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概述

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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：

MediaPlayer player=new MediaPlayer()player->setDataSource(url,header);player->prepare();player->start();...

1、构造函数下面就介绍具体的调用流程，起点为MediaPlayer.cpp

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代码如下

MediaPlayer::MediaPlayer(){    ALOGV("constructor");    mListener = NULL;    mCookie = NULL;    mStreamType = AUDIO_STREAM_MUSIC;    mCurrentPosition = -1;    mSeekPosition = -1;    mCurrentState = MEDIA_PLAYER_IDLE;    mPrepareSync =false;    mPrepareStatus = NO_ERROR;    mLoop =false;    mLeftVolume = mRightVolume =1.0;    mVideoWidth = mVideoHeight =0;    mLockThreadId =0;    mAudioSessionId = AudioSystem::newAudioSessionId();    AudioSystem::acquireAudioSessionId(mAudioSessionId);    mSendLevel =0;    mRetransmitEndpointValid =false;}

主要是基本的初始化，比较简单。

2、setDataSource

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入口代码如下

status_t MediaPlayer::setDataSource(        constchar *url, const KeyedVector
   
     *headers){    ALOGV("setDataSource(%s)", url);    status_t err = BAD_VALUE;    if(url != NULL) {        constsp
    
     & service(getMediaPlayerService());        if(service != 0) {            sp
     
       player(service->create(getpid(),this, mAudioSessionId));            if((NO_ERROR != doSetRetransmitEndpoint(player)) ||                (NO_ERROR != player->setDataSource(url, headers))) {                player.clear();            }            err = attachNewPlayer(player);        }    }    returnerr;}
     
    
   

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

sp
   
     MediaPlayerService::create(pid_t pid,const sp
    
     & client,        intaudioSessionId){    int32_t connId = android_atomic_inc(&mNextConnId);     sp
     
       c =new Client(            this, pid, connId, client, audioSessionId,            IPCThreadState::self()->getCallingUid());     ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid,         IPCThreadState::self()->getCallingUid());     wp
      
        w = c;    {        Mutex::Autolock lock(mLock);        mClients.add(w);    }    returnc;}
      
     
    
   

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

status_t MediaPlayerService::Client::setDataSource(intfd, int64_t offset, int64_t length){    ALOGV("setDataSource fd=%d, offset=%lld, length=%lld", fd, offset, length);    struct stat sb;    intret = fstat(fd, &sb);    if(ret != 0) {        ALOGE("fstat(%d) failed: %d, %s", fd, ret, strerror(errno));        returnUNKNOWN_ERROR;    }     ALOGV("st_dev  = %llu", sb.st_dev);    ALOGV("st_mode = %u", sb.st_mode);    ALOGV("st_uid  = %lu", sb.st_uid);    ALOGV("st_gid  = %lu", sb.st_gid);    ALOGV("st_size = %llu", sb.st_size);     if(offset >= sb.st_size) {        ALOGE("offset error");        ::close(fd);        returnUNKNOWN_ERROR;    }    if(offset + length > sb.st_size) {        length = sb.st_size - offset;        ALOGV("calculated length = %lld", length);    }     player_type playerType = MediaPlayerFactory::getPlayerType(this,                                                               fd,                                                               offset,                                                               length);    sp
   
     p = setDataSource_pre(playerType);    if(p == NULL) {        returnNO_INIT;    }     // now set data source    setDataSource_post(p, p->setDataSource(fd, offset, length));    returnmStatus;}
   

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。

#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) {                             \        bestScore = getDefaultPlayerType();             \    }                                                   \                                                        \    returnret; player_type MediaPlayerFactory::getPlayerType(constsp
   
    & client,                                              intfd,                                              int64_t offset,                                              int64_t length) {    GET_PLAYER_TYPE_IMPL(client, fd, offset, length);}
   

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

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

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

MediaPlayerService::MediaPlayerService(){    ***    MediaPlayerFactory::registerBuiltinFactories();}

void MediaPlayerFactory::registerBuiltinFactories() {    Mutex::Autolock lock_(&sLock);     if(sInitComplete)        return;     registerFactory_l(newStagefrightPlayerFactory(), STAGEFRIGHT_PLAYER);    registerFactory_l(newNuPlayerFactory(), NU_PLAYER);    registerFactory_l(newSonivoxPlayerFactory(), SONIVOX_PLAYER);    registerFactory_l(newTestPlayerFactory(), TEST_PLAYER);     sInitComplete =true;}

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

class StagefrightPlayerFactory :    publicMediaPlayerFactory::IFactory {  public:    virtual float scoreFactory(constsp
   
    & client,                               intfd,                               int64_t offset,                               int64_t length,                               float curScore) {        char buf[20];        lseek(fd, offset, SEEK_SET);        read(fd, buf, sizeof(buf));        lseek(fd, offset, SEEK_SET);         long ident = *((long*)buf);         // Ogg vorbis?        if(ident == 0x5367674f)// 'OggS'            return1.0;         return0.0;    }     virtual sp
    
      createPlayer() {        ALOGV(" create StagefrightPlayer");        returnnew StagefrightPlayer();    }};
    
   

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

sp
   
     MediaPlayerService::Client::setDataSource_pre(        player_type playerType){    ALOGV("player type = %d", playerType);     // create the right type of player    sp
    
      p = createPlayer(playerType);    if(p == NULL) {        returnp;    }     if(!p->hardwareOutput()) {        mAudioOutput =new AudioOutput(mAudioSessionId);        static_cast
     
      (p.get())->setAudioSink(mAudioOutput);    }     returnp;}
     
    
   

sp
   
     MediaPlayerService::Client::createPlayer(player_type playerType){    // determine if we have the right player type    sp
    
      p = mPlayer;    if((p != NULL) && (p->playerType() != playerType)) {        ALOGV("delete player");        p.clear();    }    if(p == NULL) {        p = MediaPlayerFactory::createPlayer(playerType,this, notify);    }     if(p != NULL) {        p->setUID(mUID);    }     returnp;}
    
   

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

void MediaPlayerService::Client::setDataSource_post(        constsp
   
    & p,        status_t status){    ALOGV(" setDataSource");    mStatus = status;    if(mStatus != OK) {        ALOGE("  error: %d", mStatus);        return;    }     // Set the re-transmission endpoint if one was chosen.    if(mRetransmitEndpointValid) {        mStatus = p->setRetransmitEndpoint(&mRetransmitEndpoint);        if(mStatus != NO_ERROR) {            ALOGE("setRetransmitEndpoint error: %d", mStatus);        }    }     if(mStatus == OK) {        mPlayer = p;    }}
   

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