How to test AsyncTask class using Mockito

Question

I have AsyncTask class as shown below in the code, and I am trying to test it. I coded the test cases of the AsyncTask as shown below in the testing section, but as shown in the testing code, I just tested whether or not the AsyncTask methods was called or not, and I did not tested the code in doInBackground() for example, because I do not know how to test it

Please let me know how to test AsyncTask class any guideline or hints are highly appreciated

code

public class AsyncTaskImageLoader extends AsyncTask<String, Void, RequestCreator> {

RequestCreator requCreator = null;
String picUrl = null;
private ImageView mImageView = null;
private UserAdapter.MyViewHolder mHolder = null;
ProgressBar mProgressBar = null;
Validation mValidation = null;
private Context mCtx = null;

public AsyncTaskImageLoader(Context ctx, UserAdapter.MyViewHolder holder) {
    mHolder = holder;
    mCtx = ctx;
    mValidation = new Validation(ctx);
}

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

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

@Override
protected RequestCreator doInBackground(String... params) {

    picUrl = params[0];

    if (mValidation.isValidUrl(picUrl)) {

        while (!isCancelled() && requCreator == null) {
            try {
                requCreator = mValidation.requestCreatorFromUrl(picUrl);
            } catch (Exception e) {

            }
            //the value of the delay could be changed preferably
            SystemClock.sleep(100);
        }

    }
    return requCreator;
}

@Override
protected void onPostExecute(RequestCreator requestCreator) {
    super.onPostExecute(requestCreator);

    mHolder.progressBar.setVisibility(View.GONE);
    //requestCreator.into(mHolder.imageViewAvatarOfOwner);
    mValidation.setImageOnImageView(requestCreator, mHolder.imageViewAvatarOfOwner);
}

testing:

public class AsyncTaskImageLoaderTest {

@Mock
ProgressBar mockProgressBar = null;
@Mock
AsyncTaskImageLoader mockAsyncTaskImageLoader = null;
@Mock
Context mCtx = null;


@Before
public void setUp() {
    mCtx = mock(Context.class);
    mockProgressBar = mock(ProgressBar.class);
    mockAsyncTaskImageLoader = mock(AsyncTaskImageLoader.class);
}

@Test
public void whenProgreeBarISSetToVisibleInOnPreExecute() throws Exception {
    mockProgressBar.setVisibility(View.VISIBLE);
    verify(mockProgressBar).setVisibility(View.VISIBLE);
}

@Test
public void whenOnDoInBackgroundIsCalled() throws Exception {
    String str = new String();

    mockAsyncTaskImageLoader.execute(str);
    verify(mockAsyncTaskImageLoader).execute(str);
}

@Test
public void whenOnPostExecuteIsCalled() throws Exception {
    RequestCreator mockRequestCreator = mock(RequestCreator.class);
    mockAsyncTaskImageLoader.onPostExecute(mockRequestCreator);
}
}