WeakReference keeps returning Activity reference after orientation change

Question

I want to use a WeakReference in my AsyncTask to not leak memory. With the log statements I can see, that the activity reference I get returned from the get method is indeed null if I leave the activity over the back button. However, when I rotate the device, I get returned a working reference, which means I leak memory. Any idea why this happens?

Update: It starts showing null as soon as I leave the activity AFTER rotating the device and then open a different app. Could it be that the system just does not see a necessity in cleaning up because my activity is so simple?

public class MainActivity extends AppCompatActivity {
    private ProgressBar progressBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressBar = findViewById(R.id.progress_bar);
    }

    public void startAsyncTask(View v) {
        ExampleAsyncTask exampleAsyncTask = new ExampleAsyncTask(this);
        exampleAsyncTask.execute(210);
    }

    private static class ExampleAsyncTask extends AsyncTask<Integer, Integer, String> {
        private WeakReference<MainActivity> activityReference;

        ExampleAsyncTask(MainActivity context) {
            activityReference = new WeakReference<>(context);
        }

        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            MainActivity activity = activityReference.get();
            if (activity == null || activity.isFinishing()) {
                return;
            }

            activity.progressBar.setVisibility(View.VISIBLE);
        }

        @Override
        protected String doInBackground(Integer... integers) {
            for (int i = 0; i < integers[0]; i++) {
                publishProgress((i * 100) / integers[0]);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            return "Finished";
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);

            MainActivity activity = activityReference.get();
            if (activity == null || activity.isFinishing()) {
                Log.i("activity", "activity = null");
                return;
            } else
                Log.i("activity", "activity not null");
            activity.progressBar.setProgress(values[0]);
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);

            MainActivity activity = activityReference.get();
            if (activity == null || activity.isFinishing()) {
                return;
            }

            activity.progressBar.setProgress(0);
            activity.progressBar.setVisibility(View.INVISIBLE);
            Toast.makeText(activity, s, Toast.LENGTH_SHORT).show();
        }
    }
}

Answer 1

When the activity is being destroyed, the activity object should become eligible for garbage collection. This means, that some of the next GC events would free up memory hold by the activity.

You are making an assumption, that GC have to occur immediately after you perform orientation change, which is not true. What WeakReference ensures, is that it will not be a reason to hold the object when next time GC will try to clear up memory. Sooner or later the activity object would be cleared by GC, but it will not take place instantly as you expect.

You can utilize System.gc() which will initiate a synchronous GC event to take place, but it's not advised to rely on this method in normal scenario.

    System.gc();
    MainActivity activity = activityReference.get();
    // now `activity` should be null