概述

android中的多媒体播放结构是：client-server模式

client端对应的类是：MediaPlayer -- framework/base/media/java/android/media/MediaPlayer.java

server端对应的类是：MediaPlayer->MediaPlayerService->Stagefrigthplayer->AwesomePlayer

其中mediaplayerservice是service,MediaPlayer通过binder机制与其交互。

这里Java层的MediaPlayer类与Cpp层的MediaPlayer代码交互不介绍了，自己不了解，只介绍下层的过程。

注：对于mediaplayer类的方法介绍，可参考译文

本篇主要介绍一个典型的播放调用过程

下面是一个典型的播放序列：

[html] view plain copy print ?
  1. MediaPlayer player=new MediaPlayer()
  2. player->setDataSource(url,header);
  3. player->prepare();
  4. player->start();
  5. ...

1、构造函数下面就介绍具体的调用流程，起点为MediaPlayer.cpp

代码如下

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  1. MediaPlayer::MediaPlayer()
  2. {
  3. ALOGV("constructor");
  4. mListener = NULL;
  5. mCookie = NULL;
  6. mStreamType = AUDIO_STREAM_MUSIC;
  7. mCurrentPosition = -1;
  8. mSeekPosition = -1;
  9. mCurrentState = MEDIA_PLAYER_IDLE;
  10. mPrepareSync =false;
  11. mPrepareStatus = NO_ERROR;
  12. mLoop =false;
  13. mLeftVolume = mRightVolume =1.0;
  14. mVideoWidth = mVideoHeight =0;
  15. mLockThreadId =0;
  16. mAudioSessionId = AudioSystem::newAudioSessionId();
  17. AudioSystem::acquireAudioSessionId(mAudioSessionId);
  18. mSendLevel =0;
  19. mRetransmitEndpointValid =false;
  20. }


主要是基本的初始化，比较简单。

2、setDataSource

入口代码如下

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  1. status_t MediaPlayer::setDataSource(
  2. constchar *url, const KeyedVector<String8, String8> *headers)
  3. {
  4. ALOGV("setDataSource(%s)", url);
  5. status_t err = BAD_VALUE;
  6. if(url != NULL) {
  7. constsp<IMediaPlayerService>& service(getMediaPlayerService());
  8. if(service != 0) {
  9. sp<IMediaPlayer> player(service->create(getpid(),this, mAudioSessionId));
  10. if((NO_ERROR != doSetRetransmitEndpoint(player)) ||
  11. (NO_ERROR != player->setDataSource(url, headers))) {
  12. player.clear();
  13. }
  14. err = attachNewPlayer(player);
  15. }
  16. }
  17. returnerr;
  18. }


先看下 service->create(getpid(), this, mAudioSessionId)操作，代码在mediaplayerservice.cpp中 setDataSource的操作就转成通过与mediaplayerservice的交互来完成了

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  1. sp<IMediaPlayer> MediaPlayerService::create(pid_t pid,const sp<IMediaPlayerClient>& client,
  2. intaudioSessionId)
  3. {
  4. int32_t connId = android_atomic_inc(&mNextConnId);
  6. sp<Client> c =new Client(
  7. this, pid, connId, client, audioSessionId,
  8. IPCThreadState::self()->getCallingUid());
  10. ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid,
  11. IPCThreadState::self()->getCallingUid());
  13. wp<Client> w = c;
  14. {
  15. Mutex::Autolock lock(mLock);
  16. mClients.add(w);
  17. }
  18. returnc;
  19. }


而且，Client类的继承关系为：Client->BnMediaPlayer->IMediaPlayer分析上面代码，可以看出create方法，是构造了一个Client对象，并且将此client对象添加到mediapalyerservice类的全局列表中：mClient

因此在setDataSource中的

1 sp<IMediaPlayer> player(service->create(getpid(), this , mAudioSessionId));

语句相当于

1 sp<IMediaPlayer> player( new Client(**));

