Android -- MediaPlayer内部实现简析

#define GET_PLAYER_TYPE_IMPL(a...)                          Mutex::Autolock lock_(&sLock);                                                                                  player_type ret = STAGEFRIGHT_PLAYER;//默认播放器类型                   float bestScore = 0.0;                              
	//根据当前传入的视频源,对各个播放器进行比较打分,找到合适的播放器;否则使用默认播放器StagefightPlayer     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();//根据"media.stagefright.use-awesome"属性配置,选择当前默认的播放器                       }                                                                                                               return ret;                                         \

//调用createPlayer()创建播放器实例对象
sp<MediaPlayerBase> MediaPlayerService::Client::setDataSource_pre(
        player_type playerType)
{
    ALOGV("player type = %d", playerType);

    // create the right type of player
    sp<MediaPlayerBase> p = createPlayer(playerType);
    if (p == NULL) {
        return p;
    }

    if (!p->hardwareOutput()) {//以StagefrightPlayer为例,它没有重写父类的MediaPlayerInterface::hardwareOutput()方法;返回false,表示音频不直接输出到硬件上.
        Mutex::Autolock l(mLock);
        mAudioOutput = new AudioOutput(mAudioSessionId, IPCThreadState::self()->getCallingUid(),
                mPid, mAudioAttributes);
        static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);//调用父类MediaPlayerInterface::setAudioSink()方法
    }

    return p;
}

//如果当前持有了播放器实例对象,则要判断它是否与我们需要的播放器类型相符;
//如果不相符,则删除它;再重新创建该类型的播放器实例
sp<MediaPlayerBase> MediaPlayerService::Client::createPlayer(player_type playerType)
{
    // determine if we have the right player type
    sp<MediaPlayerBase> p = mPlayer;
    if ((p != NULL) && (p->playerType() != playerType)) {
        ALOGV("delete player");
        p.clear();//重置该sp<>指针
    }
    if (p == NULL) {
        p = MediaPlayerFactory::createPlayer(playerType, this, notify, mPid);//创建播放器对象;重要:createPlayer()里面setNotifyCallback()函数调用
    }

    if (p != NULL) {
        p->setUID(mUID);
    }

    return p;
}

//找到对应视频源类型的播放器工厂实例,并创建Player对象;最后给该Player对象设置传递消息的notifyFunc回调函数
sp<MediaPlayerBase> MediaPlayerFactory::createPlayer(
        player_type playerType,
        void* cookie,
        notify_callback_f notifyFunc,
        pid_t pid) {
    sp<MediaPlayerBase> p;
    IFactory* factory;
    status_t init_result;
    Mutex::Autolock lock_(&sLock);

    if (sFactoryMap.indexOfKey(playerType) < 0) {
        ALOGE("Failed to create player object of type %d, no registered"
              " factory", playerType);
        return p;
    }

    factory = sFactoryMap.valueFor(playerType);//从sFactoryMap中获取到类型对应的Player工厂对象
    CHECK(NULL != factory);
    p = factory->createPlayer(pid);//创建对应类型的播放器实例,直接new实例对象;如创建StagefrightPlayer,期间会实例化StagefrightPlayer::(AwesomePlayer *mPlayer)成员

    if (p == NULL) {
        ALOGE("Failed to create player object of type %d, create failed",
               playerType);
        return p;
    }

    init_result = p->initCheck();//实现直接return OK;
    if (init_result == NO_ERROR) {
        p->setNotifyCallback(cookie, notifyFunc);//调用父类MediaPlayerBase::setNotifyCallback();cookie是当前调用MediaPlayerService::Client::createPlayer()的Client实例,notifyFunc这里是MediaPlayerService::Client::notify()函数指针.
    } else {
        ALOGE("Failed to create player object of type %d, initCheck failed"
              " (res = %d)", playerType, init_result);
        p.clear();
    }

