How run two AsyncTasks in one Activity?

Question

I have two Activitys (mainActivity & downloadActivity) and I have 2 AsyncTasks in downloadActivity

In downloadActivity first it execute getFileAsyncTask for reading a JSON file for adding some images and create a ListView from images, if user clicks on an image, the downloadAsyncTask was called and it starts to download something from the internet. My problem is here: when the second AsyncTask is running I go back to mainActivity and comeback again to downloadActivity the first AsyncTask wasn't called until the downloadAsyncTask completed.

public class downloadActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...
        new getFileAsyncTask().execute();
        ...
    }
    private class getFileAsyncTask extends AsyncTask<Void, Void, Void> {
        @Override
        protected Void doInBackground(Void... params) {
        //fetch a json file from internet and add images in ListView
            return null;
        }
    }
    //there is a Base Adapter class 
    //if user clicks on an image it calls this downloadAsyncTask.execute()
    private class downloadAsyncTask extends AsyncTask<Void, Integer, Void> {
        @Override
        protected void onPreExecute() {
        //download the file
        }
    }

    @Override
    protected Void doInBackground(Void... unused) {
        //download the file
    }
}

note: I want to write something like shopping apps. For example, user can download file and surf into shop to see products .

Answer 1

If you want to run multiple AsyncTasks in parallel, you can call executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR) instead of execute() on your task. By default, AsyncTasks run in serial, first come first serve.

Be careful that the two threads do not interact on the same data, this can cause some strange and hard to trace errors.