MediaPlayer代码分析(3)-播放前的准备prepare

本文分析prepare和prepareAsyn。

JNI层的prepare*

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

    // Handle the case where the display surface was set before the mp was
    // initialized. We try again to make it stick.
    sp<IGraphicBufferProducer> st = getVideoSurfaceTexture(env, thiz);
    mp->setVideoSurfaceTexture(st);

    process_media_player_call( env, thiz, mp->prepare(), "java/io/IOException", "Prepare failed." );
}

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

    // Handle the case where the display surface was set before the mp was
    // initialized. We try again to make it stick.
    sp<IGraphicBufferProducer> st = getVideoSurfaceTexture(env, thiz);
    mp->setVideoSurfaceTexture(st);

    process_media_player_call( env, thiz, mp->prepareAsync(), "java/io/IOException", "Prepare Async failed." );
}

两个函数都调用了getVideoSurfaceTexture()

static sp<IGraphicBufferProducer>
getVideoSurfaceTexture(JNIEnv* env, jobject thiz) {
    IGraphicBufferProducer * const p = (IGraphicBufferProducer*)env->GetIntField(thiz, fields.surface_texture);
    return sp<IGraphicBufferProducer>(p);
}

获取了Java层的mNativeSurfaceTexture

private int mNativeSurfaceTexture;  // accessed by native methods

接着调用了mp->prepare和mp->prepareAsync。这里我们只分析mp->prepareAsync，它的操作结果经过回调通知给Java层的。

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

它里面有一个互斥锁，之后调用prepareAsync_l()；

// must call with lock held
status_t MediaPlayer::prepareAsync_l()
{
    if ( (mPlayer != 0) && ( mCurrentState & ( MEDIA_PLAYER_INITIALIZED | MEDIA_PLAYER_STOPPED) ) ) {
        mPlayer->setAudioStreamType(mStreamType);
        mCurrentState = MEDIA_PLAYER_PREPARING;
        return mPlayer->prepareAsync();
    }
    ALOGE("prepareAsync called in state %d", mCurrentState);
    return INVALID_OPERATION;
}

首先是mp->setAudioStreamType，在IMediaPlayer.cpp中对应的transact操作是SET_AUDIO_STREAM_TYPE

case SET_AUDIO_STREAM_TYPE: {
            CHECK_INTERFACE(IMediaPlayer, data, reply);
            reply->writeInt32(setAudioStreamType((audio_stream_type_t) data.readInt32()));
            return NO_ERROR;
        } break;

对应到BnMediaPlayer侧则是MediaPlayerService::Client::setAudioStreamType

status_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;
}

AudioOutput->setAudioStreamType

void            setAudioStreamType(audio_stream_type_t streamType) { mStreamType = streamType; }

这里讲一下AudiOutput，它是class AudioOutput : public MediaPlayerBase::AudioSink，这个AudioSink将在后面AwesomePlayer里用到，也就是调用的AudioOutput。

在setDataSource_pre中我们已经将mAudioOutput设置到AwesomePlayer中了。

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()) {
        mAudioOutput = new AudioOutput(mAudioSessionId, IPCThreadState::self()->getCallingUid());
        static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
    }

    return p;
}

接着我们继续看prepareAsync函数，mp->prepareAsync()对应的到BnMediaPlayer的操作是MediaPlayerService::Client::prepareAsync

        case PREPARE_ASYNC: {
            CHECK_INTERFACE(IMediaPlayer, data, reply);
            reply->writeInt32(prepareAsync());
            return NO_ERROR;
        } break;

MediaPlayerService::Client::prepareAsync

status_t MediaPlayerService::Client::prepareAsync()
{
    ALOGV("[%d] prepareAsync", mConnId);
    sp<MediaPlayerBase> p = getPlayer();
    if (p == 0) return UNKNOWN_ERROR;
    status_t ret = p->prepareAsync();
#if CALLBACK_ANTAGONIZER
    ALOGD("start Antagonizer");
    if (ret == NO_ERROR) mAntagonizer->start();
#endif
    return ret;
}

