Android開發——MediaPlayer源碼不完整分析

因為MediaPlayer的源碼比較多，所以只能來一個不完整分析了。

前段時間在工作中遇到一個問題就是MediaPlayer的播放網絡流媒體的時候，當沒有完全下載完畢的時，我們調用seekTo的時候會觸發OnCompletionListener，你們懂的咯，這樣就直接播放下一首了。挺糾結的，所以就決定看看mediaplayer的源碼咯，希望能從中找到解決的方法。

seekTo在MediaPlayer的調用流程如下圖：

seekTo 點擊上圖看大圖

在MediaPlayer.java中的seekTo是一個native修飾的方法

   1: /**

   2:  * Seeks to specified time position.

   3:  *

   4:  * @param msec the offset in milliseconds from the start to seek to

   5:  * @throws IllegalStateException if the internal player engine has not been

   6:  * initialized

   7:  */

   8: public native void seekTo(int msec) throws IllegalStateException;

好，我們來看看此方法的JNI是如何實現的。

   1: static void android_media_MediaPlayer_seekTo(JNIEnv *env, jobject thiz, int msec)

   2: {

   3:     sp<MediaPlayer> mp = getMediaPlayer(env, thiz);//獲取MediaPlayer實例

   4:     if (mp == NULL ) {

   5:         jniThrowException(env, "java/lang/IllegalStateException", NULL);

   6:         return;

   7:     }

   8:     LOGV("seekTo: %d(msec)", msec);

   9:     status_t result = mp->seekTo(msec);//1,調用MediaPlayer的seekTo方法

  10:     process_media_player_call( env, thiz, result, NULL, NULL );//2,處理MediaPlayer方法調用的返回結果

  11: }

1，調用MediaPlayer的seekTo方法

   1: status_t MediaPlayer::seekTo_l(int msec)

   2: {

   3:     LOGV("seekTo %d", msec);

   4:     if ((mPlayer != 0) && ( mCurrentState & ( MEDIA_PLAYER_STARTED | MEDIA_PLAYER_PREPARED | MEDIA_PLAYER_PAUSED |  MEDIA_PLAYER_PLAYBACK_COMPLETE) ) ) {

   5:         if ( msec < 0 ) {

   6:             LOGW("Attempt to seek to invalid position: %d", msec);

   7:             msec = 0;

   8:         } else if ((mDuration > 0) && (msec > mDuration)) {

   9:             LOGW("Attempt to seek to past end of file: request = %d, EOF = %d", msec, mDuration);

  10:             msec = mDuration;

  11:         }

  12:         // cache duration

  13:         mCurrentPosition = msec;

  14:         if (mSeekPosition < 0) {

  15:             getDuration_l(NULL);

  16:             mSeekPosition = msec;

  17:             //調用seekTo了

  18:             return mPlayer->seekTo(msec);

  19:         }

  20:         else {

  21:             LOGV("Seek in progress - queue up seekTo[%d]", msec);

  22:             return NO_ERROR;

  23:         }

  24:     }

  25:     LOGE("Attempt to perform seekTo in wrong state: mPlayer=%p, mCurrentState=%u", mPlayer.get(), mCurrentState);

  26:     return INVALID_OPERATION;

  27: }

  28:  

  29: status_t MediaPlayer::seekTo(int msec)

  30: {

  31:     mLockThreadId = getThreadId();

  32:     Mutex::Autolock _l(mLock);

  33:     status_t result = seekTo_l(msec);

  34:     mLockThreadId = 0;

  35:     return result;

  36: }

2，處理MediaPlayer方法調用的返回結果

   1: static void process_media_player_call(JNIEnv *env, jobject thiz, status_t opStatus, const char* exception, const char *message)

   2: {

   3:     if (exception == NULL) {  // Don't throw exception. Instead, send an event.

   4:         if (opStatus != (status_t) OK) {

   5:             sp<MediaPlayer> mp = getMediaPlayer(env, thiz);

   6:             if (mp != 0) mp->notify(MEDIA_ERROR, opStatus, 0);//調用MediaPlayer的notify

   7:         }

   8:     } else {  // Throw exception!

   9:         if ( opStatus == (status_t) INVALID_OPERATION ) {

  10:             jniThrowException(env, "java/lang/IllegalStateException", NULL);

  11:         } else if ( opStatus != (status_t) OK ) {

  12:             if (strlen(message) > 230) {

  13:                // if the message is too long, don't bother displaying the status code

  14:                jniThrowException( env, exception, message);

  15:             } else {

  16:                char msg[256];

  17:                 // append the status code to the message

  18:                sprintf(msg, "%s: status=0x%X", message, opStatus);

  19:                jniThrowException( env, exception, msg);

  20:             }

  21:         }

  22:     }

  23: }

接下來看看MediaPlayer的notify方法，這個方法主要是通過判斷MediaPlayer的狀態向我們的app發送回調：

   1: void MediaPlayer::notify(int msg, int ext1, int ext2)

   2: {

   3:     LOGV("message received msg=%d, ext1=%d, ext2=%d", msg, ext1, ext2);

   4:     bool send = true;

   5:     bool locked = false;

   6:  

   7:     // TODO: In the future, we might be on the same thread if the app is

   8:     // running in the same process as the media server. In that case,

   9:     // this will deadlock.

  10:     //

  11:     // The threadId hack below works around this for the care of prepare

  12:     // and seekTo within the same process.

  13:     // FIXME: Remember, this is a hack, it's not even a hack that is applied

  14:     // consistently for all use-cases, this needs to be revisited.

