPlayer_prepareAsync},
		{"_start","()V",(void*)android_media_MediaPlayer_start},
		{"_stop","()V",(void*)android_media_MediaPlayer_stop},
		{"getVideoWidth","()I",(void*)android_media_MediaPlayer_getVideoWidth},
		{"getVideoHeight","()I",(void*)android_media_MediaPlayer_getVideoHeight},
		{"seekTo","(I)V",(void*)android_media_MediaPlayer_seekTo},
		{"_pause","()V",(void*)android_media_MediaPlayer_pause},
		{"isPlaying","()Z",(void*)android_media_MediaPlayer_isPlaying},
		{"getCurrentPosition","()I",(void*)android_media_MediaPlayer_getCurrentPosition},
		{"getDuration","()I",(void*)android_media_MediaPlayer_getDuration},	
		{"_release","()V",(void*)android_media_MediaPlayer_release},	
		{"_reset","()V",(void*)android_media_MediaPlayer_reset},	
		{"setAudioStreamType","(I)V",(void*)android_media_MediaPlayer_setAudioStreamType},	
		{"setLooping","(Z)V",(void*)android_media_MediaPlayer_setLooping},	
		{"setVolume","(FF)V",(void*)android_media_MediaPlayer_setVolume},	
		{"getFrameAt","(I)Landroid/graphics/Bitmap;",	
		(void*)android_media_MediaPlayer_getFrameAt},	
		{"native_setup","(Ljava/lang/Object;)V",	
		(void*)android_media_MediaPlayer_native_setup},	
		{"native_finalize","()V",(void*)android_media_MediaPlayer_native_finalize},	
	};

static int register_android_media_MediaPlayer(JNIEnv *env){
	return AndroidRuntime::registerNativeMethods(env,"android/media/MediaPlayer", gMethods, NELEM(gMethods));
}

jint JNI_OnLoad(JavaVM* vm, void* reserved){
		if (register_android_media_MediaPlayer(env) <0) {	
		LOGE("ERROR: MediaPlayer native registration failed\n");	
		goto bail;
	}

}

当 VM载入libmedia_jni.so档案时，就呼叫JNI_OnLoad()函数。接着，JNI_OnLoad()呼叫 register_android_media_MediaPlayer()函数。此时，就呼叫到 AndroidRuntime::registerNativeMethods()函数，向VM(即AndroidRuntime)登记 gMethods[]表格所含的本地函数了。简而言之，registerNativeMethods()函数的用途有二：

typedef struct {
	constchar* name;            /*Java中函数的名字*/
	constchar* signature;      /*描述了函数的参数和返回值*/
	void* fnPtr;               /*函数指针，指向C函数*/
} JNINativeMethod;

package com.hello.jnitest;import android.app.Activity;
import android.os.Bundle;

publicclass jnitest extends Activity {
	/** Called when the activity is first created. */	
	@Override

	publicvoid onCreate(Bundle savedInstanceState) {		
		super.onCreate(savedInstanceState);		
		setContentView(R.layout.main);		
		Nadd cal =new Nadd();		
		setTitle("The Native Add Result is "+ String.valueOf(cal.nadd(10, 19)));	
	}
}

Nadd.java文件：

package com.hello.jnitest;

publicclass Nadd {

	static {	
		System.loadLibrary("Nadd");
	}

	public native int nadd(int a, int b);
}

2.使用javah命令生成C/C++的.h文件。注意类要包含包名，路径文件夹下要包含所有包中的类，否则会报找不到类的错误。classpath参数指定到包名前一级文件夹，文件夹层次结构要符合java类的组织层次结构。

# Copyright (C) 2009 The Android Open Source Project#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := myjni
LOCAL_SRC_FILES := myjni.c
include $(BUILD_SHARED_LIBRARY)

myjni.c文件：

#include <string.h>
#include <jni.h>

jstring Java_com_hello_NdkTest_NdkTest_stringFromJNI( JNIEnv* env,jobject thiz )
{
	return (*env)->NewStringUTF(env, "Hello from My-JNI !");
}

package com.hello.NdkTest;
import android.app.Activity;

import android.os.Bundle;
import android.widget.TextView;

public class NdkTest extends Activity {

	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {	
		super.onCreate(savedInstanceState);		
		TextView  tv =new TextView(this);	
		tv.setText( stringFromJNI() );	
		setContentView(tv);
	}

	public native String  stringFromJNI();
		static {	
			System.loadLibrary("myjni");	
		}
	}
}