这里调用了AwesomePlayer的prepareAsync函数

status_t AwesomePlayer::prepareAsync() {
    ATRACE_CALL();
    Mutex::Autolock autoLock(mLock);

    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;  // async prepare already pending
    }

    mIsAsyncPrepare = true;
    return prepareAsync_l();
}

先判断mFlags是不是preparing。初始化AwesomePlayer时将mFlags赋值为0，所以这个判断不会是true，继续看下面代码

AwesomePlayer::prepareAsync_l()

status_t AwesomePlayer::prepareAsync_l() {
    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;  // async prepare already pending
    }

    if (!mQueueStarted) {
        mQueue.start();
        mQueueStarted = true;
    }

    modifyFlags(PREPARING, SET);
    mAsyncPrepareEvent = new AwesomeEvent(
            this, &AwesomePlayer::onPrepareAsyncEvent);

    mQueue.postEvent(mAsyncPrepareEvent);

    return OK;
}

首先还是判断mFlags，此时不是preparing。接着启动mQueue(类TimedEventQueue)。

之后修改mFlags的状态为PREPARING，表示现在正在准备处理文件的音视频流。

这里出现了一个AwesomeEvent，这是要闹什么？再看后面的代码，原来是定义了一个event对象，然后放到之前启动的mQueue中进行处理。
参见之前的TimedEventQueue分析文章，queue中处理的结果就是调用AwesomePlayer::onPrepareAsyncEvent函数。

继续看AwesomePlayer::onPrepareAsyncEvent

void AwesomePlayer::onPrepareAsyncEvent() {
    Mutex::Autolock autoLock(mLock);
    beginPrepareAsync_l();
}

AwesomePlayer::beginPrepareAsync_l()

void AwesomePlayer::beginPrepareAsync_l() {
    if (mFlags & PREPARE_CANCELLED) {
        ALOGI("prepare was cancelled before doing anything");
        abortPrepare(UNKNOWN_ERROR);
        return;
    }

    if (mUri.size() > 0) {
        status_t err = finishSetDataSource_l();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    if (mVideoTrack != NULL && mVideoSource == NULL) {
        status_t err = initVideoDecoder();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    if (mAudioTrack != NULL && mAudioSource == NULL) {
        status_t err = initAudioDecoder();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    modifyFlags(PREPARING_CONNECTED, SET);

    if (isStreamingHTTP()) {
        postBufferingEvent_l();
    } else {
        finishAsyncPrepare_l();
    }
}

这里我们分析文件类型的，uri的流媒体以后再说。代码分别初始化audio和video。

初始化video解码器：

AwesomePlayer::initVideoDecoder

status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {
    ATRACE_CALL();

    // Either the application or the DRM system can independently say
    // that there must be a hardware-protected path to an external video sink.
    // For now we always require a hardware-protected path to external video sink
    // if content is DRMed, but eventually this could be optional per DRM agent.
    // When the application wants protection, then
    //   (USE_SURFACE_ALLOC && (mSurface != 0) &&
    //   (mSurface->getFlags() & ISurfaceComposer::eProtectedByApp))
    // will be true, but that part is already handled by SurfaceFlinger.

#ifdef DEBUG_HDCP
    // For debugging, we allow a system property to control the protected usage.
    // In case of uninitialized or unexpected property, we default to "DRM only".
    bool setProtectionBit = false;
    char value[PROPERTY_VALUE_MAX];
    if (property_get("persist.sys.hdcp_checking", value, NULL)) {
        if (!strcmp(value, "never")) {
            // nop
        } else if (!strcmp(value, "always")) {
            setProtectionBit = true;
        } else if (!strcmp(value, "drm-only")) {
            if (mDecryptHandle != NULL) {
                setProtectionBit = true;
            }
        // property value is empty, or unexpected value
        } else {
            if (mDecryptHandle != NULL) {
                setProtectionBit = true;
            }
        }
    // can' read property value
    } else {
        if (mDecryptHandle != NULL) {
            setProtectionBit = true;
        }
    }
    // note that usage bit is already cleared, so no need to clear it in the "else" case
    if (setProtectionBit) {
        flags |= OMXCodec::kEnableGrallocUsageProtected;
    }
#else
    if (mDecryptHandle != NULL) {
        flags |= OMXCodec::kEnableGrallocUsageProtected;
    }
#endif
    ALOGV("initVideoDecoder flags=0x%x", flags);
    mVideoSource = OMXCodec::Create(
            mClient.interface(), mVideoTrack->getFormat(),
            false, // createEncoder
            mVideoTrack,
            NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);

