What prevents program from exiting?

Question

I wrote like this.

public static void main(String[] args){
    Thread thread = new Thread(() -> {
        while(true){
            try{
                // do something
                Thread.sleep(10);
            }catch(InterruptedException ex){
                System.out.println("ABC");
                break;
            }
        }
    });
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter(){
        @Override
        public void windowClosed(WindowEvent e){
            thread.interrupt();
        }
    });
    thread.start();
    frame.setVisible(true);
}

When I clicked "close" button of the window,
the program took few seconds to end.
What prevents program from exiting ?
And how can I close program immediately without using JFrame.EXIT_ON_CLOSE ?

I think my thread (while(true) thread) ends immediately
because "ABC" is displayed soon after I clicked "close" button.

Thank you.

EDIT

same thing occured without my thread.

public static void main(String[] args){
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setVisible(true);
}

EDIT2

public static void main(String[] args){
    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
        Thread[] threads = new Thread[5];
        Thread.enumerate(threads);
        System.out.println(Arrays.toString(threads));
        System.err.println(LocalDateTime.now() + " SHUTDOWN");
    }
    ));
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter(){
            @Override
            public void windowClosed(WindowEvent e){
                Thread[] threads = new Thread[5];
                Thread.enumerate(threads);
                System.out.println(Arrays.toString(threads));
                System.err.println(LocalDateTime.now() + " CLOSE");
            }
        });
    frame.setVisible(true);
}  

the output was:

[Thread[AWT-EventQueue-0,6,main], Thread[DestroyJavaVM,5,main], null, null, null]
2016-08-02T19:04:50.465 CLOSE
[Thread[DestroyJavaVM,5,main], Thread[Thread-0,5,main], null, null, null]
2016-08-02T19:04:51.762 SHUTDOWN

Answer 1

While System.exit() should generally be used with care, the Swing documentation explicitly states that it is the right approach in scenarios as shown in the question:

When the last displayable window within the Java virtual machine (VM) is disposed of, the VM may terminate. Note, however, that there can be a delay before the program exits automatically, and that under some circumstances the program might keep running. It is quicker and safer to explicitly exit the program using System.exit(int).

The following example does not show the delay between the close operation and the shutdown hook. When removing the System.exit(0) call, a delay of approx. 1,3 seconds can be observed:

package com.example.swing;

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.time.LocalDateTime;

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

public class ExitDelay {
    public static void main(String[] args){

        Runtime.getRuntime().addShutdownHook(new Thread(() -> System.err.println(LocalDateTime.now() + " SHUTDOWN") ));

        SwingUtilities.invokeLater(() -> {

            JFrame frame = new JFrame();
            frame.setSize(1280,720);
            frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            frame.addWindowListener(new WindowAdapter(){
                @Override
                public void windowClosed(WindowEvent e){
                    System.err.println(LocalDateTime.now() + " CLOSE");

                    // Do all required cleanup stuff.
                    // check that threads are done, close files, commit transactions etc.
                    // ...

                    System.exit(0);
                }
            });
            frame.setVisible(true);

        });
    }
}

Answer 2

To exit your program immediately you can use System.exit(0).

Like that:

public static void main(String[] args){
    Thread thread = new Thread(() -> {
        while(true){
            try{
                // do something
                Thread.sleep(10);
            }catch(InterruptedException ex){
                System.out.println("ABC");
                System.exit(0);
            }
        }
    });
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter(){
        @Override
        public void windowClosed(WindowEvent e){
            thread.interrupt();
        }
    });
    thread.start();
    frame.setVisible(true);
}

Answer 3

You use DISPOSE_ON_CLOSE and that makes sense because EXIT_ON_CLOSE would use System.exit to close the jvm process with all active threads. A JFrame is a complex structure that uses some native calls to register callbacks for mouse events at your operating system. So a clean exit takes some time. Additionally the Garbage Collector needs some time to detect that the AWT-Thread that runs the JFrame is completely finish with the cleanup before he kills it. So I think 1.3 seconds is acceptable for a clean finish of your application.
But if you really want to do so you could force the end with another Thread that simply checks continuously if all running threads have finished work and all JFrames have beend disappeared. And if so it calls the big evil System.exit.

final JFrame frame = new JFrame(); // make your JFrame final
final Thread thread  = new Thread(() ...  // make your Thread final

and add this end checking thread

final Thread endChecker  = new Thread(() -> {            
    while(true){
        try{
            Thread.sleep(100);
            if(!frame.isDisplayable() && !thread.isAlive())
                System.exit(0);                 
        }catch(InterruptedException ex){
            break;
        }
    }           
});
endChecker.start();