Catch Exception of AsyncTask. Need Thinking

android, exception

Solution

Change the return type of `doInBackground()` to `Object` and when you receive the result in `onPostExecute(Object result)` use the `instanceOf` operator to check if the returned result is an `Exception` or the `List<String>`.

Edit

Since the result can either be an Exception or else the proper List, you can use the following:

protected void onPostExecute(final Object result) {
    working = false;
    if(result instanceof SomeCustomException) {
        exceptionHappendedListener.exceptionHappended(result);
    }
    else{
        if (onPostExecuteListener != null) {
            onPostExecuteListener.onPostExecute(result);
        }
    }
}

Also change the following statement:

public abstract class AbstractWorkerTask<Params, Progress, Object> extends AsyncTask<Params, Progress, Object>

Problem

I want to catch exception of a thread in doInBackground and print the error message in onPostExcecute. The problem is I don't have the Throwable object in onPostExecute. How to catch Exception in non-UI thread and print the error message in UI-thread? ``` public class TestTask extends AsyncTask<Void, Void, List<String>> { @Override protected List<String> doInBackground(final Void... params) { try { ... return listOfString; } catch(SomeCustomException e) { ... return null; } } @Override protected void onPostExecute(final List<String> result) { if(result == null) { // print the error of the Throwable "e". // The problem is I don't have the Throwable object here! So I can't check the type of exception. } } } ``` Update after Arun's answer: This is my AsyncTask wrapper class. It intends to do handling Exception in doInBackground but I can't find a good solution to do it. ``` public abstract class AbstractWorkerTask<Params, Progress, Result> extends AsyncTask<Params, Progress, Result> implements Workable { protected OnPreExecuteListener onPreExecuteListener; protected OnPostExecuteListener<Result> onPostExecuteListener; protected ExceptionHappenedListener exceptionHappendedListener; private boolean working; @Override protected void onPreExecute() { if (onPreExecuteListener != null) { onPreExecuteListener.onPreExecute(); } working = true; } @Override protected void onPostExecute(final Result result) { working = false; if(/* .........*/ ) { exceptionHappendedListener.exceptionHappended(e); } if (onPostExecuteListener != null) { onPostExecuteListener.onPostExecute(result); } } @Override public boolean isWorking() { return working; } public void setOnPreExecuteListener(final OnPreExecuteListener onPreExecuteListener) { this.onPreExecuteListener = onPreExecuteListener; } public void setOnPostExecuteListener(final OnPostExecuteListener<Result> onPostExecuteListener) { this.onPostExecuteListener = onPostExecuteListener; } public void setExceptionHappendedListener(final ExceptionHappenedListener exceptionHappendedListener) { this.exceptionHappendedListener = exceptionHappendedListener; } public interface OnPreExecuteListener { void onPreExecute(); } public interface OnPostExecuteListener<Result> { void onPostExecute(final Result result); } public interface ExceptionHappenedListener { void exceptionHappended(Exception e); } } ```

Original source