    return p;
}

//StagefrightPlayer是默认播放器
class StagefrightPlayerFactory :
    public MediaPlayerFactory::IFactory {
  public:
    virtual float scoreFactory(const sp<IMediaPlayer>& /*client*/,
                               int fd,
                               int64_t offset,
                               int64_t length,
                               float /*curScore*/) {
        if (legacyDrm()) {
            sp<DataSource> 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<IMediaPlayer>& /*client*/,
                               const char* url,
                               float /*curScore*/) {
        if (legacyDrm() && !strncasecmp("widevine://", url, 11)) {
            return 1.0;
        }
        return 0.0;
    }

    virtual sp<MediaPlayerBase> createPlayer(pid_t /* pid */) {
        ALOGV(" create StagefrightPlayer");
        return new StagefrightPlayer();//实例化StagefrightPlayer对象,并初始化AwesomePlayer *mPlayer成员
    }
  private:
    bool legacyDrm() {
        char value[PROPERTY_VALUE_MAX];
        if (property_get("persist.sys.media.legacy-drm", value, NULL)
                && (!strcmp("1", value) || !strcasecmp("true", value))) {
            return true;
        }
        return false;
    }
};

init_result = p->initCheck();//实现直接return OK;
    if (init_result == NO_ERROR) {
        p->setNotifyCallback(cookie, notifyFunc);//调用父类MediaPlayerBase::setNotifyCallback();cookie是当前调用MediaPlayerService::Client::createPlayer()的Client实例,notifyFunc这里是MediaPlayerService::Client::notify()函数指针.
    } else {
        ALOGE("Failed to create player object of type %d, initCheck failed"
              " (res = %d)", playerType, init_result);
        p.clear();
    }

   void        setNotifyCallback(
            void* cookie, notify_callback_f notifyFunc) {
        Mutex::Autolock autoLock(mNotifyLock);
        mCookie = cookie; mNotify = notifyFunc;
    }

// callback mechanism for passing messages to MediaPlayer object
typedef void (*notify_callback_f)(void* cookie,
        int msg, int ext1, int ext2, const Parcel *obj);

if (!p->hardwareOutput()) {//以StagefrightPlayer为例,它没有重写父类的MediaPlayerInterface::hardwareOutput()方法;返回false,表示音频不直接输出到硬件上.
        Mutex::Autolock l(mLock);
        mAudioOutput = new AudioOutput(mAudioSessionId, IPCThreadState::self()->getCallingUid(),
                mPid, mAudioAttributes);
        static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);//调用父类MediaPlayerInterface::setAudioSink()方法
    }

// Warning: The filedescriptor passed into this method will only be valid until
// the method returns, if you want to keep it, dup it!
//实际调用AwesomePlayer实例的setDataSource()方法
status_t StagefrightPlayer::setDataSource(int fd, int64_t offset, int64_t length) {
    ALOGV("setDataSource(%d, %lld, %lld)", fd, offset, length);
    return mPlayer->setDataSource(dup(fd), offset, length);
}

StagefrightPlayer::StagefrightPlayer()
    : mPlayer(new AwesomePlayer) {
    ALOGV("StagefrightPlayer");

    mPlayer->setListener(this);
}

//p指向创建的播放器实例, status是p->setDataSource()调用结果;setDataSource()调用过程的收尾阶段
void MediaPlayerService::Client::setDataSource_post(
        const sp<MediaPlayerBase>& 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) {//设置再次传输时的终端
        mStatus = p->setRetransmitEndpoint(&mRetransmitEndpoint);
        if (mStatus != NO_ERROR) {
            ALOGE("setRetransmitEndpoint error: %d", mStatus);
        }
    }

    if (mStatus == OK) {
        mPlayer = p;//将播放器实例保存到mPlayer字段中
    }
}

status_t MediaPlayer::prepareAsync()
{
    ALOGV("prepareAsync");
    Mutex::Autolock _l(mLock);
    return prepareAsync_l();
}

void        MediaPlayerBase::sendEvent(int msg, int ext1=0, int ext2=0,
                          const Parcel *obj=NULL) {
        notify_callback_f notifyCB;
        void* cookie;
        {
            Mutex::Autolock autoLock(mNotifyLock);
            notifyCB = mNotify;
            cookie = mCookie;
        }

