如何使WakefulService（IntentService）等到MediaPlayer完成？

Question

I'm trying to make an app, which plays series of sounds using MediaPlayer, at scheduled times. To properly handle the wake lock and schedule the playback I used CommonsWare's WakefulIntentService .

Unfortunately, the IntentService's worker thread quits right after I call MediaPlayer.play() and neither MediaPlayer registered listeners are called. Instead, the exception is logged:

W/MessageQueue(6727): Handler (android.media.MediaPlayer$EventHandler) {4160d820} sending message to a Handler on a dead thread
W/MessageQueue(6727): java.lang.RuntimeException: Handler (android.media.MediaPlayer$EventHandler) {4160d820} sending message to a Handler on a dead thread
W/MessageQueue(6727):   at android.os.MessageQueue.enqueueMessage(MessageQueue.java:294)
W/MessageQueue(6727):   at android.os.Handler.sendMessageAtTime(Handler.java:473)
W/MessageQueue(6727):   at android.os.Handler.sendMessageDelayed(Handler.java:446)
W/MessageQueue(6727):   at android.os.Handler.sendMessage(Handler.java:383)
W/MessageQueue(6727):   at android.media.MediaPlayer.postEventFromNative(MediaPlayer.java:2063)
W/MessageQueue(6727):   at dalvik.system.NativeStart.run(Native Method)

As far as I understand, it is caused by the worker thread being already dead when MediaPlayer completes. If I pause the thread by means of the debugger and let the player complete, everything works fine.

In my listeners I not only release MediaPlayer's resources, but also use OnCompletionListener to do consecutive MediaPlayer.play() calls until the sound queue is empty.

I tried putting a wait loop right after the initial play() call, checking for a custom completion flag, but it seems to freeze because MediaPlayer's callbacks are called on the same thread play() was called.

The question is, how can I make the worker thread not quit before I let it do so (ie the has been processed and the onCompletion() method has been called for the last time?

Here is the code of my service:

public class SoundService extends WakefulIntentService {
    private static final TAG = "SoundService";
    private final Queue<SoundDescriptor> soundQueue = new ConcurrentLinkedQueue<SoundDescriptor>();

    private final OnCompletionListener onComediaPlayerletionListener = new OnComediaPlayerletionListener() {
        @Override
        public void onCompletion(MediaPlayer mediaPlayer) {
            mediaPlayer.reset();
            try {
                if (!playNextFromQueue(mediaPlayer)) {
                    Log.v(TAG, "Reached end of queue. Cleaning up.");
                    release();
                }
            } catch (Exception e) {
                Log.e(TAG, "Exception!", e)
                release();
            }
        }
    };
    private final OnErrorListener onErrorListener = new OnErrorListener() {
        @Override
        public boolean onError(MediaPlayer mediaPlayer, int what, int extra) {
            Log.v(TAG, "Error!!");
            release();
            return false;
        }
    };

    public SoundService() {
        // populate soundQueue
    }

    protected void doWakefulWork(Intent intent) {
        MediaPlayer mediaPlayer = new MediaPlayer();
        mediaPlayer.setOnComediaPlayerletionListener(onComediaPlayerletionListener);
        mediaPlayer.setOnErrorListener(onErrorListener);
        playNextFromQueue(mediaPlayer);
    }

    private boolean playNextFromQueue(MediaPlayer mediaPlayer) throws IllegalArgumentException, IllegalStateException, IOException {
        SoundDescriptor descriptor = soundQueue.poll();
        if (descriptor != null) {
            mediaPlayer.setDataSource(descriptor.getFileDescriptor(), descriptor.getStartOffset(), descriptor.getLength());
            descriptor.close();
            mediaPlayer.prepare();
            mediaPlayer.start();
            return true;
        } else {
            return false;
        }
    }
}

Answer 1

To properly handle the wake lock and schedule the playback I used CommonsWare's WakefulIntentService.

That's not an appropriate choice.

Unfortunately, the IntentService's worker thread quits right after I call MediaPlayer.play() and neither MediaPlayer registered listeners are called.

That's why it's not an appropriate choice. :-)

The question is, how can I make the worker thread not quit before I let it do so (ie the has been processed and the onCompletion() method has been called for the last time?

Don't use WakefulIntentService . Don't use IntentService . Use Service . Manage the WakeLock yourself, and call stopSelf() on the service when the audio is finished.

Answer 2

The problem with MediaPlayer is that it caches a Handler to the current thread when it is instantiated. In case the current thread has no Looper, it defaults to the main thread. It will use this handler to execute the listeners callbacks.

Lets say you instantiate the player and use it inside a service's thread. If you call the player methods through the service, and then this service thread dies, the player will try to call the callbacks to the registered listeners using a handler pointing to the deceased thread, and it will throw the exception shown in the question.

I've find MediaPlayer very tricky and unreliable. The simplest solution is to use the player in the main thread. They are short calls, shouldn't raise ANR's.