When exactly is the Event Dispatch Thread started?

Asked 26/5, 2012 at 21:22 Answered 26/5, 2012 at 23:6

When exactly is the EDT started? What line of code is responsible of it?

My guess is that "someSwingComponent.setVisible(true)" does the trick, but I'm not sure.

Thanks!

Labionasal answered 26/5, 2012 at 21:22 Comment(1)

The method responsible for creating the Event Dispatch Thread is EventQueue.initDispatchThread, so putting a breakpoint there may give you some insight. – Littrell 26/5, 2012 at 21:30

Q: When exactly is the EDT started? What line of code is responsible [f]of it?

The inner workings of Swing are JVM-specific. Different JVMs start the Event Dispatch Thread (EDT) based on differing criteria. In general though:

The EDT starts when it receives its first AWTEvent.

The stack traces below reaffirm this point. Take for example the following main method.

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setVisible(true);
}

In the example above, the line of code responsible for starting the EDT is frame.setVisible(true);

The above main method was executed on two different JVMs. A breakpoint was placed at EventQueue.initDispatchThread. When the breakpoint was hit, the following stack traces were noted.

Using the Mac's JDK on the AWT-AppKit thread:

EventQueue.initDispatchThread() line: 906   
EventQueue.wakeup(boolean) line: 1109   
NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method]  
NativeMethodAccessorImpl.invoke(Object, Object[]) line: 39  
DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 25  
Method.invoke(Object, Object...) line: 597  
SunToolkit.wakeupEventQueue(EventQueue, boolean) line: 348  
PostEventQueue.postEvent(AWTEvent) line: 2137   
SunToolkit.postEvent(AppContext, AWTEvent) line: 583    
SunToolkit.executeOnEventHandlerThread(PeerEvent) line: 654 
SunToolkit.executeOnEventHandlerThread(Object, Runnable) line: 631  
EventFactoryProxy.windowMoved(CWindow) line: 89

Using Oracle's JDK for Windows on the main thread:

java.awt.EventQueue.initDispatchThread() line: 861  
java.awt.EventQueue.postEventPrivate(java.awt.AWTEvent) line: 199   
java.awt.EventQueue.postEvent(java.awt.AWTEvent) line: 180  
javax.swing.RepaintManager.scheduleProcessingRunnable(sun.awt.AppContext) line: 1369    
javax.swing.RepaintManager.nativeAddDirtyRegion(sun.awt.AppContext, java.awt.Container, int, int, int, int) line: 548   
javax.swing.SwingPaintEventDispatcher.createPaintEvent(java.awt.Component, int, int, int, int) line: 45 
sun.awt.windows.WFramePeer(sun.awt.windows.WComponentPeer).postPaintIfNecessary(int, int, int, int) line: 741   
sun.awt.windows.WFramePeer(sun.awt.windows.WComponentPeer).handlePaint(int, int, int, int) line: 736    
sun.java2d.d3d.D3DScreenUpdateManager.repaintPeerTarget(sun.awt.windows.WComponentPeer) line: 274   
sun.java2d.d3d.D3DScreenUpdateManager.createScreenSurface(sun.awt.Win32GraphicsConfig, sun.awt.windows.WComponentPeer, int, boolean) line: 175  
...
sun.awt.windows.WToolkit.createFrame(java.awt.Frame) line: 383  
javax.swing.JFrame(java.awt.Frame).addNotify() line: 460    
javax.swing.JFrame(java.awt.Window).show() line: 859    
javax.swing.JFrame(java.awt.Component).show(boolean) line: 1584 
javax.swing.JFrame(java.awt.Component).setVisible(boolean) line: 1536   
javax.swing.JFrame(java.awt.Window).setVisible(boolean) line: 842   
Example.main(java.lang.String[]) line: 113

On the Mac, a call to PostEventQueue.postEvent(AWTEvent) is made. Similarly on Windows, a call to java.awt.EventQueue.postEvent(java.awt.AWTEvent) is made. Both eventually call EventQueue.initDispatchThread.

As another example, consider the following main method:

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            System.out.println("Start!");
        }
    });
}

Using the Mac's JDK on the main thread:

EventQueue.initDispatchThread() line: 906 [local variables unavailable] 
EventQueue.postEventPrivate(AWTEvent) line: 227 
EventQueue.postEvent(AWTEvent) line: 208    
EventQueue.invokeLater(Runnable) line: 1048 
SwingUtilities.invokeLater(Runnable) line: 1267 
Example.main(String[]) line: 31

Using Oracle's JDK for Windows on the main thread:

java.awt.EventQueue.initDispatchThread() line: 861  
java.awt.EventQueue.postEventPrivate(java.awt.AWTEvent) line: 199   
java.awt.EventQueue.postEvent(java.awt.AWTEvent) line: 180  
java.awt.EventQueue.invokeLater(java.lang.Runnable) line: 999   
javax.swing.SwingUtilities.invokeLater(java.lang.Runnable) line: 1267

The call to SwingUtilties.invokeLater is responsible for starting the EDT. Here again, calls to EventQueue.postEvent(AWTEvent) are made.

Thoughts on 'My guess is that "someSwingComponent.setVisible(true)" does the trick, but I'm not sure.'

Not just any call to someSwingComponent.setVisible(true) will start the EDT. For example, executing the following main method does not create the AWT-Event-Queue-0 thread:

public static void main(String[] args) {
    JLabel label = new JLabel();
    label.setVisible(true);
}

Resources

Of course, there are many resources online about the EDT.

