android - onPostExecute not being called in AsyncTask (Handler runtime exception)

Question

I have an AsyncTask that fetches some data and then updates the UI with this new data. It has been working fine for months, but I recently added a feature that displays a notification when there is new data. Now when my app is launched through the notification, sometimes I get this exception and onPostExecute is not called.

This is what happens when the app is launched:

1) Expand the UI and find views

2) Cancel the alarm (through AlarmManager) that checks for new data and reset the alarm. (This is so that if the user disables the alarm it is cancelled before the next time he/she reboots.)

3) Start the AsyncTask. If the app was launched from the notification, pass in a little bit of the data and then cancel the notification.

I'm stuck on what could be causing this exception. It seems that the exception is from the AsyncTask code, so I'm not sure how I can fix it.

Thanks!

Here is the exception:

I/My App(  501): doInBackground exiting
W/MessageQueue(  501): Handler{442ba140} sending message to a Handler on a dead thread
W/MessageQueue(  501): java.lang.RuntimeException: Handler{442ba140} sending message to a Handler on a dead thread
W/MessageQueue(  501):  at android.os.MessageQueue.enqueueMessage(MessageQueue.java:179)
W/MessageQueue(  501):  at android.os.Handler.sendMessageAtTime(Handler.java:457)
W/MessageQueue(  501):  at android.os.Handler.sendMessageDelayed(Handler.java:430)
W/MessageQueue(  501):  at android.os.Handler.sendMessage(Handler.java:367)
W/MessageQueue(  501):  at android.os.Message.sendToTarget(Message.java:348)
W/MessageQueue(  501):  at android.os.AsyncTask$3.done(AsyncTask.java:214)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask$Sync.innerSet(FutureTask.java:252)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask.set(FutureTask.java:112)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:310)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask.run(FutureTask.java:137)
W/MessageQueue(  501):  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
W/MessageQueue(  501):  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
W/MessageQueue(  501):  at java.lang.Thread.run(Thread.java:1096)

EDIT: Here is my onCreate method in my main activity (the one opened by the notification). There are some onClickListeners that I omitted to save space. I don't think they should have any effect, since the buttons they are attached to are not being pressed.

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Call the parent

    setContentView(R.layout.main); // Create the UI from the XML file

    // Find the UI elements
    controls = (SlidingDrawer) findViewById(R.id.drawer); // Contains the
    // buttons
    // comic = (ImageView) findViewById(R.id.comic); // Displays the comic
    subtitle = (TextView) findViewById(R.id.subtitleTxt); // Textbox for the
    // subtitle
    prevBtn = (Button) findViewById(R.id.prevBtn); // The previous button
    nextBtn = (Button) findViewById(R.id.nextBtn); // The next button
    randomBtn = (Button) findViewById(R.id.randomBtn); // The random button
    fetchBtn = (Button) findViewById(R.id.comicFetchBtn); // The go to specific id button
    mostRecentBtn = (Button) findViewById(R.id.mostRecentBtn); // The button to go to the most recent comic
    comicNumberEdtTxt = (EditText) findViewById(R.id.comicNumberEdtTxt); // The text box to Zooming image view setup
    zoomControl = new DynamicZoomControl();

    zoomListener = new LongPressZoomListener(this);
    zoomListener.setZoomControl(zoomControl);

    zoomComic = (ImageZoomView) findViewById(R.id.zoomComic);
    zoomComic.setZoomState(zoomControl.getZoomState());
    zoomComic.setImage(BitmapFactory.decodeResource(getResources(), R.drawable.defaultlogo));
    zoomComic.setOnTouchListener(zoomListener);

    zoomControl.setAspectQuotient(zoomComic.getAspectQuotient());

    resetZoomState();

    // enter the new id
    imm = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE); // Used to hide the soft keyboard

    Log.i(LOG_TAG, "beginning loading of first comic");
    int notificationComicNumber = getIntent().getIntExtra("comic", -1);
    Log.i(LOG_TAG, "comic number from intent: " + notificationComicNumber);
    if (notificationComicNumber == -1) {
        fetch = new MyFetcher(this, zoomComic, subtitle, controls, comicNumberEdtTxt, imm, zoomControl);
        fetch.execute(MyFetcher.LAST_DISPLAYED_COMIC);
    } else {
        fetch = new MyFetcher(this, zoomComic, subtitle, controls, comicNumberEdtTxt, imm, zoomControl);
        fetch.execute(notificationComicNumber);
        ((NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE)).cancelAll();
    }
    Log.i(LOG_TAG, "ending loading of new comic");

    Log.i(LOG_TAG, "first run checks beginning");
    // Get SharedPreferences
    prefs = getSharedPreferences("prefs", Context.MODE_PRIVATE);

    // Check if this is the first run of the app for this version
    if (prefs.getBoolean("firstRun-" + MAJOR_VERSION_NUMBER, true)) {
        prefs.edit().putBoolean("firstRun-" + MAJOR_VERSION_NUMBER, false).commit();
        firstRunVersionDialog();
    }

    // Check if this is the first run of the app
    if (prefs.getBoolean("firstRun", true)) {
        prefs.edit().putBoolean("firstRun", false).commit();
        firstRunDialog();
    }
    Log.i(LOG_TAG, "First run checks done");

            // OnClickListener s for the buttons omitted to save space

EDIT 2: I've been digging through Android source code tracking down where the exception is coming from. This is lines 456 and 457 of sendMessageAtTime in Handler:

msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);

And this is enqueueMessage from MessageQueue:

    final boolean enqueueMessage(Message msg, long when) {
        if (msg.when != 0) {
            throw new AndroidRuntimeException(msg
                    + " This message is already in use.");
        }
        if (msg.target == null && !mQuitAllowed) {
            throw new RuntimeException("Main thread not allowed to quit");
        }
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                    msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            } else if (msg.target == null) {
                mQuiting = true;
            }

            msg.when = when;
            //Log.d("MessageQueue", "Enqueing: " + msg);
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                msg.next = p;
                mMessages = msg;
                this.notify();
            } else {
                Message prev = null;
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
                msg.next = prev.next;
                prev.next = msg;
                this.notify();
            }
        }
        return true;
    }

I'm a little confused about what mQuiting is, but it looks like the previous time enqueueMessage was called msg.target was null.

Answer 1

This is due to a bug in AsyncTask in the Android framework. AsyncTask.java has the following code:

private static final InternalHandler sHandler = new InternalHandler();

It expects this to be initialized on the main thread, but that is not guaranteed since it will be initialized on whichever thread happens to cause the class to run its static initializers. I reproduced this issue where the Handler references a worker thread.

A common pattern that causes this to happen is using the class IntentService. The C2DM sample code does this.

A simple workaround is to add the following code to the application's onCreate method:

Class.forName("android.os.AsyncTask");

This will force AsyncTask to be initialized in the main thread. I filed a bug on this in the android bug database. See http://code.google.com/p/android/issues/detail?id=20915.