mediaplayer创建（一）

已于 2022-09-26 09:54:41 修改
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分类专栏： media 文章标签： android java ui
于 2022-09-21 10:07:21 首次发布
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本文链接：https://blog.youkuaiyun.com/mengxin00100/article/details/126167169
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        想了安卓多媒体的整体流程整体流程，理清mediaplayer、mediaplayerservice、mediacodec、omx之间的关系，还是从自己写个最简单的demo跟下流程，mediaplayer的播放其实很简单，主要分为如下6个步骤，代码如下：

 

（1）new 一个mediaplayer对象;

（2）调用setDataSource接口，数据流不同入参不同;

（3）调用setSurface给mediaplayer提供视频渲染区域有;

（4）调用prepareAsync接口，异步准备视频播放；

（5）调用start，开始视频播放;

 （6）调用stop，结束视频播放；

            if(mediaPlayer == null){
                mediaPlayer = new MediaPlayer();
            }
            try {
                if(!path.isEmpty()) {
                    mediaPlayer.setDataSource(path);
                    mediaPlayer.setSurface(surfaceHolder.getSurface());
                    mediaPlayer.prepareAsync();
                    mediaPlayer.setOnPreparedListener(mediaOnPreparedListener);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }

将demo跑起来后，根据demo 看看mediaplay整个过程都做了什么。

（1）先看MediaPlayer.java，先是加载了libmedia_jni.so, 看native_init的实现也只是初始化部分的动作，

    static {
        System.loadLibrary("media_jni");
        native_init();
    }
  private MediaPlayer(int sessionId) {
        super(new AudioAttributes.Builder().build(),
                AudioPlaybackConfiguration.PLAYER_TYPE_JAM_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);
        mOpenSubtitleSources = new Vector<InputStream>();

        AttributionSource attributionSource = AttributionSource.myAttributionSource();
        // set the package name to empty if it was null
        if (attributionSource.getPackageName() == null) {
            attributionSource = attributionSource.withPackageName("");
        }

        /* Native setup requires a weak reference to our object.
         * It's easier to create it here than in C++.
         */
        try (ScopedParcelState attributionSourceState = attributionSource.asScopedParcelState()) {
            native_setup(new WeakReference<MediaPlayer>(this), attributionSourceState.getParcel());
        }

        baseRegisterPlayer(sessionId);
    }

看 Mediaplayer 也只是一些初始的行为。

（2）setDataSource（MediaPlayer.java）->android_media_MediaPlayer_setDataSourceFD（android_media_MediaPlayer.cpp）->setDataSource（mediaplayer.cpp）-> setDataSource (IMediaPlayer.cpp), 如下是setDataSource的代码。看到如下代码应该就可以清楚为什么在mediaplayer后需要调用setDataSource，因为setDataSource 才是真正的开始创建播放器，这里也可以看到同mediaplayerservice的交互。

1 const sp<IMediaPlayerService> service(getMediaPlayerService()); 这一步其实是获取了BpMediaPlayerService 2 sp<IMediaPlayer> player(service->create(this, mAudioSessionId)); 这一步是通过BpMediaPlayerService 的create方法返回了BpMediaPlayer 3 attachNewPlayer 将player赋值给mPlayer

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
    ALOGV("setDataSource(%d, %" PRId64 ", %" PRId64 ")", fd, offset, length);
    status_t err = UNKNOWN_ERROR;