  15:      if (mLockThreadId != getThreadId()) {

  16:         mLock.lock();

  17:         locked = true;

  18:     }

  19:  

  20:     if (mPlayer == 0) {

  21:         LOGV("notify(%d, %d, %d) callback on disconnected mediaplayer", msg, ext1, ext2);

  22:         if (locked) mLock.unlock();   // release the lock when done.

  23:         return;

  24:     }

  25:  

  26:     switch (msg) {

  27:     case MEDIA_NOP: // interface test message

  28:         break;

  29:     case MEDIA_PREPARED://prepared結束

  30:         LOGV("prepared");

  31:         mCurrentState = MEDIA_PLAYER_PREPARED;

  32:         if (mPrepareSync) {

  33:             LOGV("signal application thread");

  34:             mPrepareSync = false;

  35:             mPrepareStatus = NO_ERROR;

  36:             mSignal.signal();

  37:         }

  38:         break;

  39:     case MEDIA_PLAYBACK_COMPLETE://播放完畢

  40:         LOGV("playback complete");

  41:         if (!mLoop) {

  42:             mCurrentState = MEDIA_PLAYER_PLAYBACK_COMPLETE;

  43:         }

  44:         break;

  45:     case MEDIA_ERROR://出錯

  46:         // Always log errors.

  47:         // ext1: Media framework error code.

  48:         // ext2: Implementation dependant error code.

  49:         LOGE("error (%d, %d)", ext1, ext2);

  50:         mCurrentState = MEDIA_PLAYER_STATE_ERROR;

  51:         if (mPrepareSync)

  52:         {

  53:             LOGV("signal application thread");

  54:             mPrepareSync = false;

  55:             mPrepareStatus = ext1;

  56:             mSignal.signal();

  57:             send = false;

  58:         }

  59:         break;

  60:     case MEDIA_INFO://logcat經常可以看到

  61:         // ext1: Media framework error code.

  62:         // ext2: Implementation dependant error code.

  63:         LOGW("info/warning (%d, %d)", ext1, ext2);

  64:         break;

  65:     case MEDIA_SEEK_COMPLETE://seek完畢

  66:         LOGV("Received seek complete");

  67:         if (mSeekPosition != mCurrentPosition) {

  68:             LOGV("Executing queued seekTo(%d)", mSeekPosition);

  69:             mSeekPosition = -1;

  70:             seekTo_l(mCurrentPosition);

  71:         }

  72:         else {

  73:             LOGV("All seeks complete - return to regularly scheduled program");

  74:             mCurrentPosition = mSeekPosition = -1;

  75:         }

  76:         break;

  77:     case MEDIA_BUFFERING_UPDATE://緩沖

  78:         LOGV("buffering %d", ext1);

  79:         break;

  80:     case MEDIA_SET_VIDEO_SIZE://設置視頻大小

  81:         LOGV("New video size %d x %d", ext1, ext2);

  82:         mVideoWidth = ext1;

  83:         mVideoHeight = ext2;

  84:         break;

  85:     default:

  86:         LOGV("unrecognized message: (%d, %d, %d)", msg, ext1, ext2);

  87:         break;

  88:     }

  89:  

  90:     sp<MediaPlayerListener> listener = mListener;

  91:     if (locked) mLock.unlock();

  92:  

  93:     // this prevents re-entrant calls into client code

  94:     if ((listener != 0) && send) {

  95:         Mutex::Autolock _l(mNotifyLock);

  96:         LOGV("callback application");

  97:         //調用監聽器，回調應用的監聽器

  98:         listener->notify(msg, ext1, ext2);

  99:         LOGV("back from callback");

 100:     }

 101: }

在監聽器的notify方法中，是通過jni“反向調用”MediaPlayer.java中的postEventFromNative，在通過mEventHandler根據不同的消息類型調用不同的監聽器。

   1: private static void postEventFromNative(Object mediaplayer_ref,

   2:                                           int what, int arg1, int arg2, Object obj)

   3:   {

   4:       MediaPlayer mp = (MediaPlayer)((WeakReference)mediaplayer_ref).get();

   5:       if (mp == null) {

   6:           return;

   7:       }

   8:  

   9:       if (mp.mEventHandler != null) {

  10:           Message m = mp.mEventHandler.obtainMessage(what, arg1, arg2, obj);

  11:           mp.mEventHandler.sendMessage(m);

  12:       }

  13:   }

OK，至此我們分析了seekTo的整個流程。其他方法的流程是很相似的，大家不妨親自去看看。

回到本文開頭的問題，通過觀察logcat得知是發生錯誤導致的，暫時未找到解決的辦法。如果你有解決方法不妨告訴我哦。

------------------------

03-20 03:08:13.302: ERROR/MediaPlayer(419): Attempt to call getDuration without a valid mediaplayer
03-20 03:08:13.365: ERROR/MediaPlayer(419): error (-38, 0)
03-20 03:08:13.372: ERROR/MediaPlayer(419): Attempt to call getDuration without a valid mediaplayer
03-20 03:08:13.512: ERROR/MediaPlayer(419): error (-38, 0)
03-20 03:08:13.522: DEBUG/dalvikvm(419): GC freed 3405 objects / 831072 bytes in 122ms
03-20 03:08:13.752: ERROR/MediaPlayer(419): Error (-38,0)
03-20 03:08:13.762: DEBUG/MultiPlayer(419): Error: -38,0
03-20 03:08:13.772: ERROR/MediaPlayer(419): Error (-38,0)
03-20 03:08:13.781: ERROR/MediaPlayer(419): pause called in state 0
03-20 03:08:13.781: ERROR/MediaPlayer(419): error (-38, 0)

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PS：“反向調用”就是在c/c++中通過jni來調用java中的方法或屬性。