Question

My program looks like this:

import java.awt.*;
import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        JFrame jf = new JFrame();
        jf.setSize(new Dimension(200, 200));
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setVisible(true);
    }
}

I’m just confused why after JVM’s quitting from main(), my program does not end instantly? I noticed that if I remove the line “jf.setVisible(true);”, it will end.

Is it implemented though techniques like garbage collecting or class destructors? I’m interested that if I want to write something similar, how could I do it.

Answer 1

The reason is that when you call setVisible(true) on the JFrame, behind the scenes a non-daemon thread is started, and the JVM will not exit until all non-daemon threads terminate.

Please have a look here for more on AWT/Swing Threading issues.
It states:

“There is at least one alive non-daemon thread while there is at least one displayable AWT or Swing component within the application (see Component.isDisplayable).”

While this is for Java 1.5, I think that it is still valid information.

Also, I believe that the Event Dispatch Thread or EDT is not a daemon thread, and so it is another thread associated with Swing that drives this.

Edit 1
This suggests that the EDT is in fact a non-Daemon thread:

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class IsEdtDaemon {
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
             JFrame frame = new JFrame();
             frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
             frame.setVisible(true);

             System.out.printf("Is the current thread the EDT thread: %b%n",  SwingUtilities.isEventDispatchThread());
             System.out.printf("Is our EDT Thread a daemon thread:    %b%n",  Thread.currentThread().isDaemon());
         }
      });
   }
}

The output from the code is:

Is the current thread the EDT thread: true
Is our EDT Thread a daemon thread: false