How to use synchronized the object in AsyncTask doInBackground()?

Question

I got the trouble with using synchronized the object in background thread. I've specified the problem in sample nutshell:

I write the simple class like that:

public class UIThreadClass {

   MyObject object;

   UIThreadClass() {
      object = new MyObject();

      object.doActionOne();
      object.doActionTwo();
  }
}

Now my task is improve the code by put two method to non-UI thread. I am using AsyncTask

For doActionOne()

private static class DoActionOneTask extends AsyncTask<Void, Void, Void> {

    private WeakReference<MyObject> wObject;

    DoActionOneTask(MyObject object) {
        wObject = new WeakReference<>(object);
    }

    @Override
    protected Void doInBackground(Void... voids) {
       if(wObject.get() != null) {
          MyObject myObject = wObject.get();
          myObject.doActionOne();
       }

       return null;
    }
}

For doActionTwo()

private static class DoActionTwoTask extends AsyncTask<Void, Void, Void> {

    private WeakReference<MyObject> wObject;

    DoActionOneTask(MyObject object) {
        wObject = new WeakReference<>(object);
    }

    @Override
    protected Void doInBackground(Void... voids) {
       if(wObject.get() != null) {
          MyObject myObject = wObject.get();
          myObject.doActionTwo();
       }

       return null;
    }
}

Now call them in UI thread

public class UIThreadClass {

   MyObject object;

   UIThreadClass() {
      object = new MyObject();

      new DoActionOneTask(object).execute();
      new DoActionTwoTask(object).execute();
  }
}

The question is:

When using AsyncTask - how to synchronized the object to make sure that the doActionTwo() alway call after doActionOne() method ?.

I have tried using synchronized (myObject) {myObject.doActionOne();} in doInBackground() but the system is warning me that myObject just local variable and it is difficult to guarantee correctness.

Can anyone help me this situation ? Thanks a lot!

AsyncTask as the name suggest it will perform the task asynchronously, You can create a separate thread and perform operations and synchronise them — Akshay Katariya, Apr 19 '18 at 05:05
instead of synchronized use ThreadPoolExecutor that will run simultaneously. for more have look [this](https://stackoverflow.com/a/47466464/5110595) — Hemant Parmar, Apr 19 '18 at 05:07
you can start your 2nd Asynctask from the first one's onPostExecute method or use ThreadPoolExecutor — himel, Apr 19 '18 at 05:08
BTW the way you use WeakReference is not safe, you should store the `wObject.get()`in a variable and perform your if-statement. Calling `wObject.get()` again can potentially give a null reference at some point. — Enzokie, Apr 19 '18 at 05:56
@AkshayKatariya: But what I want is "how to synchronized". The problem is solved, thanks — Truong Giang Dam, Apr 19 '18 at 06:39
@HemantParmar: Yeah, I know about ThreadPoolExecutor, but my task is using both of them and compare between. The problem is solved, thanks. — Truong Giang Dam, Apr 19 '18 at 06:40
@himel: thanks for you suggestion but not using onPostExecute here because of not using callback requirement. The problem is solved, thanks — Truong Giang Dam, Apr 19 '18 at 06:41
@Enzokie: Can you explain why calling wObject.get() again can potentially give a null reference, pls ? — Truong Giang Dam, Apr 19 '18 at 06:46

score 2 · Accepted Answer · answered Apr 19 '18 at 05:47

You don't need to do anything. They will execute serially as per the Android docs

Order of execution
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

Great, thanks you guy, you are right, that's my mistake not reading docs clearly. — Truong Giang Dam, Apr 19 '18 at 06:44

How to use synchronized the object in AsyncTask doInBackground()?

1 Answers1