android binder简单理解三

来源：互联网发布：中国联通网络套餐种类编辑：程序博客网时间：2024/05/16 09:21

前面两章已经把meidaserver和servicemanager的通信建立了，并且mediaserver向servicemanager注册了service media.player。现在来简单看看media的client是怎么和mediaserver通信的。

frameworks/base/media/java/android/media/MediaPlayer.java

 593     public MediaPlayer() {。。。。。。 //这些都不是我要关注的东西 609         /* Native setup requires a weak reference to our object. 610          * It's easier to create it here than in C++. 611          */ 612         native_setup(new WeakReference<MediaPlayer>(this));//这个看起来和mediaserver有关 613     }

native_setup, jni的东西，去看看jni

framework/base/media/jni/android_media_MediaPlayer.cpp

 640 static void 641 android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this) 642 { 643     ALOGV("native_setup"); 644     sp<MediaPlayer> mp = new MediaPlayer();//这个从frameworks/av/include/media/mediaplayer.h这里来，下面具体看看 645     if (mp == NULL) { 646         jniThrowException(env, "java/lang/RuntimeException", "Out of memory"); 647         return; 648     } 650     // create new listener and give it to MediaPlayer 651     sp<JNIMediaPlayerListener> listener = new JNIMediaPlayerListener(env, thiz, weak_this);//相当于注册callback， 652     mp->setListener(listener); 653 654     // Stow our new C++ MediaPlayer in an opaque field in the Java object. 655     setMediaPlayer(env, thiz, mp);//这个也简单，就是把mp保存起来 656 }

frameworks/av/include/media/mediaplayer.h

193 class MediaPlayer : public BnMediaPlayerClient,194                     public virtual IMediaDeathNotifier

可见MediaPlayer继承了IMediaDeathNotifier

new MediaPlayer(),看看它的构造函数：

frameworks/av/media/libmedia/mediaplayer.cpp

 45 MediaPlayer::MediaPlayer() 46 { 47     ALOGV("constructor"); 48     mListener = NULL; 49     mCookie = NULL; 50     mStreamType = AUDIO_STREAM_MUSIC; 51     mCurrentPosition = -1; 52     mSeekPosition = -1; 53     mCurrentState = MEDIA_PLAYER_IDLE; 54     mPrepareSync = false; 55     mPrepareStatus = NO_ERROR;56     mLoop = false; 57     mLeftVolume = mRightVolume = 1.0; 58     mVideoWidth = mVideoHeight = 0; 59     mLockThreadId = 0; 60     mAudioSessionId = AudioSystem::newAudioSessionId(); 61     AudioSystem::acquireAudioSessionId(mAudioSessionId); 62     mSendLevel = 0; 63     mRetransmitEndpointValid = false; 64 }

没做什么实际的东西，还没要和service通信。一般而言new了一个player之后会调用setDataSource(String)的，这个应该要和service通信了吧？

java层就不看了，它一定会同过jni调用下去的，直接看jni中的东西

228 static void 229 android_media_MediaPlayer_setDataSourceFD(JNIEnv *env, jobject thiz, jobject fileDescriptor, jlong offset, jlong length) 230 { 231     sp<MediaPlayer> mp = getMediaPlayer(env, thiz);//上面new的 Mediaplayer 232     if (mp == NULL ) { 233         jniThrowException(env, "java/lang/IllegalStateException", NULL); 234         return; 235     } 236  237     if (fileDescriptor == NULL) {。。。。。。241     int fd = jniGetFDFromFileDescriptor(env, fileDescriptor); 242     ALOGV("setDataSourceFD: fd %d", fd); 243     process_media_player_call( env, thiz, mp->setDataSource(fd, offset, length), "java/io/IOException", "setDataSourceFD failed." );//这个调用很特别，关于jni的原理现在还不懂，但可以知道它进入了frameworks/av/media/libmedia/mediaplayer.cpp中的 setDataSource 244 }

frameworks/av/media/libmedia/mediaplayer.cpp

157 status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)158 {159     ALOGV("setDataSource(%d, %lld, %lld)", fd, offset, length);160     status_t err = UNKNOWN_ERROR;161     const sp<IMediaPlayerService>& service(getMediaPlayerService());//嗯，这里去获取service162     if (service != 0) {163         sp<IMediaPlayer> player(service->create(this, mAudioSessionId));164         if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||165             (NO_ERROR != player->setDataSource(fd, offset, length))) {166             player.clear();166             player.clear();167         }168         err = attachNewPlayer(player);169     }170     return err;171 }

