Java detect when application is closing

Question

I learned Java for about 1 year some years ago. Now I wanted to go back to it, but my skills got a bit rusty. So I'm using Eclipse with the WindowBuilder to make an easy Form Application to get started again. I wanted to get a popup window when I exit the application because in future projects I need to execute some code when the application is being exited by another way than my own exit button (for example via the red cross or Alt+F4).

So this is my code so far:

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JButton;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;


public class Mainframe {

private JFrame frame;

/**
 * Launch the application.
 */
public static void main(String[] args) {


    Runtime.getRuntime().addShutdownHook(new Thread()
    {
        @Override
        public void run()
        {
            JOptionPane.showConfirmDialog(null, "Do you", "Message", 
                    JOptionPane.YES_NO_OPTION);
            MessageBox.infoBox("YOUR INFORMATION HERE", "TITLE BAR MESSAGE");
        }
    });

    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                Mainframe window = new Mainframe();
                window.frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });

}

/**
 * Create the application.
 */
public Mainframe() {
    initialize();
}

/**
 * Initialize the contents of the frame.
 */
private void initialize() {
    frame = new JFrame();
    frame.setBounds(100, 100, 450, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JButton btnNewButton = new JButton("New button");
    btnNewButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            System.exit(0);


        }
    });
    frame.getContentPane().add(btnNewButton, BorderLayout.SOUTH);
}



}

I used the code for the "when the Application is being exited" from this example, but no matter how I close it, no "Messagebox is appearing". When I use the code for the MessageBoxes directly after the main void, the MessageBoxes appear, so I guess I missplaced the ShutdownHook.

I would really appreciate if anybody can help me fix this problem.

Answer 1

The shutdown hook is meant to perform certain actions (such as closing database connections and shutting down threads) once the application has been shut down (be it by a System.exit call or a kill signal from the OS). It is not meant to be used for these kinds of dialogs.

You will want to add a WindowListener to your JFrame, and perform an action on the windowClosing and/or windowClosed events. In this listener, you can then display a dialog or whatever you wish.

If you wish to be able to override the closing of the window, you can find details on how to do so on this question.