    // 获取BpMediaPlayerService
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        //通过BpMediaPlayerService 的create方法返回了BpMediaPlayer
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mAttributionSource));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(fd, offset, length))) {
            player.clear();
        }
        //将player赋值给mPlayer
        err = attachNewPlayer(player);
    }
    return err;
}
IMediaDeathNotifier::getMediaPlayerService()
{
    ALOGV("getMediaPlayerService");
    Mutex::Autolock _l(sServiceLock);
    if (sMediaPlayerService == 0) {
        sp<IServiceManager> sm = defaultServiceManager();
        sp<IBinder> binder;
        do {
            binder = sm->getService(String16("media.player"));
            if (binder != 0) {
                break;
            }
            ALOGW("Media player service not published, waiting...");
            usleep(500000); // 0.5 s
        } while (true);

        if (sDeathNotifier == NULL) {
            sDeathNotifier = new DeathNotifier();
        }
        binder->linkToDeath(sDeathNotifier);
        sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
    }
    ALOGE_IF(sMediaPlayerService == 0, "no media player service!?");
    return sMediaPlayerService;
}
void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}
    virtual sp<IMediaPlayer> create(
            const sp<IMediaPlayerClient>& client, audio_session_t audioSessionId,
            const AttributionSourceState& attributionSource) {
        Parcel data, reply;
        data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
        data.writeStrongBinder(IInterface::asBinder(client));
        data.writeInt32(audioSessionId);
        data.writeParcelable(attributionSource);

        remote()->transact(CREATE, data, &reply);
        return interface_cast<IMediaPlayer>(reply.readStrongBinder());
    }

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch (code) {
        case CREATE: {
            CHECK_INTERFACE(IMediaPlayerService, data, reply);
            sp<IMediaPlayerClient> client =
                interface_cast<IMediaPlayerClient>(data.readStrongBinder());
            audio_session_t audioSessionId = (audio_session_t) data.readInt32();
            AttributionSourceState attributionSource;
            status_t status = data.readParcelable(&attributionSource);
            if (status != NO_ERROR) {
                return status;
            }
            sp<IMediaPlayer> player = create(client, audioSessionId, attributionSource);
            reply->writeStrongBinder(IInterface::asBinder(player));
            return NO_ERROR;
        } break;
    …………
    }
}

这一块也是C/S架构的核心，主要围绕C++层MediaPlayer通过BpMediaPlayerService这个proxy对象，经过IPC与远程服务端MediaPlayerService（BnMediaPlayerService）通信，完成C/S架构。（Bpxxx是一个代理外包，所有真正的工作是再Bnxxx里面做的，这里p指proxy，n指native, BpMediaPlayerService\BnMediaPlayerService都在IMediaPlayerService.cpp中实现）

当Server端接受到Client端的请求，MediaPlayerService会为每一个Client进程创建一个会话，这里就是new一个MediaPlayer::Client对象与其交互，然后这个对象再根据Client端请求的资源类型去判断创建了什么类型的Player。在mediaplayerservice:create中，创建了一个MediaPlayerService::Client实例，也就是说，MediaPlayerService会为每个client应用程序创建一个相应的MediaPlayerServece::Client，来提供服务。也就是说，mediaplayer.cpp中执行的setDataSource函数，在MediaPlayerService.cpp中，实际上只是创建了一个client，并将两者对应起来，如果再次执行setDataSource函数，会创建第二个client。同时看这个函数的返回值类型，为sp<IMediaPlayer>，而在函数的最后，return c，这个c的类型是sp<Client>，从而从侧面也证明了两者几乎是等同的关系。

sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client,
        audio_session_t audioSessionId, const AttributionSourceState& attributionSource)
{
    int32_t connId = android_atomic_inc(&mNextConnId);
    // TODO b/182392769: use attribution source util
    AttributionSourceState verifiedAttributionSource = attributionSource;
    verifiedAttributionSource.pid = VALUE_OR_FATAL(
        legacy2aidl_pid_t_int32_t(IPCThreadState::self()->getCallingPid()));
    verifiedAttributionSource.uid = VALUE_OR_FATAL(
        legacy2aidl_uid_t_int32_t(IPCThreadState::self()->getCallingUid()));

    sp<Client> c = new Client(
            this, verifiedAttributionSource, connId, client, audioSessionId);

    ALOGV("Create new client(%d) from %s, ", connId,
        verifiedAttributionSource.toString().c_str());

    wp<Client> w = c;
    {
        Mutex::Autolock lock(mLock);
        mClients.add(w);
    }
    return c;
}