    if (mVideoSource != NULL) {
        int64_t durationUs;
        if (mVideoTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {
            Mutex::Autolock autoLock(mMiscStateLock);
            if (mDurationUs < 0 || durationUs > mDurationUs) {
                mDurationUs = durationUs;
            }
        }

        status_t err = mVideoSource->start();

        if (err != OK) {
            ALOGE("failed to start video source");
            mVideoSource.clear();
            return err;
        }
    }

    if (mVideoSource != NULL) {
        const char *componentName;
        CHECK(mVideoSource->getFormat()
                ->findCString(kKeyDecoderComponent, &componentName));

        {
            Mutex::Autolock autoLock(mStatsLock);
            TrackStat *stat = &mStats.mTracks.editItemAt(mStats.mVideoTrackIndex);

            stat->mDecoderName = componentName;
        }

        static const char *kPrefix = "OMX.Nvidia.";
        static const char *kSuffix = ".decode";
        static const size_t kSuffixLength = strlen(kSuffix);

        size_t componentNameLength = strlen(componentName);

        if (!strncmp(componentName, kPrefix, strlen(kPrefix))
                && componentNameLength >= kSuffixLength
                && !strcmp(&componentName[
                    componentNameLength - kSuffixLength], kSuffix)) {
            modifyFlags(SLOW_DECODER_HACK, SET);
        }
    }

    return mVideoSource != NULL ? OK : UNKNOWN_ERROR;
}

initVideoDecoder的核心就是OMXCodec::Create。

initAudioDecoder()

status_t AwesomePlayer::initAudioDecoder() {
    ATRACE_CALL();

    sp<MetaData> meta = mAudioTrack->getFormat();

    const char *mime;
    CHECK(meta->findCString(kKeyMIMEType, &mime));
    // Check whether there is a hardware codec for this stream
    // This doesn't guarantee that the hardware has a free stream
    // but it avoids us attempting to open (and re-open) an offload
    // stream to hardware that doesn't have the necessary codec
    audio_stream_type_t streamType = AUDIO_STREAM_MUSIC;
    if (mAudioSink != NULL) {
        streamType = mAudioSink->getAudioStreamType();
    }

    mOffloadAudio = canOffloadStream(meta, (mVideoSource != NULL),
                                     isStreamingHTTP(), streamType);

    if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_RAW)) {
        ALOGV("createAudioPlayer: bypass OMX (raw)");
        mAudioSource = mAudioTrack;
    } else {
        // If offloading we still create a OMX decoder as a fall-back
        // but we don't start it
        mOmxSource = OMXCodec::Create(
                mClient.interface(), mAudioTrack->getFormat(),
                false, // createEncoder
                mAudioTrack);

        if (mOffloadAudio) {
            ALOGV("createAudioPlayer: bypass OMX (offload)");
            mAudioSource = mAudioTrack;
        } else {
            mAudioSource = mOmxSource;
        }
    }

    if (mAudioSource != NULL) {
        int64_t durationUs;
        if (mAudioTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {
            Mutex::Autolock autoLock(mMiscStateLock);
            if (mDurationUs < 0 || durationUs > mDurationUs) {
                mDurationUs = durationUs;
            }
        }

        status_t err = mAudioSource->start();

        if (err != OK) {
            mAudioSource.clear();
            mOmxSource.clear();
            return err;
        }
    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_QCELP)) {
        // For legacy reasons we're simply going to ignore the absence
        // of an audio decoder for QCELP instead of aborting playback
        // altogether.
        return OK;
    }

    if (mAudioSource != NULL) {
        Mutex::Autolock autoLock(mStatsLock);
        TrackStat *stat = &mStats.mTracks.editItemAt(mStats.mAudioTrackIndex);
        const char *component;
        if (!mAudioSource->getFormat()
                ->findCString(kKeyDecoderComponent, &component)) {
            component = "none";
        }

        stat->mDecoderName = component;
    }

    return mAudioSource != NULL ? OK : UNKNOWN_ERROR;
}

initAudioDecoder的核心函数同样是OMXCodec::Create.