"The Event Dispatch Thread" from The Java Tutorials's "Lesson: Concurrency in Swing"
"Threads and Swing" from the Sun Developer Network
"Swing Threads" from the Java Glossary at mindprod.com
"How does the event dispatch thread work?" on Stack Overflow

Littrell answered 26/5, 2012 at 23:6 Comment(0)

EDIT: You guys are right; The EDT is not started directly on startup. I did some debugging and this is what I found:

The Event Dispatch Thread is lazly started whenever a component requests access to the event queue by calling Toolkit.getEventQueue(). This can be done when Component.show() is called (the same as Component.setVisible()) but there are also other calls which can trigger this initialization like Component.repaint(). Once a reference to the eventqueue is obtained a job can be added to it with EventQueue.postEvent(). This method checks if the EDT exists and if it doesn't it creates it with initDispatchThread().

The only way to prevent it from starting is to start the JVM in headless mode (which disables AWT all together) with the "-Djava.awt.headless=true " flag. But thats basically the only low level interaction you can have with it.

The setVisible method of a Component should always be invoked on the EDT (just like any other modification you make to an Swing / AWT component). You use the EDT by telling Java to execute code on the EDT. The easiest way todo this is to use SwingUtilities.invokeLater(). This schedules your Thread (your Runnable implementation) to be executed from the EDT. This is the only kind of interaction you as a developer should have with the EDT. You shouldn't have any kind of low level interaction with the EDT like pausing or aborting the thread.

Inconsolable answered 26/5, 2012 at 22:16 Comment(1)

I don't believe this statement is true: "The Event Dispatch Thread is started during JVM startup." At application startup, the AWT-EventQueue-0 thread does not exist yet. Pieter is right; usually a call to JFrame.setVisible or a similar call starts this thread. – Littrell 26/5, 2012 at 22:22

you are right setVisible for any AWT / Swing container starting EDT, non safe way
Initial Thread is about the safest way
if all events are done in the present EDT, then isDispatchThread returns false
in the case that isDispatchThread returns false, then any Swing thread safe methods could alive the EDT, most safest way is invoke that from invokeLater()

code for test

import java.awt.Color;
import java.awt.EventQueue;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;

public class IsThereEDT {

    private ScheduledExecutorService scheduler;
    private AccurateScheduledRunnable periodic;
    private ScheduledFuture<?> periodicMonitor;
    private int taskPeriod = 30;
    private SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
    private Date dateRun;
    private JFrame frame1 = new JFrame("Frame 1");

    public IsThereEDT() {
        scheduler = Executors.newSingleThreadScheduledExecutor();
        periodic = new AccurateScheduledRunnable() {

            private final int ALLOWED_TARDINESS = 200;
            private int countRun = 0;
            private int countCalled = 0;
            private int maxCalled = 10;

            @Override
            public void run() {
                countCalled++;
                if (countCalled < maxCalled) {
                    if (countCalled % 3 == 0) {
                        SwingUtilities.invokeLater(new Runnable() {

                            @Override
                            public void run() {
                                System.out.println("Push a new event to EDT");
                                frame1.getContentPane().setBackground(Color.red);
                                isThereReallyEDT();
                            }
                        });
                    } else {
                        if (this.getExecutionTime() < ALLOWED_TARDINESS) {
                            countRun++;
                            isThereReallyEDT(); // non on EDT
                        }
                    }
                } else {
                    System.out.println("Terminating this madness");
                    System.exit(0);
                }
            }
        };
        periodicMonitor = scheduler.scheduleAtFixedRate(periodic, 0, taskPeriod, TimeUnit.SECONDS);
        periodic.setThreadMonitor(periodicMonitor);
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                isThereReallyEDT();
                frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame1.getContentPane().add(new JLabel("Hello in frame 1"));
                frame1.pack();
                frame1.setLocation(100, 100);
                frame1.setVisible(true);
            }
        });
        try {
            Thread.sleep(500);
        } catch (InterruptedException ex) {
            Logger.getLogger(IsThereEDT.class.getName()).log(Level.SEVERE, null, ex);
        }
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame frame2 = new JFrame("Frame 2");
                frame2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame2.getContentPane().add(new JLabel("Hello in frame 2"));
                frame2.pack();
                frame2.setLocation(200, 200);
                frame2.setVisible(true);
                isThereReallyEDT();
            }
        });
    }

    private void isThereReallyEDT() {
        dateRun = new java.util.Date();
        System.out.println("                         Time at : " + sdf.format(dateRun));
        if (EventQueue.isDispatchThread()) {
            System.out.println("EventQueue.isDispatchThread");
        } else {
            System.out.println("There isn't Live EventQueue.isDispatchThread, why any reason for that ");
        }
        if (SwingUtilities.isEventDispatchThread()) {
            System.out.println("SwingUtilities.isEventDispatchThread");
        } else {
            System.out.println("There isn't Live SwingUtilities.isEventDispatchThread, why any reason for that ");
        }
        System.out.println();
    }

    public static void main(String[] args) {
        IsThereEDT isdt = new IsThereEDT();
    }
}

abstract class AccurateScheduledRunnable implements Runnable {

    private ScheduledFuture<?> thisThreadsMonitor;

    public void setThreadMonitor(ScheduledFuture<?> monitor) {
        this.thisThreadsMonitor = monitor;
    }

    protected long getExecutionTime() {
        long delay = -1 * thisThreadsMonitor.getDelay(TimeUnit.MILLISECONDS);
        return delay;
    }
}

Gagliano answered 26/5, 2012 at 22:57 Comment(0)

Q: When exactly is the EDT started? What line of code is responsible [f]of it?

Thoughts on 'My guess is that "someSwingComponent.setVisible(true)" does the trick, but I'm not sure.'

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