How to close a Java Swing application from the cod

Question

What is the proper way to terminate a Swing application from the code, and what are the pitfalls?

I'd tried to close my application automatically after a timer fires. But just calling dispose() on the JFrame didn't do the trick - the window vanished but the application did not terminate. However when closing the window with the close button, the application does terminate. What should I do?

Answer 1

Your JFrame default close action can be set to "DISPOSE_ON_CLOSE" instead of EXIT_ON_CLOSE (why people keep using EXIT_ON_CLOSE is beyond me).

If you have any undisposed windows or non-daemon threads, your application will not terminate. This should be considered a error (and solving it with System.exit is a very bad idea).

The most common culprits are java.util.Timer and a custom Thread you've created. Both should be set to daemon or must be explicitly killed.

If you want to check for all active frames, you can use Frame.getFrames(). If all Windows/Frames are disposed of, then use a debugger to check for any non-daemon threads that are still running.

Answer 2

I guess a EXIT_ON_CLOSE

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

before System.exit(0) is better since you can write a Window Listener to make some cleaning operations before actually leaving the app.

That window listener allows you to defined:

public void windowClosing(WindowEvent e) {
    displayMessage("WindowListener method called: windowClosing.");
    //A pause so user can see the message before
    //the window actually closes.
    ActionListener task = new ActionListener() {
        boolean alreadyDisposed = false;
        public void actionPerformed(ActionEvent e) {
            if (frame.isDisplayable()) {
                alreadyDisposed = true;
                frame.dispose();
            }
        }
    };
    Timer timer = new Timer(500, task); //fire every half second
    timer.setInitialDelay(2000);        //first delay 2 seconds
    timer.setRepeats(false);
    timer.start();
}

public void windowClosed(WindowEvent e) {
    //This will only be seen on standard output.
    displayMessage("WindowListener method called: windowClosed.");
}

Answer 3

The following program includes code that will terminate a program lacking extraneous threads without explicitly calling System.exit(). In order to apply this example to applications using threads/listeners/timers/etc, one need only insert cleanup code requesting (and, if applicable, awaiting) their termination before the WindowEvent is manually initiated within actionPerformed().

For those who wish to copy/paste code capable of running exactly as shown, a slightly-ugly but otherwise irrelevant main method is included at the end.

public class CloseExample extends JFrame implements ActionListener {

    private JButton turnOffButton;

    private void addStuff() {
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        turnOffButton = new JButton("Exit");
        turnOffButton.addActionListener(this);
        this.add(turnOffButton);
    }

    public void actionPerformed(ActionEvent quitEvent) {
        /* Iterate through and close all timers, threads, etc here */
        this.processWindowEvent(
                new WindowEvent(
                      this, WindowEvent.WINDOW_CLOSING));
    }

    public CloseExample() {
        super("Close Me!");
        addStuff();
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                CloseExample cTW = new CloseExample();
                cTW.setSize(200, 100);
                cTW.setLocation(300,300);
                cTW.setVisible(true);
            }
        });
    }
}