        if (notifyCB) notifyCB(cookie, msg, ext1, ext2, obj);
    }

//用于向MediaPlayer对象传递消息;参数void* cookie实际指向当前的Client实例;msg参数是事件类型信息
void MediaPlayerService::Client::notify(
        void* cookie, int msg, int ext1, int ext2, const Parcel *obj)
{
    Client* client = static_cast<Client*>(cookie);//首先得到当前的Client对象
    if (client == NULL) {
        return;
    }

    sp<IMediaPlayerClient> c;
    {
        Mutex::Autolock l(client->mLock);
        c = client->mClient;//得到该Client保存的IMediaPlayerClient对象信息,它实际指向一个Native MediaPlayer实例
        if (msg == MEDIA_PLAYBACK_COMPLETE && client->mNextClient != NULL) {//如果当前事件是MEDIA_PLAYBACK_COMPLETE,表明当前播放已经结束；并且我们设置了下一个mNextClient
            if (client->mAudioOutput != NULL)                               //就需要切换执行mNextClient;效果就是当前播放结束后,会自动切换到下一个播放
                client->mAudioOutput->switchToNextOutput();
            client->mNextClient->start();
            client->mNextClient->mClient->notify(MEDIA_INFO, MEDIA_INFO_STARTED_AS_NEXT, 0, obj);//并调用notify()方法,向外告知MEDIA_INFO_STARTED_AS_NEXT事件
        }
    }

    if (MEDIA_INFO == msg &&
        MEDIA_INFO_METADATA_UPDATE == ext1) {//msg为MEDIA_INFO的情况
        const media::Metadata::Type metadata_type = ext2;

        if(client->shouldDropMetadata(metadata_type)) {
            return;
        }

        // Update the list of metadata that have changed. getMetadata
        // also access mMetadataUpdated and clears it.
        client->addNewMetadataUpdate(metadata_type);
    }

    if (c != NULL) {
        ALOGV("[%d] notify (%p, %d, %d, %d)", client->mConnId, cookie, msg, ext1, ext2);
        c->notify(msg, ext1, ext2, obj);//调用这一次的MediaPlayerService::Client实例对应的Native MediaPlayer实例的notify()方法
    }
}

//向应用程序反馈当前状态变化的回调事件,设置当前MediaPlayer的状态
void MediaPlayer::notify(int msg, int ext1, int ext2, const Parcel *obj)
{
    ALOGV("message received msg=%d, ext1=%d, ext2=%d", msg, ext1, ext2);
    bool send = true;
    bool locked = false;

    // TODO: In the future, we might be on the same thread if the app is
    // running in the same process as the media server. In that case,
    // this will deadlock.
    //
    // The threadId hack below works around this for the care of prepare,
    // seekTo and start within the same process.
    // FIXME: Remember, this is a hack, it‘s not even a hack that is applied
    // consistently for all use-cases, this needs to be revisited.
    if (mLockThreadId != getThreadId()) {
        mLock.lock();
        locked = true;
    }

    // Allows calls from JNI in idle state to notify errors
    if (!(msg == MEDIA_ERROR && mCurrentState == MEDIA_PLAYER_IDLE) && mPlayer == 0) {
        ALOGV("notify(%d, %d, %d) callback on disconnected mediaplayer", msg, ext1, ext2);
        if (locked) mLock.unlock();   // release the lock when done.
        return;
    }