再往回看mediaplayer.cpp的setDataSource，在service->create获取mediaplayerservice创建的

client后执行player（即client）->setDataSource，这里才开始创建真正的player，在MediaPlayerFactory通过最高分数获取最匹配播放器类型，目前MediaPlayerFactory还保留的medialayer有NuPlayerFactory、TestPlayerFactory、IC厂商自己的播放器。

status_t MediaPlayerService::Client::setDataSource(int fd, int64_t offset, int64_t length)
{
…………
    player_type playerType = MediaPlayerFactory::getPlayerType(this,
                                                               fd,
                                                               offset,
                                                               length);
    sp<MediaPlayerBase> p = setDataSource_pre(playerType);
    if (p == NULL) {
        return NO_INIT;
    }

    // now set data source
    return mStatus = setDataSource_post(p, p->setDataSource(fd, offset, length));
}

根据获取的播放器类型创建对应的播放器，安装原生的话是创建Nuplayer。

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->hardwareOutput()) {
        mAudioOutput = new AudioOutput(mAudioSessionId, mAttributionSource,
                mAudioAttributes, mAudioDeviceUpdatedListener);
        static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
    }
    return p;
}

sp<MediaPlayerBase> MediaPlayerService::Client::createPlayer(player_type playerType)
{
    // determine if we have the right player type
    sp<MediaPlayerBase> p = getPlayer();
    if ((p != NULL) && (p->playerType() != playerType)) {
        ALOGV("delete player");
        p.clear();
    }
    if (p == NULL) {
        p = MediaPlayerFactory::createPlayer(playerType, mListener,
            VALUE_OR_FATAL(aidl2legacy_int32_t_pid_t(mAttributionSource.pid)));
    }

    if (p != NULL) {
        p->setUID(VALUE_OR_FATAL(aidl2legacy_int32_t_uid_t(mAttributionSource.uid)));
    }

    return p;
}


status_t NuPlayerDriver::setDataSource(int fd, int64_t offset, int64_t length) {
    ALOGV("setDataSource(%p) file(%d)", this, fd);
    Mutex::Autolock autoLock(mLock);

    if (mState != STATE_IDLE) {
        return INVALID_OPERATION;
    }

    mState = STATE_SET_DATASOURCE_PENDING;

    mPlayer->setDataSourceAsync(fd, offset, length);

    while (mState == STATE_SET_DATASOURCE_PENDING) {
        mCondition.wait(mLock);
    }

    return mAsyncResult;
}

void NuPlayer::setDataSourceAsync(int fd, int64_t offset, int64_t length) {
    sp<AMessage> msg = new AMessage(kWhatSetDataSource, this);

    sp<AMessage> notify = new AMessage(kWhatSourceNotify, this);

    sp<GenericSource> source =
            new GenericSource(notify, mUIDValid, mUID, mMediaClock);

    ALOGV("setDataSourceAsync fd %d/%lld/%lld source: %p",
            fd, (long long)offset, (long long)length, source.get());

    status_t err = source->setDataSource(fd, offset, length);

    if (err != OK) {
        ALOGE("Failed to set data source!");
        source = NULL;
    }

    msg->setObject("source", source);
    msg->post();
    mDataSourceType = DATA_SOURCE_TYPE_GENERIC_FD;
}

首先mediaplayer.cpp中发出setDataSource的函数 ---> 到达IMediaPlayer的Bp端 ---> 传到IMediaPlayer的Bn端 ---> 到达MediaPlayerService::Client端的MediaPlayerService::Client::setDataSource函数 ---> NuPlayerDriver::setDataSource ---> NuPlayer: setDataSourceAsync

这里就基本将mediaplayer跟mediaplayerservice的交互理清了。

mediaplayer跟mediaplayerservice 交互可以看如下的博客，写的极好：

2. Android MultiMedia框架完全解析 - MediaPlayer的C/S架构与Binder机制实现_yanbixing123的博客-优快云博客

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