Android MediaPlayer Playback流程分析
16lz
2021-01-23
一个最简单的Media Playback过程如下，仅通过五个步骤就可以完成播放。
String url = "http://........"; // your URL hereMediaPlayer mediaPlayer = new MediaPlayer();mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);mediaPlayer.setDataSource(url);mediaPlayer.prepare(); // might take long! (for buffering, etc)mediaPlayer.start();初始化MediaPlayer
创建MediaPlayer对象，并完成Java层和Native层的初始化。
frameworks/base/media/java/android/media/MediaPlayer.javapublic class MediaPlayer implements SubtitleController.Listener{...... static { System.loadLibrary("media_jni"); native_init(); // native 初始化 } ...... public MediaPlayer() { Looper looper; if ((looper = Looper.myLooper()) != null) { mEventHandler = new EventHandler(this, looper); } else if ((looper = Looper.getMainLooper()) != null) { mEventHandler = new EventHandler(this, looper); } else { mEventHandler = null; } mTimeProvider = new TimeProvider(this); mOutOfBandSubtitleTracks = new Vector(); mOpenSubtitleSources = new Vector(); mInbandSubtitleTracks = new SubtitleTrack[0]; IBinder b = ServiceManager.getService(Context.APP_OPS_SERVICE); mAppOps = IAppOpsService.Stub.asInterface(b); /* Native setup requires a weak reference to our object. * It's easier to create it here than in C++. */ native_setup(new WeakReference(this)); // native层setup } ....... 可以看到，MediaPlayer通过静态初始化的方式对native层进行初始化。MediaPlayer对媒体控制的动作都是在底层实现的，所以必须在MediaPlayer创建时优先初始化底层。
frameworks/base/media/jni/android_media_MediaPlayer.cppstatic voidandroid_media_MediaPlayer_native_init(JNIEnv *env) { jclass clazz; clazz = env->FindClass("android/media/MediaPlayer"); if (clazz == NULL) { return; } fields.context = env->GetFieldID(clazz, "mNativeContext", "J"); if (fields.context == NULL) { return; } fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative", "(Ljava/lang/Object;IIILjava/lang/Object;)V"); if (fields.post_event == NULL) { return; } fields.surface_texture = env->GetFieldID(clazz, "mNativeSurfaceTexture", "J"); if (fields.surface_texture == NULL) { return; } clazz = env->FindClass("android/net/ProxyInfo"); if (clazz == NULL) { return; } fields.proxyConfigGetHost = env->GetMethodID(clazz, "getHost", "()Ljava/lang/String;"); fields.proxyConfigGetPort = env->GetMethodID(clazz, "getPort", "()I"); fields.proxyConfigGetExclusionList = env->GetMethodID(clazz, "getExclusionListAsString", "()Ljava/lang/String;");}native_init()获取Java类MediaPlayer和ProxyInfo中的一些方法和变量。
- 将native MediaPlayer的对象保存到Java MediaPlayer的mNativeContext中。
- 获取Java MediaPlayer的postEventFromNative()方法，用于发送事件。
- 将native video surface保存到Java MediaPlayer的mNativeSurfaceTexture中。
- 获取Java ProxyInfo的getHost()、getPort()、getExclusionListAsString()方法。
之后在MediaPlayer的构造函数中初始化一些媒体播放的必要元素，像消息、时间、字幕、权限等。最后调用native_setup()对native层进行设置。
frameworks/base/media/jni/android_media_MediaPlayer.cppstatic void android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this){ ALOGV("native_setup"); sp mp = new MediaPlayer(); if (mp == NULL) { jniThrowException(env, "java/lang/RuntimeException", "Out of memory"); return; } // create new listener and give it to MediaPlayer sp listener = new JNIMediaPlayerListener(env, thiz, weak_this); mp->setListener(listener); // Stow our new C++ MediaPlayer in an opaque field in the Java object. setMediaPlayer(env, thiz, mp);} Setup过程完成一下工作。
- 创建Native MeidaPlayer对象。
- 创建Listener上报Native事件，这里调用postEventFromNative()。
- 存储Native MediaPlayer对象到mNativeContext中，以便Java层使用。
设置声音系统