    switch (msg) {
    case MEDIA_NOP: // interface test message
        break;
    case MEDIA_PREPARED://符合当前的情况，处理Prepare完成的情况
        ALOGV("prepared");
        mCurrentState = MEDIA_PLAYER_PREPARED;
        if (mPrepareSync) {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = NO_ERROR;
            mSignal.signal();
        }
        break;
    case MEDIA_PLAYBACK_COMPLETE:
        ALOGV("playback complete");
        if (mCurrentState == MEDIA_PLAYER_IDLE) {
            ALOGE("playback complete in idle state");
        }
        if (!mLoop) {
            mCurrentState = MEDIA_PLAYER_PLAYBACK_COMPLETE;
        }
        break;
    case MEDIA_ERROR:
        // Always log errors.
        // ext1: Media framework error code.
        // ext2: Implementation dependant error code.
        ALOGE("error (%d, %d)", ext1, ext2);
        mCurrentState = MEDIA_PLAYER_STATE_ERROR;
        if (mPrepareSync)
        {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = ext1;
            mSignal.signal();
            send = false;
        }
        break;
    case MEDIA_INFO:
        // ext1: Media framework error code.
        // ext2: Implementation dependant error code.
        if (ext1 != MEDIA_INFO_VIDEO_TRACK_LAGGING) {
            ALOGW("info/warning (%d, %d)", ext1, ext2);
        }
        break;
    case MEDIA_SEEK_COMPLETE:
        ALOGV("Received seek complete");
        if (mSeekPosition != mCurrentPosition) {
            ALOGV("Executing queued seekTo(%d)", mSeekPosition);
            mSeekPosition = -1;
            seekTo_l(mCurrentPosition);
        }
        else {
            ALOGV("All seeks complete - return to regularly scheduled program");
            mCurrentPosition = mSeekPosition = -1;
        }
        break;
    case MEDIA_BUFFERING_UPDATE:
        ALOGV("buffering %d", ext1);
        break;
    case MEDIA_SET_VIDEO_SIZE:
        ALOGV("New video size %d x %d", ext1, ext2);
        mVideoWidth = ext1;
        mVideoHeight = ext2;
        break;
    case MEDIA_TIMED_TEXT:
        ALOGV("Received timed text message");
        break;
    case MEDIA_SUBTITLE_DATA:
        ALOGV("Received subtitle data message");
        break;
    case MEDIA_META_DATA:
        ALOGV("Received timed metadata message");
        break;
    default:
        ALOGV("unrecognized message: (%d, %d, %d)", msg, ext1, ext2);
        break;
    }

	//JNIMediaPlayerListener继承自MediaPlayerListener,并实现了notify()方法;声明实现在android_media_MediaPlayer.cpp中
    sp<MediaPlayerListener> listener = mListener;//mListener保存了MediaPlayer实例创建时初始化的JNIMediaPlayerListener监听对象
    if (locked) mLock.unlock();

    // this prevents re-entrant calls into client code
    if ((listener != 0) && send) {
        Mutex::Autolock _l(mNotifyLock);
        ALOGV("callback application");
        listener->notify(msg, ext1, ext2, obj);//调用JNIMediaPlayerListener类实例的notify()方法
        ALOGV("back from callback");
    }
}

case MEDIA_PREPARED:
        ALOGV("prepared");
        mCurrentState = MEDIA_PLAYER_PREPARED;
        if (mPrepareSync) {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = NO_ERROR;
            mSignal.signal();
        }
        break;

//回调MediaPlayer.java中的postEventFromNative()方法,反馈Native层发生的事件;postEventFromNative()会EventHandler(运行在MediaPalyer的线程中)
//发送附带msg参数的消息,EventHandler判断当前的事件类型,如MEDIA_PREPARED等,最后调用应用程序设置的相关回调处理相应的事件信息
void JNIMediaPlayerListener::notify(int msg, int ext1, int ext2, const Parcel *obj)
{
    JNIEnv *env = AndroidRuntime::getJNIEnv();
    if (obj && obj->dataSize() > 0) {
        jobject jParcel = createJavaParcelObject(env);
        if (jParcel != NULL) {
            Parcel* nativeParcel = parcelForJavaObject(env, jParcel);
            nativeParcel->setData(obj->data(), obj->dataSize());
            env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                    msg, ext1, ext2, jParcel);
            env->DeleteLocalRef(jParcel);
        }
    } else {
        env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                msg, ext1, ext2, NULL);
    }
    if (env->ExceptionCheck()) {
        ALOGW("An exception occurred while notifying an event.");
        LOGW_EX(env);
        env->ExceptionClear();
    }
}

