Android之Media播放器源码分析(framework——native)

一、概述：      

      声明：下面我们看的是Android 4.0版本下的Media播放器的框架，我们首先从一个简单的media播放器apk源码入手，从Java->JNI->C/C++一步步研究Android是如何通过Java一个MediaPlayer实现解码到屏幕的输出。

     通常在Android中播放视频用到的是MediaPlayer类，展示视频使用的是SurfaceView控件。

二、apk实现：

     我们首先在main.xml布局文件中添加用于视频画面绘制的SurfaceView控件：

<SurfaceView android:layout_width="fill_parent"android:layout_height="240dip"android:id="@+id/surfaceView"/>

     例如我们编写一个简单的视频播放器调用的常用方法如下。       

SurfaceView surfaceView = (SurfaceView)this.findViewById(R.id.surfaceView);  
surfaceView.getHolder().setFixedSize(720, 576);  //设置分辨率   
/*下面设置Surface不维护自己的缓冲区，而是等待屏幕的渲染引擎将内容推送到用户面前*/  
surfaceView.getHolder().setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);  
/* new 一个播放器 mediaPlayer */
MediaPlayer mediaPlayer = new MediaPlayer();  
mediaPlayer.reset();	//重置为初始状态   
mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);  
/* 设置Video影片以SurfaceHolder播放 */  
mediaPlayer.setDisplay(surfaceView.getHolder());  
mediaPlayer.setDataSource("/mnt/sdcard/test.ts");  
mediaPlayer.prepare();		//缓冲    
mediaPlayer.start();		//播放   

mediaPlayer.pause();		//暂停播放   
mediaPlayer.start();		//恢复播放   
mediaPlayer.stop();			//停止播放   
mediaPlayer.release();		//释放资源 

三、源码分析       

       我们首先来看MediaPlayer类，Java层的MediaPlayer.java位于frameworks/base/media/java/android/media/目录下：

public class MediaPlayer
{
	...
	static {
		System.loadLibrary("media_jni");
		native_init();
	}
	
	private int mNativeContext;
	private int mNativeSurfaceTexture;
	private int mListenerContext;
	
	private SurfaceHolder mSurfaceHolder;
	private EventHandler mEventHandler;
	
	
	public MediaPlayer() {
		Looper loop;
		if((loop = Looper.myLooper()) != null) {
			mEventHandler = new EventHandler(this, looper);
		} else if((looper = Looper.getMainLooper()) != null) {
			mEventHandler = new EventHandler(this. looper);
		} else {
			mEventHandler = null;
		}
		native_setup(new WeakReference<MediaPlayer>(this));
	}
	....
}

(一) static代码块

      首先会加载libmedia_jni.so库，调用native_init()方法，对应JNI接口为

android_media_MediaPlayer_native_init(JNIEnv* env)
{
	jclass clazz;
	
	class = env->FindClass("android/media/MediaPlayer");
	
	fields.context = env->GetFieldID(clazz, "mNativeContext");	// Java类中保存JNI层的mediaplayer对象
	
	/* JNI 事件通知Java，static 函数 */
	fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative", "(Ljava/lang/Object;IIILjava/lang/Object;)V");

	fields.surface_texture = env->GetFieldID(clazz, "mNativeSurfaceTexture", "I");
	
	jclass surface = env->FindClass("android/view/Surface");
	
	fields.bitmapClazz = env->FindClass("android/graphics/Bitmap");
	
	fields.bitmapContstructor = env->GetMethodID(fields.bitmapClazz, "<init>", "(I[BZ[BI)V");	// 找到Bitmap的构造函数
}

(二) 构造方法 MediaPlayer

       这里在MediaPlayer构造函数中会new 一个 EventHandler，其中EventHandler是MediaPlayer的一个内部类，继承于Handler。用于处理各种消息：MEDIA_PREPARED、MEDIA_PLAYBACK_COMPLETE、MEDIA_BUFFERING_UPDATE、MEDIA_SEEK_COMPLETE、MEDIA_SET_VIDEO_SIZE、MEDIA_ERROR、MEDIA_INFO、MEDIA_TIMED_TEXT、MEDIA_NOP等消息，对此分别调用接口OnPreparedListener的onPrepared()、OnCompletionListener的onCompletion()、OnBufferingUpdateListener的onBufferingUpdate()。。。等方法来处理，而这些方法我们都可以通过实现相应的接口来处理。

