是否可以同时运行多个 AsyncTask？

Question

I've two activities in my application. In my Activity A I'm using one AsyncTask and second Activity B also using another one AsyncTask. In my Activity A I've upload some data to server and in my Activity B I'm trying to download some other data from server. Both these are running in AsyncTask. My problem is when I trying to download data from server in Activity B onPreExecute() method was called but doInBackground() method was not called it is waiting up to the first Activity A's doInBackground() action finished. Why it is happened? Is it possible to run more than one background actions at a same time..

In Activity A

ImageButton submit_button = (ImageButton) findViewById(R.id.submit_button);

submit_button.setOnClickListener(new OnClickListener() 
    {
        public void onClick(View record_button) 
        {
                       new Save_data().execute();
        }
       });
class Save_data extends AsyncTask<String, Integer, Integer> 
 {
    protected void onPreExecute() 
    {

}
    protected Integer doInBackground(String... arg0) 
{
     //uploading data here
    }

 }

In my Activity B

ImageButton get_button = (ImageButton) findViewById(R.id.get_button);
    get_button.setOnClickListener(new OnClickListener() 
        {
            public void onClick(View record_button) 
            {
                           new download_process().execute();
            }
           });
    class download_process extends AsyncTask<String, integer, integer>
     {
        protected void onPreExecute() 
        {
       Log.e("pre-execute","has been called");//This Log works well
    }
         protected Integer doInBackground(String... arg0) 
    {
         //downloading data here
        }   
     }

Answer 1

use executor as follows

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
    new Save_data().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, location);
} else {
    new Save_data().execute(location);
}

See this

Answer 2

Yes, it is, but since Honeycomb, there's a change in the way AsyncTask is handled on Android. From HC+ they are executed sequentially, instead of being fired in parallel, as it used to be. Solution for this is to run AsyncTask using THREAD_POOL_EXECUTOR executor explicitely. You can create helper class for that:

public class AsyncTaskTools {
    public static <P, T extends AsyncTask<P, ?, ?>> void execute(T task) {
        execute(task, (P[]) null);
    }
    
    @SuppressLint("NewApi")
    public static <P, T extends AsyncTask<P, ?, ?>> void execute(T task, P... params) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, params);
        } else {
            task.execute(params);
        }
    }
}

and then you can simply call:

AsyncTaskTools.execute( new MyAsyncTask() );

or with params (however I rather suggest passign params via task constructor):

AsyncTaskTools.execute( new MyAsyncTask(), <your params here> );