JavaFX app that uses Executor hangs on quit

Issue

I'm trying to make a clone of Android's AsyncTask to use in a JavaFX app. Here's the code I've come up with:

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

abstract public class AsyncTask<Param, Result>
{
    private Param param;
    private static Executor executor;

    public AsyncTask()
    {
        if (executor == null)
            executor = Executors.newSingleThreadExecutor();
    }

    protected void onPreExecute()
    {

    }

    protected Result doInBackground(Param param)
    {
        return null;
    }

    protected void onPostExecute(Result result)
    {

    }

    final public void execute(Param param)
    {
        this.param = param;

        onPreExecute();

        Task t = new Task();
        executor.execute(t);
    }

    private class Task implements Runnable
    {
        public void run()
        {
            Result result = doInBackground(param);
            onPostExecute(result);
        }
    }
}

I can use this class in my JavaFX app and it works fine except for one thing: when I close the main window, the JVM hangs rather than exiting cleanly. I have to force-quit the app.

I think the problem is related to the Executor. Because I don't issue a shutdown(), the Executor hangs waiting for more tasks to execute. Because AsyncTask is a wrapper for Java's Executor, the class that extends AsyncTask won't have direct access to the Executor and therefore can't issue a shutdown(). How can I do an orderly shutdown of the Executor?

Solution

You either need to shutdown the executor from the Application.stop method or you make sure the Executor's thread won't prevent the JVM from shutting down by using a daemon thread:

if (executor == null) {
    executor = Executors.newSingleThreadExecutor(r -> {
        Thread t = new Thread(r);
        t.setDaemon(true);
        return t;
    });
}

Answered By - fabian