因为AwesomePlayer底层使用的是OMX的编解码器，所以在获取音视频信息是就需要使用OMX模块。

OMX部分下一篇将解释一下。

回到beginPrepareAsync_l。

void AwesomePlayer::beginPrepareAsync_l() {
    if (mFlags & PREPARE_CANCELLED) {
        ALOGI("prepare was cancelled before doing anything");
        abortPrepare(UNKNOWN_ERROR);
        return;
    }

    if (mUri.size() > 0) {
        status_t err = finishSetDataSource_l();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    if (mVideoTrack != NULL && mVideoSource == NULL) {
        status_t err = initVideoDecoder();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    if (mAudioTrack != NULL && mAudioSource == NULL) {
        status_t err = initAudioDecoder();

        if (err != OK) {
            abortPrepare(err);
            return;
        }
    }

    modifyFlags(PREPARING_CONNECTED, SET);

    if (isStreamingHTTP()) {
        postBufferingEvent_l();
    } else {
        finishAsyncPrepare_l();
    }
}

因为我们是文件，所以继续走finishAsyncPrepare_l

finishAsyncPrepare_l

void AwesomePlayer::finishAsyncPrepare_l() {
    if (mIsAsyncPrepare) {
        if (mVideoSource == NULL) {
            notifyListener_l(MEDIA_SET_VIDEO_SIZE, 0, 0);
        } else {
            notifyVideoSize_l();
        }

        notifyListener_l(MEDIA_PREPARED);
    }

    mPrepareResult = OK;
    modifyFlags((PREPARING|PREPARE_CANCELLED|PREPARING_CONNECTED), CLEAR);
    modifyFlags(PREPARED, SET);
    mAsyncPrepareEvent = NULL;
    mPreparedCondition.broadcast();

    if (mAudioTearDown) {
        if (mPrepareResult == OK) {
            if (mExtractorFlags & MediaExtractor::CAN_SEEK) {
                seekTo_l(mAudioTearDownPosition);
            }

            if (mAudioTearDownWasPlaying) {
                modifyFlags(CACHE_UNDERRUN, CLEAR);
                play_l();
            }
        }
        mAudioTearDown = false;
    }
}

首先通知上层，发送MEDIA_SET_VIDEO_SIZE消息。

MediaPlayer.cpp

    case MEDIA_SET_VIDEO_SIZE:
        ALOGV("New video size %d x %d", ext1, ext2);
        mVideoWidth = ext1;
        mVideoHeight = ext2;

MediaPlayer.java

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

之后是通知MEDIA_PREPARED

MediaPlayer.cpp

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

MediaPlayer.java

    case MEDIA_PREPARED:
        scanInternalSubtitleTracks();
        if (mOnPreparedListener != null)
            mOnPreparedListener.onPrepared(mMediaPlayer);
        return;

接着清楚AwesomePlayer的状态，并设置为PREPARED。

然后通过mPreparedCondition.broadcast();，发送广播。通知等待这个条件的地方。

status_t AwesomePlayer::prepare_l() {
    if (mFlags & PREPARED) {
        return OK;
    }

    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;
    }

    mIsAsyncPrepare = false;
    status_t err = prepareAsync_l();

    if (err != OK) {
        return err;
    }

    while (mFlags & PREPARING) {
        mPreparedCondition.wait(mLock);
    }

    return mPrepareResult;
}

原来在prepare_l中函数一直等待这个条件的执行。prepareAsync_l中启动的mQueue消息队列使得执行进入另一个pthread线程，如果希望两个线程同步，那么可以使用condition方式进行通知。

prepare_l函数最后返回mPrepareResult，mPrepareResult在finishAsyncPrepare_l中mPrepareResult = OK;。

至此prepare的流程已经分析完成。下一篇将分析OMX模块。