      重点还是native_setup(new WeakReference<MediaPlayer>(this))这句话，调用的JNI方法：

android_media_MediaPlayer_setup(JNIEnv* env, JObject thiz, jobject weak_this)
{
	/* 这里参数中：thiz代码Java层的MediaPlayer对象，weak_this表示对Java层MediaPlayer对象的弱引用*/
	
	/* 这里首先在JNI层 new 一个 MediaPlayer对象 */
	sp<MediaPlayer> mp = new MediaPlayer();
	
	// Create new listener and give it to MediaPlayer
	sp<JNIMediaPlayerListener> listener = new JNIMediaPlayerListener(env, thiz, weak_this);
	mp->setListener(listener);
	
	setMediaPlayer(env, thiz, mp);	// Stow out new C++ MediaPlayer
}

class JNIMediaPlayerListener : public MediaPlayerListener
{
public:
	JNIMediaPlayerListener(JNIEnv* env, jobject thiz, jobject weak_thiz);
	~JNIMediaPlayerListener();
	virtual void notify(int msg, int ext1, int ext2, const Parcel *obj = NULL);
private:
	JNIMediaPlayerListener();
	jclass mClass;		//	对 MediaPlayer类的引用
	jobject mObject; 	// 对Java层MediaPlayer对象的弱引用
}

void JNIMediaPlayerListener::notify(int msg, int ext1, int ext2, const Parcel *obj = NULL)
{
	JNIEnv *env = AndroidRuntime::getJNIEnvt();
	if(obj && obj->dataSize() > 0)
	{
		jbyteArray jArray = env->NewByteArray(obj->dataSize());
		jbyte *nArray = env->GetByteArrayElements(jArray, NULL);
		memcpy(nArray, obj->data(), obj->dataSize());
		env->ReleaseByteArrayElements(jArray, nArray, 0);
		env->CallStaticVoidMethod(mClass, fields.post_event, mObject, msg, ext1, ext2, jArray);
		env->DeleteLocalRef(jArray);
	}
	else
		env->CallStaticVoidMethod(mClass, fields.post_event, mObject, msg, ext1, ext2, NULL);
}

private static void postEventFromNative(Object mediaplayer_ref, int what, int arg1, int arg2, Object obj)
{
	MediaPlayer mp = (MediaPlayer)((WeakReference)mediaplayer_ref).get();
	Message m = mp.mEventHandler.obtainMessage(what, arg1, arg2, obj);
	mp.mEventHandler.sendMessage(m);
}
最后是通过EventHandler来处理。

现在我们看看构造JNI层的MediaPlayer类：定义在frameworks/base/include/media/mediaplayer.h中

class MediaPlayer : public BnMediaPlayerClient, public virtual IMediaDeathNotifier
{
public:
	MediaPlayer();
	~MediaPlayer();
	
	void died();
	void disconnect();
	
	status_t setDataSource(const char* url, const KeyedVector<String8, String8> *headers);
	...
	status_t setVideoSurfaceTexture(const sp<ISurfaceTexture>& surfaceTexture);
	status_t setListener(const sp<MediaPlayerListener>& listener);
	status_t prepare();
	status_t start();
	status_t stop();
	status_t pause();
	...
	void notify(int msg, int ext1, int ext2, const Parcel &obj = NULL);
static sp<IMemory> decode(const char* url, uint32_t *pSampleRate, int *pNumChannels, int *pFormat);
	status_t setAudioSessionID(int sessionId);
	....
private:
	sp<IMediaPlayer> mPlayer;	// 对应着MediaPlayerService内部类Client在客户端的代理，相当于BpMediaPlayer
	thread_id_t mLockThreadId;
	sp<MediaPlayerListener> mListener;
	....
};

 new 完MediaPlayer对象之后，设置其监听变量mListener = listener，然后将其保存到Java层对象中。

static sp<MediaPlayer> setMediaPlayer(JNIEnv* env, jobject thiz, const sp<MediaPlayer>& player)
{
	Mutex::Autolock l(sLock);
	sp<MediaPlayer> old = (MediaPlayer*)env>GetIntField(thiz, fields.context);
	if(player.get())
		player->incStrong(thiz);	// 增加player对象强引用计数
	if(old != 0)
		old->decStrong(thiz);		// 对原来的Java层保存的JNI层MediaPlayer对象减少强引用计数
	// 将新的player对象保存到Java层的thiz对象的fields.context对应的变量mNativeContext中
	env->SetIntField(thiz, fields.context, (int)player.get());
	return old;
}