Android系统将定义了不同的Audio Stream类型，用来适用不同的应用场景。像STREAM_VOICE_CALL用于打电话场景，STREAM_SYSTEM用于系统声音，STREAM_RING用于电话铃声，STREAM_MUSIC用于播放音乐等。每种Stream类型都是独立的，有自己的状态和音量。在使用Player前需要选择一个Stream类型，音乐播放和视频播放时基本上采用的都是STREAM_MUSIC。
Java层的setAudioStreamType()直接调用的Native的setAudioStreamType()，而Native的这个方法也很简单，就是赋值给mStreamType。
frameworks/av/media/libmedia/mediaplayer.cppstatus_t MediaPlayer::setAudioStreamType(audio_stream_type_t type){ ALOGV("MediaPlayer::setAudioStreamType"); Mutex::Autolock _l(mLock); if (mStreamType == type) return NO_ERROR; if (mCurrentState & ( MEDIA_PLAYER_PREPARED | MEDIA_PLAYER_STARTED | MEDIA_PLAYER_PAUSED | MEDIA_PLAYER_PLAYBACK_COMPLETE ) ) { // Can't change the stream type after prepare ALOGE("setAudioStream called in state %d", mCurrentState); return INVALID_OPERATION; } // cache mStreamType = type; return OK; }mStreamType会在Player Prepare过程中被设置生效，这也就是为什么必须在Player开始前设置Stream类型。
frameworks/av/media/libmedia/mediaplayer.cppstatus_t MediaPlayer::prepareAsync_l(){ if ( (mPlayer != 0) && ( mCurrentState & ( MEDIA_PLAYER_INITIALIZED | MEDIA_PLAYER_STOPPED) ) ) { mPlayer->setAudioStreamType(mStreamType); if (mAudioAttributesParcel != NULL) { mPlayer->setParameter(KEY_PARAMETER_AUDIO_ATTRIBUTES, *mAudioAttributesParcel); } mCurrentState = MEDIA_PLAYER_PREPARING; return mPlayer->prepareAsync(); } ALOGE("prepareAsync called in state %d", mCurrentState); return INVALID_OPERATION;}上面的过程又远程调用了MediaPlayerService的setAudioStreamType()，真正的Audio系统设置就发生在这里。Audio系统比较复杂，这里不再展开。
frameworks/av/media/libmediaplayerservice/MediaPlayerService.cppstatus_t MediaPlayerService::Client::setAudioStreamType(audio_stream_type_t type){ ALOGV("[%d] setAudioStreamType(%d)", mConnId, type); // TODO: for hardware output, call player instead Mutex::Autolock l(mLock); if (mAudioOutput != 0) mAudioOutput->setAudioStreamType(type); return NO_ERROR;}初始化底层播放器
Android MediaPlayer使用”工厂模式“来管理底层播放器。只要按MediaPlayerFactory
定义接口进行实现，并注册到sFactoryMap中，就可以调用自己的Player。原生Android提供了四个可用的播放器：NuPlayer、StagefrightPlayer、SonivoxPlayer和TestPlayer，厂商也可以增加客制化的播放器。在播放前需要根据评分来选择使用播放器，这个选择过程就是在setDataSource()中完成的。
Java层MediaPlayer实现了多个setDataSource()，但最终都是通过两个JNI接口调用到native中。
- nativeSetDataSource：处理网络流媒体。
- _setDataSource：处理本地的媒体文件。
frameworks/base/media/jni/android_media_MediaPlayer.cppstatic void android_media_MediaPlayer_setDataSourceAndHeaders( JNIEnv *env, jobject thiz, jobject httpServiceBinderObj, jstring path, jobjectArray keys, jobjectArray values) { sp mp = getMediaPlayer(env, thiz);...... sp httpService; if (httpServiceBinderObj != NULL) { sp binder = ibinderForJavaObject(env, httpServiceBinderObj); httpService = interface_cast(binder); } status_t opStatus = mp->setDataSource( httpService, pathStr, headersVector.size() > 0? &headersVector : NULL);......}static voidandroid_media_MediaPlayer_setDataSourceFD(JNIEnv *env, jobject thiz, jobject fileDescriptor, jlong offset, jlong length){ sp mp = getMediaPlayer(env, thiz);...... int fd = jniGetFDFromFileDescriptor(env, fileDescriptor); ALOGV("setDataSourceFD: fd %d", fd); process_media_player_call( env, thiz, mp->setDataSource(fd, offset, length), "java/io/IOException", "setDataSourceFD failed." );} JNI中又调用Native层MediaPlayer的setDataSource()方法，最终的远程调用到MediaPlayerService的setDataSource()。
