Catching Ctrl+C in Java

Question

Is it possible to catch the Ctrl+C signal in a java command-line application? I'd like to clean up some resources before terminating the program.

Answer 1

You can attach a shutdown hook to the VM which gets run whenever the VM shuts down:

The Java virtual machine shuts down in response to two kinds of events:

• The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or

• The virtual machine is terminated in response to a user interrupt, such as typing Ctrl+C, or a system-wide event, such as user logoff or system shutdown.

The thread you pass as shutdown hook has to follow several rules, though, so read the linked documentation carefully to avoid any problems. This includes ensuring thread-safety, quick termination of the thread, etc.

Also, as commenter Jesper points out, shutdown hooks are guaranteed to run on normal shutdown of the VM but if the VM process is terminated forcibly they don't. This can happen if native code screws up or if you forcibly kill the process (kill -9, taskkill /f).

But in those scenarios all bets are off anyway, so I wouldn't waste too much thought on it.

Answer 2

Just for quick console testing purposes...

Runtime.getRuntime().addShutdownHook(new Thread() {
        public void run() {
            try {
                Thread.sleep(200);
                System.out.println("Shouting down ...");
                //some cleaning up code...

            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    });