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。

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

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voidMediaPlayerService::instantiate(){
defaultServiceManager()->addService(
String16("media.player"),newMediaPlayerService());
}

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

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publicMediaPlayer(){
Looperlooper;
if((looper=Looper.myLooper())!=null){
mEventHandler=newEventHandler(this,looper);
}elseif((looper=Looper.getMainLooper())!=null){
mEventHandler=newEventHandler(this,looper);
}else{
mEventHandler=null;
}
/*Nativesetuprequiresaweakreferencetoourobject.
*It'seasiertocreateitherethaninC++.
*/
native_setup(newWeakReference<MediaPlayer>(this));
}

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

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staticvoid
android_media_MediaPlayer_native_setup(JNIEnv*env,jobjectthiz,jobjectweak_this)
{
ALOGV("native_setup");
sp<MediaPlayer>mp=newMediaPlayer();
if(mp==NULL){
jniThrowException(env,"java/lang/RuntimeException","Outofmemory");
return;
}
//createnewlistenerandgiveittoMediaPlayer
sp<JNIMediaPlayerListener>listener=newJNIMediaPlayerListener(env,thiz,weak_this);
mp->setListener(listener);
//StowournewC++MediaPlayerinanopaquefieldintheJavaobject.
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。

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status_tMediaPlayer::setDataSource(constsp<IStreamSource>&source)
{
ALOGV("setDataSource");
status_terr=UNKNOWN_ERROR;
constsp<IMediaPlayerService>&service(getMediaPlayerService());
if(service!=0){
sp<IMediaPlayer>player(service->create(getpid(),this,mAudioSessionId));
if((NO_ERROR!=doSetRetransmitEndpoint(player))||
(NO_ERROR!=player->setDataSource(source))){
player.clear();
}
err=attachNewPlayer(player);
}
returnerr;
}

【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。

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virtualsp<IMediaPlayer>create(
pid_tpid,constsp<IMediaPlayerClient>&client,intaudioSessionId){
Parceldata,reply;
data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
data.writeInt32(pid);
data.writeStrongBinder(client->asBinder());
data.writeInt32(audioSessionId);
remote()->transact(CREATE,data,&reply);
returninterface_cast<IMediaPlayer>(reply.readStrongBinder());
}

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

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status_tBnMediaPlayerService::onTransact(
uint32_tcode,constParcel&data,Parcel*reply,uint32_tflags)
{
switch(code){
caseCREATE:{
CHECK_INTERFACE(IMediaPlayerService,data,reply);
pid_tpid=data.readInt32();
sp<IMediaPlayerClient>client=
interface_cast<IMediaPlayerClient>(data.readStrongBinder());
intaudioSessionId=data.readInt32();
sp<IMediaPlayer>player=create(pid,client,audioSessionId);
reply->writeStrongBinder(player->asBinder());
returnNO_ERROR;
}break;
caseMAKE_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。

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sp<IMediaPlayer>MediaPlayerService::create(pid_tpid,constsp<IMediaPlayerClient>&client,
intaudioSessionId)
{
int32_tconnId=android_atomic_inc(&mNextConnId);
sp<Client>c=newClient(
this,pid,connId,client,audioSessionId,
IPCThreadState::self()->getCallingUid());
ALOGV("Createnewclient(%d)frompid%d,uid%d,",connId,pid,
IPCThreadState::self()->getCallingUid());
wp<Client>w=c;
{
Mutex::Autolocklock(mLock);
mClients.add(w);
}
returnc;
}

【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。 [cpp] view plain copy

status_tMediaPlayerService::Client::setDataSource(intfd,int64_toffset,int64_tlength)
{
ALOGV("setDataSourcefd=%d,offset=%lld,length=%lld",fd,offset,length);
structstatsb;
intret=fstat(fd,&sb);
if(ret!=0){
ALOGE("fstat(%d)failed:%d,%s",fd,ret,strerror(errno));
returnUNKNOWN_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("offseterror");
::close(fd);
returnUNKNOWN_ERROR;
}
if(offset+length>sb.st_size){
length=sb.st_size-offset;
ALOGV("calculatedlength=%lld",length);
}
//Untilre-transmitfunctionalityisaddedtotheexistingcoreandroid
//players,weusethespecialAAHTXplayerwheneverwewereconfiguredfor
//retransmission.
player_typeplayerType=getPlayerType(fd,offset,length);
sp<MediaPlayerBase>p=setDataSource_pre(playerType);
if(p==NULL){
returnNO_INIT;
}
//nowsetdatasource
setDataSource_post(p,p->setDataSource(fd,offset,length));
returnmStatus;
}

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sp<MediaPlayerBase>MediaPlayerService::Client::setDataSource_pre(
player_typeplayerType)
{
ALOGV("playertype=%d",playerType);
//createtherighttypeofplayer
sp<MediaPlayerBase>p=createPlayer(playerType);
if(p==NULL){
returnp;
}
if(!p->hardwareOutput()){
mAudioOutput=newAudioOutput(mAudioSessionId);
static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
}
returnp;
}

[cpp] view plain copy

sp<MediaPlayerBase>MediaPlayerService::Client::createPlayer(player_typeplayerType)
{
//determineifwehavetherightplayertype
sp<MediaPlayerBase>p=mPlayer;
if((p!=NULL)&&(p->playerType()!=playerType)){
ALOGV("deleteplayer");
p.clear();
}
if(p==NULL){
p=android::createPlayer(playerType,this,notify);
}
if(p!=NULL){
p->setUID(mUID);
}
returnp;
}

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staticsp<MediaPlayerBase>createPlayer(player_typeplayerType,void*cookie,
notify_callback_fnotifyFunc)
{
sp<MediaPlayerBase>p;
switch(playerType){
caseSONIVOX_PLAYER:
ALOGV("createMidiFile");
p=newMidiFile();
break;
caseSTAGEFRIGHT_PLAYER:
ALOGV("createStagefrightPlayer");
p=newStagefrightPlayer;
break;
caseNU_PLAYER:
ALOGV("createNuPlayer");
p=newNuPlayerDriver;
break;
caseTEST_PLAYER:
ALOGV("CreateTestPlayerstub");
p=newTestPlayerStub();
break;
caseAAH_RX_PLAYER:
ALOGV("create[email protected]RXPlayer");
p=createAAH_RXPlayer();
break;
caseAAH_TX_PLAYER:
ALOGV("create[email protected]TXPlayer");
p=createAAH_TXPlayer();
break;
default:
ALOGE("Unknownplayertype:%d",playerType);
returnNULL;
}
if(p!=NULL){
if(p->initCheck()==NO_ERROR){
p->setNotifyCallback(cookie,notifyFunc);
}else{
p.clear();
}
}
if(p==NULL){
ALOGE("Failedtocreateplayerobject");
}
returnp;
}

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

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