Question

There are certain configuration files that the user should save before closing the browser. Is there a way to detect if the user is trying to close the browser and to warn them to first save the files?

Answer 1

You should define a public method inside your applet and invoke it using Javascript before the user closes a page:
window.onbeforeunload = document.YourApplet.YourMethod(event);
where YourApplet is your applet’s name attribute: <applet name="YourApplet" ...
and YourMethod is the method you defined in your applet.