When exactly is the Event Dispatch Thread started?

Question

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.<

Answer 1

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.

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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