Is there a way to set up two or more event dispatch threads (EDT)?

Is Java capable of creating more than one EDT at a time?

I'm experimenting with setting up EDT and how it works in updating the content of a "heavy duty" panel with potentially a dozen of panels embedded inside and with hundreds of components altogether. Currently I have

        public void run() {
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    panel.update();
                }
            });
        }

I've looked at the following posts:

Measuring "busyness" of the event dispatching thread

How does the event dispatch thread work?

Java Event-Dispatching Thread explanation

http://en.wiki2.org/wiki/Event_dispatching_thread

and so forth.

I sort of understand that if there are, say a dozen of events, that an single EDT has to handle, Java already has an internal scheduling mechanism to group/prioritize these events.

According to http://docs.oracle.com/javase/tutorial/uiswing/concurrency/dispatch.html

"This is necessary because most Swing object methods are not "thread safe": invoking them from multiple threads risks thread interference or memory consistency errors."

So what if I create a 2nd EDT with new Thread(new Runnable() { ... }.start() below?

Will java automatically merge the two EDTs back to one for fear of thread safety?

       new Thread(new Runnable() {
        public void run() {
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    panel.update();
                }
            });
        }
    }).start();

The following applies in case one uses the Sun toolkit. I am not sure about other Java implementations (have not tested it).

One can have multiple EDT threads provided the toolkit is initialized from multiple threads from separate thread groups.

PoC code:

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;

public class Test {
    public static void main(String[] argv) {
        for (int i = 0; i < 5; i++) {
            // the separate thread group is needed at least for the Sun toolkit
            ThreadGroup tg = new ThreadGroup("Test Group " + i);
            Thread t = new Thread(tg, new Runnable() {
                @Override
                public void run() {
                    startApp();
                }
            }, "Test " + i);
            t.start();
        }
    }
    
    private static void startApp() {
        sun.awt.SunToolkit.createNewAppContext();
        final JFrame frm = new JFrame(Thread.currentThread().getName()) {
            @Override
            public void setVisible(boolean b) {
                super.setVisible(b);
                System.out.println("Closed");
                if (!b) dispose();
            }
        };
        final JTextArea ta = new JTextArea();
        frm.add(ta);
        JButton btn = new JButton("Dialog");
        // Showing a modal dialog will block only this frame
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                JOptionPane.showMessageDialog(frm, "Test message from " + frm.getTitle());
            }
        });
        frm.add(btn, BorderLayout.SOUTH);
        frm.setPreferredSize(new Dimension(300, 300));
        frm.pack();
        frm.show();
        Thread t = new Thread(new Runnable() {
            int i = 0;
            @Override
            public void run() {
                try {
                    while (true) {
                        i++;
                        if (!frm.isVisible()) break;
                        SwingUtilities.invokeAndWait(new Runnable() {
                            @Override
                            public void run() {
                                try {
                                    ta.getDocument().insertString(0, "Test " + i + " " +
                                        Thread.currentThread().hashCode()  + // This is to show that we are actually on a different thread as invokeAndWait is static and one may suspect a different behaviour
                                        "\n", null);
                                } catch (Throwable t) {
                                    t.printStackTrace();
                                }
                            }
                        });
                        Thread.sleep(1000 + (int)(Math.random() * 500));
                    }
                    Thread.sleep(4000); // This is just to show that the deamon thread might not have ended before the toolkit calls System.exit
                } catch (Throwable t) {
                    t.printStackTrace();
                }
                System.out.println("Thread " + Thread.currentThread().getName() + " exit");
            }
        });
        t.setDaemon(true);
        t.start();
    }
}

The above code demonstrates opening 5 frames on separate EDTs. When each frame is closed and disposed the respective EDT ends. When all the frames are closed and disposed the app will exit (automatically).

Notes:

• one should not use (mix) components created in one EDT from another EDT;
• closing the modal dialog from the example has the side effect of not returning the focus to the parent frame (as one would expect) but to any of the others.

I use the above approach in order to measure the height of a wrap enabled JTextArea that is populated with a multitude of different texts that might take some time and during that time I do not want to block the main UI. The result (the measured heights) is used on the main UI.

Tested with Java 1.6.0_25 and 1.8.0_60.