AsyncTask called from Handler will not execute doInBackground

Question

The application I'm working on is using a background thread to download a list of images through a API, then display the images in a slideshow.

There is a background task (currently AsyncTask) to periodically fetch the new images.

I'm not getting any error messages about the wrong Thread etc, it's just that the AsyncTasks's second instance will not run the doInBackground method.

Here is some code from the Activity:

private DownloadTask mDownloadTask = null;
private Handler mHandler;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            if(mDownloadTask != null) {
                mDownloadTask.cancel(true);
            }
            mDownloadTask = new DownloadTask();
            mDownloadTask.execute((Void[]) null);
        }
    };

    mDownloadTask = new DownloadTask();
    mDownloadTask.execute((Void[]) null);
}

The DownloadTask looks like this:

@Override
protected List<String> doInBackground(Void... voids) {
     // Download list of URLs from server, etc.
}

@Override
protected void onPostExecute(List<String> urls) {
    mHandler.sendEmptyMessageDelayed(111, 5000);
}

The handler will be called, the onPreExecute (in the AsyncTask) will be called and the initial run of DownloadTask (right in onCreate) also works.

According to this question: Android SDK AsyncTask doInBackground not running (subclass) , it might be SDK15 related.

Thanks for any hints.

Update As I received comments that the Handler might not be in the UI thread (which is weird, as Thread.currentThread is the same both in onCreate and the Handlers handleMessage method, I revised the handleMessage method to:

mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                if(mDownloadTask != null) {
                    mDownloadTask.cancel(true);
                }
                mDownloadTask = new DownloadTask();
                mDownloadTask.execute((Void[]) null);
            }
        });
    }
};

Still without success.

Update the full DownloadTask class

class DownloadTask extends AsyncTask<Void, Void, List<String>> {

    @Override
    protected void onPreExecute() {
        // Cancel the animation.
        if (mSlideshowAnimation != null) {
            mSlideshowAnimation.cancel(true);
        }

        mImageView1.setVisibility(View.GONE);
        mImageView2.setVisibility(View.GONE);
        animate(mProgressBar).alpha(1.0f).setDuration(500).start();

        Log.d(TAG, "Download preparation done.");
    }

    @Override
    protected List<String> doInBackground(Void... voids) {
        Log.d(TAG, "Download");
        SharedPreferences s = getSharedPreferences("access", Context.MODE_PRIVATE);
        String token = s.getString("token", null);

        Log.d(TAG, "Downloading slideshows.");

        List<String> urls = new ArrayList<String>();
        Slideshow[] slideshows = new Api(SlideshowActivity.this).getSlideshows(token);
        for (Slideshow slideshow : slideshows) {
            urls.addAll(slideshow.getAllPhotoUrls());
        }

        Log.d(TAG, "Downloading slideshows: " + slideshows.length);

        for (String url : urls) {
            try {
                url = Api.HOST + url;

                if (!Cache.fileExists(Cache.getCacheFilenameForUrl(SlideshowActivity.this, url))) {
                    Cache.cacheStream(SlideshowActivity.this, HttpHelper.download(SlideshowActivity.this, url), url);
                } else {
                    Log.d(TAG, "Cached: " + url);
                }
            } catch (IOException e) {
                Log.e(TAG, "Error while downloading.", e);
            }
        }

        Log.d(TAG, "Downloading slideshows finished.");

        return urls;
    }

    @Override
    protected void onPostExecute(List<String> urls) {
        Log.d(TAG, "download successful");
        animate(mProgressBar).alpha(0.0f).setDuration(500).start();

        mCurrentImageIndex = -1;
        mImageUrls = urls;

        mSlideshowAnimation = new SlideshowAnimation();
        mSlideshowAnimation.execute((Void[]) null);

        mHandler.sendEmptyMessageDelayed(111, 5000);
    }
}

One AsyncTask object can be executed only once. So try changing your code and instead using object of AsyncTask class directly use it as new DownloadTask().execute(); — MKJParekh
@Frankenstein that's what I'm doing in mDownloadTask = new DownloadTask(); both times? — Sebastian Roth
Do not write .execute((Void[]) null). It can be written .execute(). — Hauleth
@Hauleth can be, but creates compiler warnings for unchecked varargs parameter. — Sebastian Roth

Sebastian Roth Sebastian Roth · Accepted Answer · 2012-05-02T02:20:15

Thanks to a helpful discussion with Waqas (thank you!) I finally uncovered the error in my code. In fact all above written is correct and works as is. Issue with my case was that the second task blocked the first one and vice versa.

It was perhaps a coincidence to find this post on Google Groups: http://groups.google.com/group/android-developers/browse_thread/thread/f0cd114c57ceefe3?tvc=2&q=AsyncTask+in+Android+4.0 . Recommend that everyone involved in threading reads this discussion carefully.

AsyncTask switched the Threading model to a serial executor (again), which is not compatible with my approach of having 2 AsyncTasks it seems.

Finally I switched the handling of the "Download" to a classic Thread and used the Handler to post messages to cancel the Slideshow if neccessary. Using the Handlers sendEmptyMessageDelayed I will simple recreate the Download Thread after a while to refresh the data.

Thanks to all comments & answers.

AsyncTask called from Handler will not execute doInBackground

2 Answers