Multiple Async Task Hanging the App

Question

In my android App, I use multiple AsyncTask using THREAD_POOL_EXECUTOR which makes the tasks run in parallel. Sometime the app hangs. Below is the code I use.

1. Could you please let me know how to fine tune so as to avoid any hanging issue ?

2. How to find the point in which app is hanging ?

   new fetchInitialCoinsParallel().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, url);
   
   prefCoinList = getPrefCoin();
   if(prefCoinList.size()>0){
       for(int i=0;i<prefCoinList.size();i++){
           new fetchAltCoinsParallel().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, url);
       }
   }
   
   public class fetchAltCoinsParallel extends AsyncTask<String, String, String> {
   @Override
   protected void onPreExecute() {
   }
   
   protected String doInBackground(String... params) {
       try {
           InputStream is = getDataFromURL(params[0]);
           if(is!=null){
               BufferedReader br = new BufferedReader(new InputStreamReader(is));
               synchronized(this){
                   brList.add(br);
               }
   
           }else{
               prefCoinNotLoadedTimeOutCount=prefCoinNotLoadedTimeOutCount+1;
           }
   
   
           if(brList.size()==prefCoinList.size()-prefCoinNotLoadedTimeOutCount){
               try {
                   loadAltCoins(getAltCoinDataParallel());
               } catch (IOException e) {
                   e.printStackTrace();
               }
   
               maingame.dataReady=true;
           }
   
       } catch (IOException e) {
           e.printStackTrace();
       } 
       return null;
   }
   
   protected void onPostExecute(String params) {
   }
   protected void onProgressUpdate(String... progress) {
   }
   

   }

Thread details

Answer 1

Check the AsyncTaskLoader concept. This feature is supported by Android community introduced in API level 11 along with Honeycomb features.

AsyncTaskLoader solved the lot of limitations & workaround solutions of the AsyncTask.java

Official : https://developer.android.com/reference/android/content/AsyncTaskLoader.html

Good Sample: https://medium.com/google-developers/making-loading-data-on-android-lifecycle-aware-897e12760832

public class JsonAsyncTaskLoader extends AsyncTaskLoader<List<String>> {
    // You probably have something more complicated
    // than just a String. Roll with me
    private List<String> mData;

    public JsonAsyncTaskLoader(Context context) {
        super(context);
    }

    @Override
    protected void onStartLoading() {
        if (mData != null) {
            // Use cached data
            deliverResult(mData);
        } else {
            // We have no data, so kick off loading it
            forceLoad();
        }
    }

    @Override
    public List<String> loadInBackground() {
        // This is on a background thread
        // Good to know: the Context returned by getContext()
        // is the application context
        File jsonFile = new File(
                getContext().getFilesDir(), "downloaded.json");
        List<String> data = new ArrayList<>();
        // Parse the JSON using the library of your choice
        // Check isLoadInBackgroundCanceled() to cancel out early
        return data;
    }

    @Override
    public void deliverResult(List<String> data) {
        // We’ll save the data for later retrieval
        mData = data;
        // We can do any pre-processing we want here
        // Just remember this is on the UI thread so nothing lengthy!
        super.deliverResult(data);
    }
}

Answer 2

Instead of using Async Tasks, i recommend using RXJAVA for this purpose.

Here is the disadvantage given for Async Task: https://stackoverflow.com/a/9654445/9100553

Using RxJava will solve this problem, here is an perfect blog which can solve your problem of Multi Threading using RxJava.

http://www.nurkiewicz.com/2017/09/idiomatic-concurrency-flatmap-vs.html

(Read the second half) Both flatmap and parallel operators would be useful in your case.