    /*
     * Called from native code when an interesting event happens.  This method
     * just uses the EventHandler system to post the event back to the main app thread.
     * We use a weak reference to the original MediaPlayer object so that the native
     * code is safe from the object disappearing from underneath it.  (This is
     * the cookie passed to native_setup().)
     */
    private static void postEventFromNative(Object mediaplayer_ref,
                                            int what, int arg1, int arg2, Object obj)
    {
        MediaPlayer mp = (MediaPlayer)((WeakReference)mediaplayer_ref).get();//拿到给弱引用指向的MediaPlayer实例
        if (mp == null) {
            return;
        }

        if (what == MEDIA_INFO && arg1 == MEDIA_INFO_STARTED_AS_NEXT) {//如果是自动切换到下一个播放的事件或是MEDIA_INFO
            // this acquires the wakelock if needed, and sets the client side state
            mp.start();//就直接start()
        }
        if (mp.mEventHandler != null) {//最后将该事件发送到EventHandler中处理
            Message m = mp.mEventHandler.obtainMessage(what, arg1, arg2, obj);//此时what是MEDIA_PREPARED
            mp.mEventHandler.sendMessage(m);
        }
    }

    private class EventHandler extends Handler
    {
        private MediaPlayer mMediaPlayer;

        public EventHandler(MediaPlayer mp, Looper looper) {
            super(looper);
            mMediaPlayer = mp;
        }

        @Override
        public void handleMessage(Message msg) {
            if (mMediaPlayer.mNativeContext == 0) {
                Log.w(TAG, "mediaplayer went away with unhandled events");
                return;
            }
            switch(msg.what) {
            case MEDIA_PREPARED:
                try {
                    scanInternalSubtitleTracks();
                } catch (RuntimeException e) {
                    // send error message instead of crashing;
                    // send error message instead of inlining a call to one rror
                    // to avoid code duplication.
                    Message msg2 = obtainMessage(
                            MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, MEDIA_ERROR_UNSUPPORTED, null);
                    sendMessage(msg2);
                }
                if (mOnPreparedListener != null)
                    mOnPreparedListener.onPrepared(mMediaPlayer);
                return;

            case MEDIA_PLAYBACK_COMPLETE:
                if (mOnCompletionListener != null)
                    mOnCompletionListener.onCompletion(mMediaPlayer);
                stayAwake(false);
                return;

            case MEDIA_STOPPED:
                {
                    TimeProvider timeProvider = mTimeProvider;
                    if (timeProvider != null) {
                        timeProvider.onStopped();
                    }
                }
                break;

            case MEDIA_STARTED:
            case MEDIA_PAUSED:
                {
                    TimeProvider timeProvider = mTimeProvider;
                    if (timeProvider != null) {
                        timeProvider.onPaused(msg.what == MEDIA_PAUSED);
                    }
                }
                break;

            case MEDIA_BUFFERING_UPDATE:
                if (mOnBufferingUpdateListener != null)
                    mOnBufferingUpdateListener.onBufferingUpdate(mMediaPlayer, msg.arg1);
                return;

            case MEDIA_SEEK_COMPLETE:
                if (mOnSeekCompleteListener != null) {
                    mOnSeekCompleteListener.onSeekComplete(mMediaPlayer);
                }
                // fall through

            case MEDIA_SKIPPED:
                {
                    TimeProvider timeProvider = mTimeProvider;
                    if (timeProvider != null) {
                        timeProvider.onSeekComplete(mMediaPlayer);
                    }
                }
                return;

            case MEDIA_SET_VIDEO_SIZE:
                if (mOnVideoSizeChangedListener != null) {
                    mOnVideoSizeChangedListener.onVideoSizeChanged(
                        mMediaPlayer, msg.arg1, msg.arg2);
                }
                return;

            case MEDIA_ERROR:
                Log.e(TAG, "Error (" + msg.arg1 + "," + msg.arg2 + ")");
                boolean error_was_handled = false;
                if (mOnErrorListener != null) {
                    error_was_handled = mOnErrorListener.onError(mMediaPlayer, msg.arg1, msg.arg2);
                }
                if (mOnCompletionListener != null && ! error_was_handled) {
                    mOnCompletionListener.onCompletion(mMediaPlayer);
                }
                stayAwake(false);
                return;