即player最终是用Client对象来初始化，可以直接认为player==client

[说明]c++功力较差，此处可能不严谨

紧接着执行player->setDataSource(url, headers)，即Clients::setDataSource

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  1. status_t MediaPlayerService::Client::setDataSource(intfd, int64_t offset, int64_t length)
  2. {
  3. ALOGV("setDataSource fd=%d, offset=%lld, length=%lld", fd, offset, length);
  4. struct stat sb;
  5. intret = fstat(fd, &sb);
  6. if(ret != 0) {
  7. ALOGE("fstat(%d) failed: %d, %s", fd, ret, strerror(errno));
  8. returnUNKNOWN_ERROR;
  9. }
  11. ALOGV("st_dev = %llu", sb.st_dev);
  12. ALOGV("st_mode = %u", sb.st_mode);
  13. ALOGV("st_uid = %lu", sb.st_uid);
  14. ALOGV("st_gid = %lu", sb.st_gid);
  15. ALOGV("st_size = %llu", sb.st_size);
  17. if(offset >= sb.st_size) {
  18. ALOGE("offset error");
  19. ::close(fd);
  20. returnUNKNOWN_ERROR;
  21. }
  22. if(offset + length > sb.st_size) {
  23. length = sb.st_size - offset;
  24. ALOGV("calculated length = %lld", length);
  25. }
  27. player_type playerType = MediaPlayerFactory::getPlayerType(this,
  28. fd,
  29. offset,
  30. length);
  31. sp<MediaPlayerBase> p = setDataSource_pre(playerType);
  32. if(p == NULL) {
  33. returnNO_INIT;
  34. }
  36. // now set data source
  37. setDataSource_post(p, p->setDataSource(fd, offset, length));
  38. returnmStatus;
  39. }


2.1 getPlayerType代码比较清晰，首选是选择播放器类型，之后调用setDataSource_pre以及setDataSource_post下面分别来看。

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  1. #define GET_PLAYER_TYPE_IMPL(a...) \
  2. Mutex::Autolock lock_(&sLock); \
  3. \
  4. player_type ret = STAGEFRIGHT_PLAYER; \
  5. float bestScore =0.0; \
  6. \
  7. for(size_t i = 0; i < sFactoryMap.size(); ++i) { \
  8. \
  9. IFactory* v = sFactoryMap.valueAt(i); \
  10. float thisScore; \
  11. CHECK(v != NULL); \
  12. thisScore = v->scoreFactory(a, bestScore); \
  13. if(thisScore > bestScore) { \
  14. ret = sFactoryMap.keyAt(i); \
  15. bestScore = thisScore; \
  16. } \
  17. } \
  18. \
  19. if(0.0 == bestScore) { \
  20. bestScore = getDefaultPlayerType(); \
  21. } \
  22. \
  23. returnret;
  25. player_type MediaPlayerFactory::getPlayerType(constsp<IMediaPlayer>& client,
  26. intfd,
  27. int64_t offset,
  28. int64_t length) {
  29. GET_PLAYER_TYPE_IMPL(client, fd, offset, length);
  30. }


这里如果所有播放器都不合适，则选择默认播放器这里流程是，从播放器仓库中选取最佳播放器，通过scoreFactory来确定

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  1. player_type MediaPlayerFactory::getDefaultPlayerType() {
  2. char value[PROPERTY_VALUE_MAX];
  3. if(property_get("media.stagefright.use-nuplayer", value, NULL)
  4. && (!strcmp("1", value) || !strcasecmp("true", value))) {
  5. returnNU_PLAYER;
  6. }
  8. returnSTAGEFRIGHT_PLAYER;
  9. }


这里有几种类型的播放器，会在mediaplayerservice服务类 的构造函数中来注册，代码如下代码中可以看出，一般情况下，STAGEFRIGHT_PLAYER为默认播放器

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  1. MediaPlayerService::MediaPlayerService()
  2. {
  3. ***
  4. MediaPlayerFactory::registerBuiltinFactories();
  5. }
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  1. void MediaPlayerFactory::registerBuiltinFactories() {
  2. Mutex::Autolock lock_(&sLock);
  4. if(sInitComplete)
  5. return;
  7. registerFactory_l(newStagefrightPlayerFactory(), STAGEFRIGHT_PLAYER);
  8. registerFactory_l(newNuPlayerFactory(), NU_PLAYER);
  9. registerFactory_l(newSonivoxPlayerFactory(), SONIVOX_PLAYER);
  10. registerFactory_l(newTestPlayerFactory(), TEST_PLAYER);
  12. sInitComplete =true;
  13. }

