Can we execute the java program after calling System.exit(0) ?

Yes, By adding shutdown hook

void addShutdownHook(Thread hook) registers a new JVM shutdown hook.

When JVM shuts down ?

1. Normal exit

   * When the last non-daemon thread exits

   * When System.exit() invoked

2. Interrupted by User

    * When pressing ^C

    * System-wide event like, Shutdown, Reboot, Log off

Why Shutdown hook is useful ?

Shutdown hook can be used to :

• Perform resource cleanup like, closing log file, sending some alerts
• Save the state when JVM shuts down

Implementation of Shutdown hook

A shutdown hook is an initialized but un-started thread.

Once the shutdown sequence has begun, 

• It can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
• It is impossible to register a new shutdown hook or de-register a previously-registered hook, Otherwise it will cause an IllegalStateException.

Example

Defining Shutdown hook as a thread

class ShutDownHook extends Thread {

    public void run(){ 

        System.out.println("Shutdown hook tasks - Cleanup, Save state..."); 

    }

}

Inside main method, Add the shutdown hook / thread

Runtime runtime = Runtime.getRuntime();

runtime.addShutdownHook(new ShutDownHook());

Example (same using anonymous class)

Runtime runtime = Runtime.getRuntime();

runtime.addShutdownHook(new Thread() {

    public void run(){ 

        System.out.println("Shutdown hook tasks - Cleanup, Save state..."); 

    }

});