Component Event Handlers (Listeners)

GUI components support the lists of callbacks, called Listeners, which are typically populated when the components are created. EDT executes the listeners when user excitates the components somehow (button is clicked, mouse is moved, item is selected, focus is lost, component resized and so on.)

Timer

For short tasks that must access/modify GUI periodically or at specific time, javax.swing.Timer is used. It can be considered as an invisible GUI component, whose listeners are registered to fire at specific time(s).

Equivalents

• System.Windows.Forms.Timer - .NET Framework
• flash.utils.Timer - Adobe Flash

Requests from other threads

Other application threads can pass some code to be executed in the event dispatching thread by means of SwingUtilities helper classes (or EventQueue if you are doing AWT). The submitted code must be wrapped with a Runnable object. Two methods of these classes allow:

• synchronous code execution (SwingUtilities.invokeAndWait(Runnable) or EventQueue.invokeAndWait(Runnable))
• and asynchronous code execution (SwingUtilities.invokeLater(Runnable) or EventQueue.invokeLater(Runnable))

from the event dispatching thread.

The method invokeAndWait() should never be called from the event dispatching thread—it will throw an exception. The method SwingUtilities.isEventDispatchThread() or EventQueue.isDispatchThread() can be called to determine if the current thread is the event dispatching thread.

The code supplied by means of the invokeLater and invokeAndWait to the EDT must be as quick as possible to prevent freezing. They are normally intended to deliver the result of a lengthy computation to the GUI (user).

Worker design pattern

Both execution of a task in another thread and presenting the results in the EDT can be combined by means of worker design pattern. The javax.swing.SwingWorker class, developed by Sun Microsystems, is an implementation of the worker design pattern, and as of Java 6 is part of standard Swing distribution. SwingWorker is normally invoked from EDT-executed event Listener to perform a lengthy task in order not to block the EDT.

SwingWorker<Document, Void> worker = new SwingWorker<Document, Void>() {
    public Document doInBackground() throws IOException {
        return loadXML(); // heavy task
    }
    
    public void done() {
        try {
            Document doc = get();
            display(doc);
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
};
worker.execute();

doOutside {
    def doc = loadXML() // heavy task
    edt { display(doc) }
}

Modal Execution

SwingWorker is normally created for a lengthy tasks by EDT while handling callback (Listener) events. Spawning a worker thread, EDT proceeds handling current message without waiting the worker to complete. Often, this is not desirable.

Often, your EDT handles a GUI component action, which demands the user to make a choice by means of another dialog, like JFileChooser, which pops up, stays responsive while user picks its option and action proceeds with selected file only after "OK" button is pressed. You see, this takes time (user responds in matter of seconds) and you need a responsive GUI (the messages are still pumped in EDT) during all this time while EDT is blocking (it does not handle newer, e.g. JFileChooser, messages in the queue before the dialog is closed and current component action is finished). The vicious cycle is broken through EDT entering a new message loop, which dispatches the messages as per normal until "modal dialog is over" arrives and normal message processing resumes from the blocked position in the component action.

The open source Foxtrot project emulates the Swing message loop pumping to provide the "synchronous" execution mechanism for arbitrary user tasks, which proceeds only after the worker completes the task.

  button.addActionListener(new ActionListener()
  {
	 public void actionPerformed(ActionEvent e)
	 {
		button.setText("Sleeping...");

		String text = null;
		try
		{
		   text = (String)Worker.post(new Task()
		   {
			  public Object run() throws Exception
			  {
				 Thread.sleep(10000);
				 return "Slept !";
			  }
		   });
		}
		catch (Exception x) ...

		button.setText(text);

		somethingElse();
	 }
  });

Since Java 1.7, Java provides standard solution for custom secondary message loops by exposing createSecondaryLoop() in system EventQueue().