            case MEDIA_INFO:
                switch (msg.arg1) {
                case MEDIA_INFO_VIDEO_TRACK_LAGGING:
                    Log.i(TAG, "Info (" + msg.arg1 + "," + msg.arg2 + ")");
                    break;
                case MEDIA_INFO_METADATA_UPDATE:
                    try {
                        scanInternalSubtitleTracks();
                    } catch (RuntimeException e) {
                        Message msg2 = obtainMessage(
                                MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, MEDIA_ERROR_UNSUPPORTED, null);
                        sendMessage(msg2);
                    }
                    // fall through

                case MEDIA_INFO_EXTERNAL_METADATA_UPDATE:
                    msg.arg1 = MEDIA_INFO_METADATA_UPDATE;
                    // update default track selection
                    if (mSubtitleController != null) {
                        mSubtitleController.selectDefaultTrack();
                    }
                    break;
                case MEDIA_INFO_BUFFERING_START:
                case MEDIA_INFO_BUFFERING_END:
                    TimeProvider timeProvider = mTimeProvider;
                    if (timeProvider != null) {
                        timeProvider.onBuffering(msg.arg1 == MEDIA_INFO_BUFFERING_START);
                    }
                    break;
                }

                if (mOnInfoListener != null) {
                    mOnInfoListener.onInfo(mMediaPlayer, msg.arg1, msg.arg2);
                }
                // No real default action so far.
                return;
            case MEDIA_TIMED_TEXT:
                if (mOnTimedTextListener == null)
                    return;
                if (msg.obj == null) {
                    mOnTimedTextListener.onTimedText(mMediaPlayer, null);
                } else {
                    if (msg.obj instanceof Parcel) {
                        Parcel parcel = (Parcel)msg.obj;
                        TimedText text = new TimedText(parcel);
                        parcel.recycle();
                        mOnTimedTextListener.onTimedText(mMediaPlayer, text);
                    }
                }
                return;

            case MEDIA_SUBTITLE_DATA:
                if (mOnSubtitleDataListener == null) {
                    return;
                }
                if (msg.obj instanceof Parcel) {
                    Parcel parcel = (Parcel) msg.obj;
                    SubtitleData data = new SubtitleData(parcel);
                    parcel.recycle();
                    mOnSubtitleDataListener.onSubtitleData(mMediaPlayer, data);
                }
                return;

            case MEDIA_META_DATA:
                if (mOnTimedMetaDataAvailableListener == null) {
                    return;
                }
                if (msg.obj instanceof Parcel) {
                    Parcel parcel = (Parcel) msg.obj;
                    TimedMetaData data = TimedMetaData.createTimedMetaDataFromParcel(parcel);
                    parcel.recycle();
                    mOnTimedMetaDataAvailableListener.onTimedMetaDataAvailable(mMediaPlayer, data);
                }
                return;

            case MEDIA_NOP: // interface test message - ignore
                break;

            default:
                Log.e(TAG, "Unknown message type " + msg.what);
                return;
            }
        }
    }

case MEDIA_PREPARED:
                try {
                    scanInternalSubtitleTracks();
                } catch (RuntimeException e) {
                    // send error message instead of crashing;
                    // send error message instead of inlining a call to one rror
                    // to avoid code duplication.
                    Message msg2 = obtainMessage(
                            MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, MEDIA_ERROR_UNSUPPORTED, null);
                    sendMessage(msg2);
                }
                if (mOnPreparedListener != null)
                    mOnPreparedListener.onPrepared(mMediaPlayer);
                return;

    /**
     * Register a callback to be invoked when the media source is ready
     * for playback.
     *
     * @param listener the callback that will be run
     */
    public void setOnPreparedListener(OnPreparedListener listener)
    {
        mOnPreparedListener = listener;
    }