因为MediaPlayer继承了IMediaDeathNotifier，其实getMediaPlayerService()是在IMediaDeathNotifier实现的。

frameworks/av/media/libmedia/IMediaDeathNotifier.cpp

35 IMediaDeathNotifier::getMediaPlayerService() 36 { 37     ALOGV("getMediaPlayerService"); 38     Mutex::Autolock _l(sServiceLock); 39     if (sMediaPlayerService == 0) { 40         sp<IServiceManager> sm = defaultServiceManager();//这个上一篇有说了 41         sp<IBinder> binder; 42         do { 43             binder = sm->getService(String16("media.player"));/*这个和上一篇的流程基本一样只是ADD_SERVICE_TRANSACTION变成了CHECK_SERVICE_TRANSACTION。这会导致最终servicemanager进入service_manager.c 233     switch(txn->code) {234     case SVC_MGR_GET_SERVICE:235     case SVC_MGR_CHECK_SERVICE:236         s = bio_get_string16(msg, &len);237         ptr = do_find_service(bs, s, len, txn->sender_euid);238         if (!ptr)返回的binder就是经过转换后的<span><span class="keyword">new</span><span> MediaPlayerService()</span></span>*/ 44             if (binder != 0) { 45                 break; 45                 break; 46             } 47             ALOGW("Media player service not published, waiting..."); 48             usleep(500000); // 0.5 s 49         } while (true); 50  51         if (sDeathNotifier == NULL) { 52             sDeathNotifier = new DeathNotifier(); 53         } 54         binder->linkToDeath(sDeathNotifier); 55         sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);/*经过interface_cast之后，binder就变成了BpMediaPlayerService，经过如下interface_cast<IMediaPlayerService>(MediaPlayerService)-->IMediaPlayerService::asInterface(MediaPlayerService)-->(/frameworks/native/include/binder/IInterface.h)89     android::sp<I##INTERFACE> I##INTERFACE::asInterface(                \90             const android::sp<android::IBinder>& obj)                   \ -->android::sp<IMediaPlayerService> IMediaPlayerService::asInterface(&MediaPlayerService){//经过代入的代码 92         android::sp<I##MediaPlayerService> intr;                                 \ 93         if (obj != NULL) {                                              \ 94             intr = static_cast<IMediaPlayerServcie*>(  /*进这里obj（IBinder）和IMediaPlayerService的descriptor当然不一样,intr==null*/                        \ 95                 obj->queryLocalInterface(                               \ 96                         IMediaPlayerService::descriptor).get());               \ 97             if (intr == NULL) {                                         \ 98                 intr = new BpMediaPlayerService(IBinder);                          <pre name="code" class="cpp">\ 99             }

100 }
101 return intr;//就在这里返回了BpMediaPlayerService

}
*/

56 } 57 ALOGE_IF(sMediaPlayerService == 0, "no media player service!?"); 58 return sMediaPlayerService; 59 }

所以sMediaPlayerService其实就是BpMediaPlayerService

BpMediaPlayerService

继承BpInterface

 54 class BpMediaPlayerService: public BpInterface<IMediaPlayerService> 55 { 56 public: 57     BpMediaPlayerService(const sp<IBinder>& impl) 58         : BpInterface<IMediaPlayerService>(impl) 59     { 60     }

 62 template<typename INTERFACE> 63 class BpInterface : public INTERFACE, public BpRefBase 64 { 65 public: 66                                 BpInterface(const sp<IBinder>& remote); 67

到这里remote被赋值为IBinder

终于从const sp<IMediaPlayerService>& service(getMediaPlayerService())出来了，返回了一个 BpMediaPlayerService对象

回到了setDataSource函数，那接着看，

163 sp<IMediaPlayer> player(service->create(this, mAudioSessionId));

这句就变成了sp<IMediaPlayer> player(BpMediaPlayerService->create(this, mAudioSessionId))

 70     virtual sp<IMediaPlayer> create( 71             const sp<IMediaPlayerClient>& client, int audioSessionId) { 72         Parcel data, reply; 73         data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor()); 74         data.writeStrongBinder(client->asBinder()); 75         data.writeInt32(audioSessionId); 76  77         remote()->transact(CREATE, data, &reply); 78         return interface_cast<IMediaPlayer>(reply.readStrongBinder()); 79     }

因为remote是IBinder,所以: IBinder->transact,当然IBinder的transact函数只在它的继承函数BpBinder实现，所以又变成了，BpBinder->transcat,

159 status_t BpBinder::transact(160     uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)161 {162     // Once a binder has died, it will never come back to life.163     if (mAlive) {164         status_t status = IPCThreadState::self()->transact(165             mHandle, code, data, reply, flags);//mHandle又是什么？这个当然是刚刚获取MediaPlayerService时返回的handle了这样在驱动中就可以找到MediaPlayerService了 166         if (status == DEAD_OBJECT) mAlive = 0;167         return status;168     }169 170     return DEAD_OBJECT;171 }

这样就到了MediaServicePlayer端的create，

 260 sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client, 261         int audioSessionId) 262 { 263     pid_t pid = IPCThreadState::self()->getCallingPid(); 264     int32_t connId = android_atomic_inc(&mNextConnId); 265  266     sp<Client> c = new Client( 267             this, pid, connId, client, audioSessionId, 268             IPCThreadState::self()->getCallingUid()); 269  270     ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid, 271          IPCThreadState::self()->getCallingUid()); 272  273     wp<Client> w = c; 274     { 275         Mutex::Autolock lock(mLock); 276         mClients.add(w); 277     } 278     return c; 279 }

就这样MediaPlayerService的client就和service 通信上了。

0 0