How to execute code when JFrame is closing after the [X] button is pressed?

The dispose() method is only called when the default close operation is set to JFrame.DISPOSE_ON_CLOSE (setDefaultCloseOperation (JFrame.DISPOSE_ON_CLOSE)). When the default close operation is set to JFrame.EXIT_ON_CLOSE, the JVM and everything else is shutdown and there is no need to dispose. When it is set to DO_NOTHING_ON_CLOSE or HIDE_ON_CLOSE, it means that you do not want the dispose() to be called and indeed it is not called. If you do not specifically set the default close operation value, by default it is HIDE_ON_CLOSE.

Therefore, overriding the dispose() method does not guarantee that your code is executed. The correct way to do this is either to add a WindowListener to the JFrame or override the processWindowEvent method. If you would like the user to decide if he/she truly wants to close the frame, the method to override is processWindowEvent. If you just want to do some setting/cleaning, you may override either of these methods.

1. Add a WindowListener to the JFrame

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class WindowCloseTest extends JFrame {
    public WindowCloseTest() {
        this.add(new JLabel("WindowCloseTest"));
        setSize(300, 200);
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e){
                //your code to be executed before window is closed.
                //not the place to opt out closing window
                System.out.println("exiting1...");
                super.windowClosing(e);
                System.out.println("exiting1.1...");
            }
        });
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                WindowCloseTest test = new WindowCloseTest();
                test.setVisible(true);
            }
        });
    }
}

2. Override the processWindowEvent method

import java.awt.event.WindowEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;

public class WindowCloseTest extends JFrame {
    public WindowCloseTest() {
        this.add(new JLabel("WindowCloseTest"));
        setSize(300, 200);
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    }

    @Override
    protected void processWindowEvent(WindowEvent e) {
        if (e.getID() == WindowEvent.WINDOW_CLOSING) {
            int opt = JOptionPane.showConfirmDialog(null,
                    "Are you sure to close this window?", "Confirm",
                    JOptionPane.YES_NO_OPTION,
                    JOptionPane.QUESTION_MESSAGE);
            if (opt == JOptionPane.NO_OPTION) {
                return;
            }
            System.out.println("exiting2.2...");
            //your code to be executed before window is closed.
        }
       
        super.processWindowEvent(e);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                WindowCloseTest test = new WindowCloseTest();
                test.setVisible(true);
            }
        });
    }
}

References:
1. How to execute code when JInternalFrame is closing programmatically
2. How to execute code when a JInternalFrame is closing after the [X] button is pressed

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