Android——Mediaplay 框架调用

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