（三）设置播放器参数

static void android_media_MediaPlayer_reset(JNIEnv *env, jobject thiz)
{
	sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
	process_media_player_call(env, thiz, mp->reset(), NULL, NULL);
}

public vodi setDisplay(SurfaceHolder sh)
{
	mSurfaceHolder = sh;
	Surface surface;
	surface = sh.getSurface();
	_setVideoSurface(surface);		// 调用JNI函数
	updateSurfaceScreenOn();
}

 这里的SurfaceHolder实际是一个接口，需要继承类去实现，定义frameworks/base/core/java/android/view/SurfaceHolder.java

      我们看到注释介绍：Abstract interface to someone holding a display surface. Allows you to control the surface size and format, edit the pixels in the surface, and monitor changes to the surface.

         原来是一个抽象接口类，里面封装了Surface，主要用于对Surface的控制操作如：改变大小、格式、像素等。

_setVideoSurface()在JNI层实现为：

static void setVideoSurface(JNIEnv* env, jobject thiz, jobject jsurface, jboolean mediaPlayerMustBeAlive = true)
{
	sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
	decVideoSurfaceRef(env, thiz);	// 减少Java类中mNativeSurfaceTexture保存的JNI层对之前ISurfaceTexture对象的弱引用

	// Surface 是Android中比较复杂的一个模块，我们以后再分析，现在只需要知道是一块显示区域就行了。
	sp<ISurfaceTexture> new_st;
	sp<Surface> surface(Surface_getSurface(env, jsurface));
	new_st = surface->getSurfaceTexture();
	new_st->incStrong(thiz);
	
	env->SetIntField(thiz, fields.surface_texture, (int)new_st.get());	// 重新设置到Java类中mNativeSurfaceTexture保存的JNI层对象的引用
	mp->setVideoSurfaceTexture(new_st);
	
}

 （四） 设置播放源setDataSource

setDataSource()设置播放源，对应的JNI函数为：
static void android_media_MediaPlayer_setDataSource(JNIEnv *env, jobject thiz, jstring path)
{
	android_media_MediaPlayer_setDataSourceAndHeaders(env, thiz, path, NULL, NULL);
}

调用的是mp.setDataSource(path, NULL);

status_t MediaPlayer::setDataSource(const char* url, const KeyedVector<String8, String8>* headers)
{
	const sp<IMediaPlayerService>& service(getMediaPlayerService());
	sp<IMediaPlayer> player(service->create(getpid(), this, mAudioSessionId));
	player->setDataSource(url, headers);
	attachNewPlayer(player);
}

这里首先 MediaPlayer的基类IMediaDeathNotifier的static函数getMediaPlayerService()函数获取MediaPlayerService的代理BpMediaPlayerService，然后通过Binder通信调用IMediaPlayerService的create()函数，传入的参数分别为当前线程的PID，当前MediaPlayer对象和前面获取的mAudioSessionId，返回一个IMediaPlayer对象实际类型为MediaPlayerService内部类型Client。

sp<IMediaPlayerService> IMediaDeathNotifier::sMediaPlayerService;
const sp<IMediaPlayerService>& IMediaDeathNotifier::getMediaPlayerService()
{
	if(sMediaPlayerService.get() == 0) 
	{
		sp<IServiceManager> sm = defaultServiceManager();
		sp<IBinder> binder;
		do {
			binder = sm->getService(String16("media.player"));
			if(binder != 0)
				break;
			usleep(500000);	// 0.5 s
		} while(true);
		
		if(sDeathNotifier == NULL)
			sDeathNotifier = new DeathNotifier();
	}
	binder->linkToDeath(sDeathNotifier);
	sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
	return sMediaPlayerService;
}

这里调用的service->create(getpid(), this, mAudioSessionId); 是通过Binder通信调用到MediaPlayerService::create()。

