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PODetermining scope of Event Dispatch Thread
 

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  1. PODetermining scope of Event Dispatch Thread
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    <p>I am new to swing and still learning the ins and outs of it. I wrote a basic code and started experimenting with EDT. Here is the code:</p> <pre><code>public class SwingDemo2 extends Thread implements ActionListener { JLabel jl; SwingDemo2() { JFrame jfr = new JFrame("Swing Event Handling"); jfr.setSize(250, 100); jfr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); jl = new JLabel("Press a button!", SwingConstants.CENTER); System.out.println("After Label: " + SwingUtilities.isEventDispatchThread()); JButton jb1 = new JButton("OK"); jb1.setActionCommand("OK"); jb1.addActionListener(this); JButton jb2 = new JButton("Reset"); jb2.setActionCommand("Reset"); jb2.addActionListener(this); jfr.add(jl, BorderLayout.NORTH); jfr.add(jb1, BorderLayout.WEST); jfr.add(jb2, BorderLayout.EAST); System.out.println("After adding: " + SwingUtilities.isEventDispatchThread()); jfr.setVisible(true); } public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { public void run() { System.out.println("In main: " + SwingUtilities.isEventDispatchThread()); new SwingDemo2(); } }); } public void actionPerformed(ActionEvent ae) { if (ae.getActionCommand() == "OK") { System.out.println("In OK: " + SwingUtilities.isEventDispatchThread()); jl.setText("You pressed Ok"); } else if (ae.getActionCommand() == "Reset") { System.out.println("In Reset: " + SwingUtilities.isEventDispatchThread()); jl.setText("You pressed Reset"); } } } </code></pre> <p>I added a few <code>isEventDispatchThread()</code> methods to verify the thread I am in. Apart from the GUI the messages in console were:</p> <pre><code>In main: true After Label: true After adding: true In OK: true In Reset: true </code></pre> <p>It seems all the time I was in the EDT. My question is, after the <code>jfr.setVisible(true)</code> statement shouldn't the <code>SwingDemo2()</code> constructor return to <code>main()</code> and shouldn't that be the end of EDT?</p> <p>I waited for many seconds before I first pressed any button in the GUI so why is still my event handling being done in EDT? Shouldn't that have given the EDT enough time to terminate?</p> <p>Thanx in advance!</p>
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    1. USSurender Thakran
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    1. COJust try to write one `isEventDispatchThread()` outside `invokeLater()`, in your main method, you will get an idea, how separate they are. When a Swing Application runs, three threads are started. **1. Main Thread :** from which your application runs. **2. Toolkit Thread :** This thread is in charge of capturing the system events, like Keyboard key presses or mouse movements. Though, this thread never runs application code. Captured events are passed over to the third thread, the EDT. **3. EDT - Event Dispatch Thread :** is in charge of dispatching the events captured by the toolkit thread.
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    2. COEDT is the thread on which you interact with your Swing.
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    3. CO@GagandeepBali **Another silly question:** How many EDTs does a swing app has? When i tried to call `invokeLater` **again** after the already present one, the program compiled and ran perfectly. Did it ran in the same EDT or a new one was created?
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