问题:

I was wondering if there is a way, by clicking on the "X", to let the program perform some code before closing the JFrame. The setDefaultCloseOperation() method takes only an integer.

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

回答1:

You might be interested in using a WindowListener. The WindowListener tutorial.

回答2:

this.addWindowListener(new WindowAdapter(){
                public void windowClosing(WindowEvent e){
                    int i=JOptionPane.showConfirmDialog(null, "Seguro que quiere salir?");
                    if(i==0)
                        System.exit(0);//cierra aplicacion
                }
            });

回答3:

@Jeffrey has a good answer, but you should consider what you're trying to do. If you really want to do something upon the closing of a frame, then a WindowListener is the way to go. However, if you're looking for a place to do some cleanup and graceful shutdown stuff, then you probably want a shutdown hook instead. Using a WindowListener, the code will only be triggered, as you said, by the user "clicking on the X". But what if the user starts the app in the foreground of a terminal and kills it with Ctrl+C? What if the user kills the process from the command line or from a task manager?