@frameworks/base/media/libmediaplayerservice/MediaPlayerService.cpp

class MediaPlayerService : public BnMediaPlayerService
{
	class Client;
	
	class AudioOutput : public MediaPlayerBase::AudioSink
	{
	public:
		AudioOutput(int sessionId);
		virtual ~AutioOutput();
		
		virtual status_t open(uint32_t sampleRate, int channelCount, int format, int bufferCount, AudioCallback cb, void *cookie);
		virtual void start();
		virtual ssize_t write(const void* buffer, size_t size);
		...
	private:
		AudioTrack* mTrack;
		AudioCallback mCallback;
		...
	};
	class AudioCache : public MediaPlayserBase::AudioSink
	{
		...
	};
public:
	static void instantiate();	// start the MediaPlayerService
	virtual sp<IMediaPlayer> create(pid_t pid, cosnt sp<IMediaPlayerClient>& client, int audioSessionId);
	virtual sp<IOMX> getOMX();
	...
private:
	virtual MediaPlayerService();
	virtual ~MediaPlayerService();
	SortedVector< wp<Client> > mClient;
	SortedVector< wp<MediaRecorderClient> > mMediaRecorderClients;
	sp<IOMX> mOMX;

	// 有个内部类 Client
	class Client : public BnMediaPlayer 
	{
		public:
			virtual status_t setVideoSurfaceTexture(const sp<ISurfaceTexture>& surfaceTexture);
			virtual status_t prepareAsync();
			virtual status_t start();
			...
			sp<MediaPlayerBase> createPlayer(player_type playerType);
			virtual status_t setDataSource(const char* url, ...);
			static void notify(void* cookie, int msg, int ext1, int ext2, const Parcel *obj);
		private:
			friend class MediaPlayerService;
			Client(const sp<MediaPlayerService>& service, pid_t pid, int32_t connId, const sp<IMediaPlayerClient>& client, 
				int audioSessionId, uid_t uid);
			sp<MediaPlayerBase> mPlayer;
			sp<MediaPlayerService> mService;
			sp<IMediaPlayerClient> mClient;
		
	};
};

这里看下MediaPlayerService的create函数：

sp<IMediaPlayer> create(pid_t pid, cosnt sp<IMediaPlayerClient>& client, int audioSessionId)
{
	int32_t connId = android_atomic_inc(&mNextConnId);
	
	// 穿入参数分别为：当前MediaPlayerService对象、客户端进程pid、客户端MediaPlayer对象的引用、audioSessionId等
	sp<Client> c = new Client(this, pid, client, audioSessionId, IPCThreadState::self()->getCallingUid());
	
	wp<Client> w = c; // Client的构造函数就是一些简单的给变量赋值操作了
	mClients.add(w);
	return c;
}

        分析到这里我们在Java中调用setDataSource()的时候底层MediaPlayer类首先通过Binder获取MediaPlayerService的代理BpMediaPlayerService，然后调用其create函数，由MediaPlayerService来处理，返回一个匿名Binder对象Client，返回类型为IMediaPlayer，保存在MediaPlayer类的sp<IMediaPlayer> mPlayer中。后续我们就可以直接使用这个mPlayer对象了，调用它的setDataSource、prepare、start、stop等。

       下面我们可以直接跳到MediaPlayerService内部类Client里面去看setDataSource():

MediaPlayerService::Client::setDataSource(const char* url, const KeyedVector<String8, String8> *headers)
{
	if(strncmp(url, "http://", 7) == 0 || strncmp(url, "https://", 8)==0 || strncmp(url, "rtsp://", 7) == 0)
	{
		checkPermission("android.permission.INTERNET");	// 检查是否具有网络权限
	}
	
	if(strncmp(url, "content://", 10) == 0)
	{
		String16 url16(url);
		int fd = android::openContentProviderFile(url16);
		setDataSource(fd, 0, 0x7ffffffffLL);
		close(fd);
		return mStatus;
	} else {
		player_type playerType = getPlayerType(url);	// 根据URL获取播放器类型
		
		sp<MediaPlayerBase> p = createPlayer(playerType);	// 根据播放器类型创建播放器
		// 这里我们分析Android的StagefrightPlayer播放器
		
		if(!p->hardwareOutput()) {
			mAudioOutput = new AudioOutput(mAudioSessionId);
			static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
		}
	}
	
	mStatus = p->setDataSource(url, headers);
	mPlayer = p;
	return mStatus;
}

Android之Media播放器源码分析(framework——native),,5-wow.com