That old chestnut: Swing Threading - EventQueue

Question

First of all, I'm sorry if this topic has been done to death already but I really have searched through the forums and tutorials and not found a solution that I can get to work.
Scenario (the usual):
- Swing GUI application
- Button fires off 'IntensiveProcess'
- Display a 'MessageBox' while 'IntensiveProcess' is running
Problem:
MessageBox is not displayed (or is displayed but its contents are not painted - depending on which [incorrect] implementation I try) until IntensiveProcess has finished.
I have tried all kinds of different ways of accomplishing this but I can't for the life of me get it to work:
The implementations I have tried involved various combinations of:
- EventQueue.invokeLater()
- EventQueue.invokeAndWait()
- inner and annonymous-inner Runnable classes
As far as I understand it, the following strategy should work:
public class MySwingApp extends JFrame
    //create button with Action: DoButtonAction
    private class DoButtonAction extends AbstractAction
        public void actionPerformed(ActionEvent e)
            // ~~~ IMPLEMENTAION 1  ~~~
            EventQueue.invokeLater(new ShowMessageBox());
            EventQueue.invokeLater(new IntensiveProcess());
            // ~~~ IMPLEMENTAION 2 ~~~
            EventQueue.invokeLater(new ShowMessageBox());
            //cannot call invokeAndWait() from Event Dispatcher thread so:
            (new Thread(new Runnable()
                public void run()
                    EventQueue.invokeAndWait(new IntensiveProcess());
            )).start();
            // ~~~ IMPLEMENTAION 3  ~~~
            EventQueue.invokeLater(new IntensiveProcess());
            //cannot call invokeAndWait() from Event Dispatcher thread so:
            (new Thread(new Runnable()
                public void run()
                    EventQueue.invokeAndWait(new ShowMessageBox());
            )).start();
            //dispose of message box if user didn't close it
    private class ShowMessageBox implements Runnable
        //display MessageBox...
    private class IntensiveProcess implements Runnable
        //execute intensive process...
}None of the above 3 implementations yield the correct result.
Can anyone please point out where I'm going wrong?
Thanks all.
John

Answer 1

I have implemented your suggestion but sadly I am
getting the same result: the entire GUI hangs until
IntensiveProcess has completed, and only then is the
message box displayed.Hmm, with the intenstive process in its own thread (construct() runs in a thread other than EDT) the GUI shouldn't hang like that, I'm not sure why that is. Maybe try invokeAndWait instead. Or, did you try displaying the message box before creating the SwingWorker? Make sure the message box is not modal if you create it before creating SwingWorker.
I can't grasp why the EventDispatcher thread is
getting tied up with IntensiveProcess. I thought
calling EventQueue.invokeLater() (or SwingWorker) is
supposed allow the executing thread (in my case the
EventDispatcher thread) to continue. So why is the
GUI not getting updated and the message box being
displayed until AFTER IntensiveProcess finishes?As I understand it, invokeLater means stuff this Runnable into the event queue and run it when its turn comes up, while invokeAndWait runs the Runnable immediately. Neither method will wait for another thread to finish like you're expecting, that's what SwingWorker does.
For some background info, the SwingWorker's construct() method creates a new non-EDT thread, which is where you're supposed to do time-intensive background work, and the code inside the finished() method will be run in the EDT, which is where you're supposed to update Swing components. The major benefit is the happens-before guarantee, construct() will run to completion before finished() is invoked. Maybe you already knew that, but it's important to know so I wanted to make sure.