The callback methods defined by AsyncTask dictate that we cannot throw checked exceptions, so we must wrap any code that throws checked exceptions with try/catch blocks. Unchecked exceptions that propagate out of AsyncTask's methods will crash our application, so we must test carefully and handle these if necessary.

For the callback methods that run on the main thread—onPreExecute, onProgressUpdate, onPostExecute, and onCancelled—we can catch exceptions in the method and directly update the user interface to alert the user.

Of course, exceptions are likely to arise in our doInBackground method too, as this is where the bulk of the work of AsyncTask is done, but unfortunately, we can't update the user interface from doInBackground. A simple solution is to have doInBackground return an object that may contain either the result or an exception, as follows:

static class Result<T> {
  private T actual;
  private Exception exc;
}
@Override
protected final Result<T> doInBackground(Void... params) {
  Result<T> result = new Result<T>();
  try {
    result.actual = calculateResult();
  } catch (Exception exc) {
    result.exc = exc;
  }
    return result;
}

protected abstract T calculateResult() throws Exception;

Now we can check in onPostExecute for the presence of an Exception in the Result object. If there is one, we can deal with it, perhaps by alerting the user; otherwise, we just use the actual result as normal.

@Override
protected final void onPostExecute(Result<R> result) {
    if (result.exc != null) {
        // … alert the user …
    } else {
        // … success, continue as normal …
    }
}

In the downloadable code, you'll find a handy SafeAsyncTask class that takes care of exception handling, and makes it easy to deal with exceptions or results.