frameworks/av/media/libmediaplayerservice/MediaPlayerService.cppstatus_t MediaPlayerService::Client::setDataSource( const sp &httpService, const char *url, const KeyedVector *headers){......if (strncmp(url, "content://", 10) == 0) { // get a filedescriptor for the content Uri and // pass it to the setDataSource(fd) method String16 url16(url); int fd = android::openContentProviderFile(url16); if (fd < 0) { ALOGE("Couldn't open fd for %s", url); return UNKNOWN_ERROR; } setDataSource(fd, 0, 0x7fffffffffLL); // this sets mStatus close(fd); return mStatus;} else { player_type playerType = MediaPlayerFactory::getPlayerType(this, url); ..... sp p = setDataSource_pre(playerType); if (p == NULL) { return NO_INIT; } setDataSource_post(p, p->setDataSource(httpService, url, headers)); return mStatus;}}status_t MediaPlayerService::Client::setDataSource(int fd, int64_t offset, int64_t length){ ...... player_type playerType = MediaPlayerFactory::getPlayerType(this, fd, offset, length); sp p = setDataSource_pre(playerType); if (p == NULL) { return NO_INIT; } // now set data source setDataSource_post(p, p->setDataSource(fd, offset, length)); return mStatus;}status_t MediaPlayerService::Client::setDataSource( const sp &source) { // create the right type of player player_type playerType = MediaPlayerFactory::getPlayerType(this, source); sp p = setDataSource_pre(playerType); if (p == NULL) { return NO_INIT; } // now set data source setDataSource_post(p, p->setDataSource(source)); return mStatus;} setDataSource()虽然具体实现不同，但最主要的步骤都是相同的。
- getPlayerType：选择合适的底层播放器。
- setDataSource_pre：创建底层播放器。
- setDataSource_post：调用底层播放器的setDataSource()，设置数据源。
播放器的选择过程也比较简单，就是根据参数寻找评分最高的播放器。每个播放器都会实现scoreFactory()方法，为自己打分。分值最高的播放器有最高优先级。
frameworks/av/media/libmediaplayerservice/MediaPlayerFactory.cpp#define GET_PLAYER_TYPE_IMPL(a...) \ Mutex::Autolock lock_(&sLock); \ \ player_type ret = STAGEFRIGHT_PLAYER; \ float bestScore = 0.0; \ \ for (size_t i = 0; i < sFactoryMap.size(); ++i) { \ \ IFactory* v = sFactoryMap.valueAt(i); \ float thisScore; \ CHECK(v != NULL); \ thisScore = v->scoreFactory(a, bestScore); \ if (thisScore > bestScore) { \ ret = sFactoryMap.keyAt(i); \ bestScore = thisScore; \ } \ } \ \ if (0.0 == bestScore) { \ ret = getDefaultPlayerType(); \ } \ \ return ret;player_type MediaPlayerFactory::getPlayerType(const sp& client, const char* url) { GET_PLAYER_TYPE_IMPL(client, url);}player_type MediaPlayerFactory::getPlayerType(const sp& client, int fd, int64_t offset, int64_t length) { GET_PLAYER_TYPE_IMPL(client, fd, offset, length);}player_type MediaPlayerFactory::getPlayerType(const sp& client, const sp &source) { GET_PLAYER_TYPE_IMPL(client, source);} 以StagefrightPlayer为例，看一下scoreFactory()的实现。
frameworks/av/media/libmediaplayerservice/MediaPlayerFactory.cppclass StagefrightPlayerFactory : public MediaPlayerFactory::IFactory { public: virtual float scoreFactory(const sp& /*client*/, int fd, int64_t offset, int64_t length, float /*curScore*/) { if (legacyDrm()) { sp source = new FileSource(dup(fd), offset, length); String8 mimeType; float confidence; if (SniffWVM(source, &mimeType, &confidence, NULL /* format */)) { return 1.0; } } if (getDefaultPlayerType() == STAGEFRIGHT_PLAYER) { char buf[20]; lseek(fd, offset, SEEK_SET); read(fd, buf, sizeof(buf)); lseek(fd, offset, SEEK_SET); uint32_t ident = *((uint32_t*)buf); // Ogg vorbis? if (ident == 0x5367674f) // 'OggS' return 1.0; } return 0.0; } virtual float scoreFactory(const sp& /*client*/, const char* url, float /*curScore*/) { if (legacyDrm() && !strncasecmp("widevine://", url, 11)) { return 1.0; } return 0.0; }...... 可以看到，如果媒体为Widevine DRM格式，StagefrightPlayer会给出更高的打分。Ogg vorbis也会返回高分。
准备Player和播放
选择好底层播放器后，使用prepare()做播放前的准备，之后使用start()开始播发。这两个操作的调用流程类似，最终都是调用到底层播放器的相关接口。这里不分析播放器的具体操作，每个播放器的操作都不同，需要针对播放器做分析。调用流程如下图所示。
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