从registerBuiltinFactories的实现可以看出共注册了四个播放器类型。后面会介绍几个播放器的用途等


下面挑选STAGEFRIGHT_PLAYER类型作为例子看下


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  1. class StagefrightPlayerFactory :
  2. publicMediaPlayerFactory::IFactory {
  3. public:
  4. virtual float scoreFactory(constsp<IMediaPlayer>& client,
  5. intfd,
  6. int64_t offset,
  7. int64_t length,
  8. float curScore) {
  9. char buf[20];
  10. lseek(fd, offset, SEEK_SET);
  11. read(fd, buf, sizeof(buf));
  12. lseek(fd, offset, SEEK_SET);
  14. long ident = *((long*)buf);
  16. // Ogg vorbis?
  17. if(ident == 0x5367674f)// 'OggS'
  18. return1.0;
  20. return0.0;
  21. }
  23. virtual sp<MediaPlayerBase> createPlayer() {
  24. ALOGV(" create StagefrightPlayer");
  25. returnnew StagefrightPlayer();
  26. }
  27. };


比较简单，就是一些判断条件，这里还提供了createPlayer，返回实际的播放器对象，后面用的着。

2.2 setDataSource_pre

具体代码如下

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  1. sp<MediaPlayerBase> MediaPlayerService::Client::setDataSource_pre(
  2. player_type playerType)
  3. {
  4. ALOGV("player type = %d", playerType);
  6. // create the right type of player
  7. sp<MediaPlayerBase> p = createPlayer(playerType);
  8. if(p == NULL) {
  9. returnp;
  10. }
  12. if(!p->hardwareOutput()) {
  13. mAudioOutput =new AudioOutput(mAudioSessionId);
  14. static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
  15. }
  17. returnp;
  18. }
[html] view plain copy print ?
  1. sp<MediaPlayerBase> MediaPlayerService::Client::createPlayer(player_type playerType)
  2. {
  3. // determine if we have the right player type
  4. sp<MediaPlayerBase> p = mPlayer;
  5. if((p != NULL) && (p->playerType() != playerType)) {
  6. ALOGV("delete player");
  7. p.clear();
  8. }
  9. if(p == NULL) {
  10. p = MediaPlayerFactory::createPlayer(playerType,this, notify);
  11. }
  13. if(p != NULL) {
  14. p->setUID(mUID);
  15. }
  17. returnp;
  18. }


主要是创建播放器

上面看具体StagefrightPlayerFactory 实现的时候，代码中实现了createPlayer，此处就是调用的此方法

因此等价于return (new StagefrightPlayer()) ;

2.3 setDataSource_post

上面的代码的主要目的是找到对应的播放器，下面主要是执行具体的setdatasource操作

这里需要注意的是 实际的工作是在调用的时候完成的

1 setDataSource_post(p, p->setDataSource(fd, offset, length));

调用的时候，执行了实际播放器的setDataSource方法，而实际的setDataSource_post 仅仅是将播放器保存在本地成员mPlayer中，代码如下

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  1. void MediaPlayerService::Client::setDataSource_post(
  2. constsp<MediaPlayerBase>& p,
  3. status_t status)
  4. {
  5. ALOGV(" setDataSource");
  6. mStatus = status;
  7. if(mStatus != OK) {
  8. ALOGE(" error: %d", mStatus);
  9. return;
  10. }
  12. // Set the re-transmission endpoint if one was chosen.
  13. if(mRetransmitEndpointValid) {
  14. mStatus = p->setRetransmitEndpoint(&mRetransmitEndpoint);
  15. if(mStatus != NO_ERROR) {
  16. ALOGE("setRetransmitEndpoint error: %d", mStatus);
  17. }
  18. }
  20. if(mStatus == OK) {
  21. mPlayer = p;
  22. }
  23. }


下面就进入实际播放器stagefritplayer中看具体实现了（此处以stagefrightplayer为例）

通过上面的介绍，熟悉了如何从MediaPlayer（Java类）一步一步调用到实际工作的类StageFrigtPlayer

这里只是以setDataSource为例，其他方法如prepare等类似。

下面这张图表示了调用流程

总结下几种播放器的区别

StagefrightPlayer： 默认播放器，本地文件基本都使用其播放

NuPlayerDriver：主要用于播放网络视频,http https rtsp等

SonivoxPlayer：用于播放midi等类型的音乐

下一篇着重分析stagefrigtplayer的主要结构

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