Why inner AsyncTask class should be static or leaks might occur? and solution for update UI

The reason is

  • When Activity is destroyed, AsyncTask (both static or non-static) still running
  • If inner class is non-static (AsyncTask) class, it will have reference to the outer class (Activity).
  • If a object has no references point to it, Garbage Collected will release it. If a object is unused and Garbage Collected can not release it => leak memory

=> If AsyncTask is non-static, Activity won't release event it is destroyed => leak

Solution for update UI after make AsyncTask as static class without leak

  1. Use WeakReference like
public class NoLeakAsyncTaskActivity extends AppCompatActivity {
    private ExampleAsyncTask asyncTask;

    @Override 
    protected void onCreate(Bundle savedInstanceState) {
        ...

        // START AsyncTask
        asyncTask = new ExampleAsyncTask();
        asyncTask.execute();
    }

    static class ExampleAsyncTask extends AsyncTask<Void, Void, Integer> {
        private WeakReference<NoLeakAsyncTaskActivity> activityReference;
        
        ExampleAsyncTask(NoLeakAsyncTaskActivity context) {
            activityReference = new WeakReference<>(context);
        }
        
        @Override
        protected Integer doInBackground(Void... voids) {
            ...
            return null;
        }

        @Override
        protected void onPostExecute(Integer value) {
            ...
            NoLeakAsyncTaskActivity activity = activityReference.get();
            if (activity == null || activity.isFinishing()) return;

            // modify the activity's UI
            TextView textView = activity.findViewById(R.id.textview);
            textView.setText(result);
        }
    }
}
  1. Send and remove the Activity reference
public class NoLeakAsyncTaskActivity extends AppCompatActivity {
    private ExampleAsyncTask asyncTask;

    @Override 
    protected void onCreate(Bundle savedInstanceState) {
        ...

        // START AsyncTask
        asyncTask = new ExampleAsyncTask();
        asyncTask.setListener(new ExampleAsyncTask.ExampleAsyncTaskListener() {
            @Override
            public void onExampleAsyncTaskFinished(Integer value) {
                // update UI in Activity here
            }
        });
        asyncTask.execute();
    }

    @Override
    protected void onDestroy() {
        asyncTask.setListener(null); // PREVENT LEAK AFTER ACTIVITY DESTROYED
        super.onDestroy();
    }

    static class ExampleAsyncTask extends AsyncTask<Void, Void, Integer> {
        private ExampleAsyncTaskListener listener;

        @Override
        protected Integer doInBackground(Void... voids) {
            ...
            return null;
        }

        @Override
        protected void onPostExecute(Integer value) {
            super.onPostExecute(value);
            if (listener != null) {
                listener.onExampleAsyncTaskFinished(value);
            }
        }

        public void setListener(ExampleAsyncTaskListener listener) {
            this.listener = listener;
        }

        public interface ExampleAsyncTaskListener {
            void onExampleAsyncTaskFinished(Integer value);
        }
    }
}

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