Android多媒体开发【4】-- 上层mediaPlayer 是如何调到Stagefright的【2】【图解】

基于android 4.1.1 源码

【1】mediaserver 启动后会把media相关一些服务添加到servicemanager中，其中就有mediaPlayerService.这样应用启动前，系统就有了mediaPlayerService这个服务程序。   

int main(int argc, char** argv)
{
    sp proc(ProcessState::self());
    sp sm = defaultServiceManager();
    ALOGI("ServiceManager: %p", sm.get());
    AudioFlinger::instantiate();
    MediaPlayerService::instantiate();
    CameraService::instantiate();
    AudioPolicyService::instantiate();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}

【2】应用层 mediaPlayer=new MediaPlayer();  调用SDK中 MediaPlayer.java (frameworks\base\media\java\android\media\MediaPlayer.java)

  public 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;
        }

        /* Native setup requires a weak reference to our object.
         * It's easier to create it here than in C++.
         */
        native_setup(new WeakReference(this));
    }

通过JNI方式调用到framework层 android_media_MediaPlayer.cpp(\frameworks\base\media\jni\android_media_MediaPlayer.cpp)

static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
    ALOGV("native_setup");
    sp mp = new MediaPlayer();
    if (mp == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    // create new listener and give it to MediaPlayer
    sp listener = new JNIMediaPlayerListener(env, thiz, weak_this);
    mp->setListener(listener);

    // Stow our new C++ MediaPlayer in an opaque field in the Java object.
    setMediaPlayer(env, thiz, mp);
}

继而调用mediaplayer.cpp(frameworks\av\media\libmedia\mediaplayer.cpp)

【3】在整个应用程序的进程中，mediaplayer.cpp 中 setDataSource会从service manager中获得mediaPlayerService 服务，然后通过服务来创建player。这个player就是播放器的真实实例。

status_t MediaPlayer::setDataSource(const sp &source)
{
    ALOGV("setDataSource");
    status_t err = UNKNOWN_ERROR;
    const sp& service(getMediaPlayerService());
    if (service != 0) {
        sp player(service->create(getpid(), this, mAudioSessionId));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

【4】通过 getMediaPlayerService 得到的service其实是 BpMediaPlayerService，这是和 mediaPlayerService 进程中的BnMediaPlayerService 相对应负责binder通讯。BpMediaPlayerService中的create其实通过binder机制将CREATE消息发送出去。

 virtual sp create(
            pid_t pid, const sp& client, int audioSessionId) {
        Parcel data, reply;
        data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
        data.writeInt32(pid);
        data.writeStrongBinder(client->asBinder());
        data.writeInt32(audioSessionId);

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

在对面的 BnMediaPlayerService中，通过onTransact（）来接受这些消息。并把结果返回。

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch (code) {
        case CREATE: {
            CHECK_INTERFACE(IMediaPlayerService, data, reply);
            pid_t pid = data.readInt32();
            sp client =
                interface_cast(data.readStrongBinder());
            int audioSessionId = data.readInt32();
            sp player = create(pid, client, audioSessionId);
            reply->writeStrongBinder(player->asBinder());
            return NO_ERROR;
        } break;
        case MAKE_CRYPTO:
}

当发现是CREATE才真正调用了MediaPlayerService 中的create函数。在create函数中其实是创建了一个MediaPlayerService::Client的实例，也就是 说 MediaPlayerService会为每个client应用进程创建一个相应的MediaPlayerService::Client的实例，来提供服务。

sp MediaPlayerService::create(pid_t pid, const sp& client,
        int audioSessionId)
{
    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);
    }
    return c;
}

【5】这样 mediaplayer.cpp就得到了一个player的实例，对他来说这个实例和本地的其他类的实例没什么用法上的区别，殊不知其实这个实例是运行在另外一个进程中。实现这种假象的就是binder机制。获得这个实例后继续player->setDataSource().在 MediaPlayerService的进程中他的实际函数中，才会真正的创建Stagefright的具体实例。

status_t MediaPlayerService::Client::setDataSource(int fd, int64_t offset, int64_t length)
{
    ALOGV("setDataSource fd=%d, offset=%lld, length=%lld", fd, offset, length);
    struct stat sb;
    int ret = fstat(fd, &sb);
    if (ret != 0) {
        ALOGE("fstat(%d) failed: %d, %s", fd, ret, strerror(errno));
        return UNKNOWN_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);
        return UNKNOWN_ERROR;
    }
    if (offset + length > sb.st_size) {
        length = sb.st_size - offset;
        ALOGV("calculated length = %lld", length);
    }

    // Until re-transmit functionality is added to the existing core android
    // players, we use the special AAH TX player whenever we were configured for
    // retransmission.
    player_type playerType = getPlayerType(fd, offset, length);
    sp p = setDataSource_pre(playerType);
    if (p == NULL) {
        return NO_INIT;
    }

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

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) {
        return p;
    }

    if (!p->hardwareOutput()) {
        mAudioOutput = new AudioOutput(mAudioSessionId);
        static_cast(p.get())->setAudioSink(mAudioOutput);
    }

    return p;
}

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 = android::createPlayer(playerType, this, notify);
    }

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

    return p;
}

static sp createPlayer(player_type playerType, void* cookie,
        notify_callback_f notifyFunc)
{
    sp p;
    switch (playerType) {
        case SONIVOX_PLAYER:
            ALOGV(" create MidiFile");
            p = new MidiFile();
            break;
        case STAGEFRIGHT_PLAYER:
            ALOGV(" create StagefrightPlayer");
            p = new StagefrightPlayer;
            break;
        case NU_PLAYER:
            ALOGV(" create NuPlayer");
            p = new NuPlayerDriver;
            break;
        case TEST_PLAYER:
            ALOGV("Create Test Player stub");
            p = new TestPlayerStub();
            break;
        case AAH_RX_PLAYER:
            ALOGV(" create A@H RX Player");
            p = createAAH_RXPlayer();
            break;
        case AAH_TX_PLAYER:
            ALOGV(" create A@H TX Player");
            p = createAAH_TXPlayer();
            break;
        default:
            ALOGE("Unknown player type: %d", playerType);
            return NULL;
    }
    if (p != NULL) {
        if (p->initCheck() == NO_ERROR) {
            p->setNotifyCallback(cookie, notifyFunc);
        } else {
            p.clear();
        }
    }
    if (p == NULL) {
        ALOGE("Failed to create player object");
    }
    return p;
}

在上面中已经看不到opencore的影子了，creaPlayer 中会根据类型来创建播放器的实例。Stagefright的实例就是在这里创建的。

下一步我们能真正进入到Stagefright里了