为什么这个基于Swing的程序不终止呢?

import javax.swing.JFrame;
import javax.swing.JOptionPane;

final class App extends JFrame {
    private App() {
        super("App");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        JOptionPane.showMessageDialog(this, "App works");
        pack();
    }

    public static void main(final String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new App().setVisible(true);
            }
        });
    }
}