Question

I have a standalone application in which I have to prompt the user with an confirm dialog box to save the changes made by him when he tries to shutdown the system by start-->shutdown.

I came to know that by using signalhandlers we can do it.
Can some one help me how to use signal handlers

Answer 1

Update May 2012 (2 and half years later)

Trejkaz comments:

On current versions of Java this signal handling code fails because the “INT” signal is “reserved by the VM or the OS”.
Additionally, none of the other valid signal names actually fire when something requests the application to close (I just painstakingly tested all of the ones I could find out about…)
The shutdown hook mostly works but we find that in our case it isn’t firing, so the next step is obviously to resort to registering a handler behind the JVM’s back

The chapter “Integrating Signal and Exception Handling” of the “Troubleshooting Guide for HotSpot VM” mentions the signals “SIGTERM, SIGINT, SIGHUP” only for Solaris OS and Linux.
Only Exception Handling on Windows are mentioned.

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Original answer (Sept 2009)

a ShutdownHook should be able to handle that case

Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
    public void run() {
        // what you want to do
    }
}));

(with caveats)
See also:

• Java signal handling and termination (link is dead, see archive or mirror)
• java exit signal handling:

as an illustration of simple signal handling:

public class Aaarggh {
  public static void main(String[] args) throws Exception {
    Signal.handle(new Signal("INT"), new SignalHandler () {
      public void handle(Signal sig) {
        System.out.println(
          "Aaarggh, a user is trying to interrupt me!!");
        System.out.println(
          "(throw garlic at user, say `shoo, go away')");
      }
    });
    for(int i=0; i<100; i++) {
      Thread.sleep(1000);
      System.out.print('.');
    }
  }
}