    /**
     * Starts or resumes playback. If playback had previously been paused,
     * playback will continue from where it was paused. If playback had
     * been stopped, or never started before, playback will start at the
     * beginning.
     *
     * @throws IllegalStateException if it is called in an invalid state
     */
    public void start() throws IllegalStateException {
        if (isRestricted()) {
            _setVolume(0, 0);
        }
        stayAwake(true);
        _start();
    }

    private native void _start() throws IllegalStateException;

static void
android_media_MediaPlayer_start(JNIEnv *env, jobject thiz)
{
    ALOGV("start");
    sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
    if (mp == NULL ) {
        jniThrowException(env, "java/lang/IllegalStateException", NULL);
        return;
    }
    process_media_player_call( env, thiz, mp->start(), NULL, NULL );
}

status_t MediaPlayer::start()
{
    ALOGV("start");

    status_t ret = NO_ERROR;
    Mutex::Autolock _l(mLock);

    mLockThreadId = getThreadId();

    if (mCurrentState & MEDIA_PLAYER_STARTED) {//MediaPlayer当前状态是MEDIA_PLAYER_STARTED,则直接返回
        ret = NO_ERROR;
    } else if ( (mPlayer != 0) && ( mCurrentState & ( MEDIA_PLAYER_PREPARED |
                    MEDIA_PLAYER_PLAYBACK_COMPLETE | MEDIA_PLAYER_PAUSED ) ) ) {//能执行start操作的状态
        mPlayer->setLooping(mLoop);//mLoop是一个布尔值,它表示当前是否需要循环播放;它的初始值是FALSE,上层可以设置该值
        mPlayer->setVolume(mLeftVolume, mRightVolume);//设置音量
        mPlayer->setAuxEffectSendLevel(mSendLevel);
        mCurrentState = MEDIA_PLAYER_STARTED;//将MediaPlayer的状态设置为MEDIA_PLAYER_STARTED
        ret = mPlayer->start();//调用MediaPlayerService::Client的start()
        if (ret != NO_ERROR) {
            mCurrentState = MEDIA_PLAYER_STATE_ERROR;
        } else {
            if (mCurrentState == MEDIA_PLAYER_PLAYBACK_COMPLETE) {
                ALOGV("playback completed immediately following start()");
            }
        }
    } else {
        ALOGE("start called in state %d", mCurrentState);
        ret = INVALID_OPERATION;
    }

    mLockThreadId = 0;

    return ret;
}

status_t MediaPlayerService::Client::start()
{
    ALOGV("[%d] start", mConnId);
    sp<MediaPlayerBase> p = getPlayer();
    if (p == 0) return UNKNOWN_ERROR;
    p->setLooping(mLoop);
    return p->start();
}

status_t StagefrightPlayer::start() {
    ALOGV("start");

    return mPlayer->play();
}

void AwesomePlayer::notifyIfMediaStarted_l() {
    if (mMediaRenderingStartGeneration == mStartGeneration) {
        mMediaRenderingStartGeneration = -1;
        notifyListener_l(MEDIA_STARTED);
    }
}

    /**
     * Same factory method as {@link #create(Context, int)} but that lets you specify the audio
     * attributes and session ID to be used by the new MediaPlayer instance.
     * @param context the Context to use
     * @param resid the raw resource id (<var>R.raw.<something></var>) for
     *              the resource to use as the datasource
     * @param audioAttributes the {@link AudioAttributes} to be used by the media player.
     * @param audioSessionId the audio session ID to be used by the media player,
     *     see {@link AudioManager#generateAudioSessionId()} to obtain a new session.
     * @return a MediaPlayer object, or null if creation failed
     */
    public static MediaPlayer create(Context context, int resid,
            AudioAttributes audioAttributes, int audioSessionId) {
        try {
            AssetFileDescriptor afd = context.getResources().openRawResourceFd(resid);
            if (afd == null) return null;

            MediaPlayer mp = new MediaPlayer();// 1

            final AudioAttributes aa = audioAttributes != null ? audioAttributes :
                new AudioAttributes.Builder().build();
            mp.setAudioAttributes(aa);
            mp.setAudioSessionId(audioSessionId);

            mp.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());// 2
            afd.close();
            mp.prepare();// 3
            return mp;
        } catch (IOException ex) {
            Log.d(TAG, "create failed:", ex);
            // fall through
        } catch (IllegalArgumentException ex) {
            Log.d(TAG, "create failed:", ex);
           // fall through
        } catch (SecurityException ex) {
            Log.d(TAG, "create failed:", ex);
            // fall through
        }
        return null;
    }