What is addShutdownHook method in java?

• addShutdownHook method registers a new virtual-machine shutdown hook.
• A shutdown hook is a initialized but unstarted thread. 
• When JVM starts its shutdown it will start all registered shutdown hooks in some unspecified order and let them run concurrently. 

When JVM (Java virtual machine)  shuts down >
When the last non-daemon thread finishes, or when the System.exit is called.

Once JVM’s shutdown has begun new shutdown hook cannot be registered neither  previously-registered hook can be de-registered. Any attempt made to do any of these operations causes an IllegalStateException.

Example:

public class AddShutDownHookTest extends Thread{

   public static void main(String[] args) throws InterruptedException {   

          System.out.println("main thread started");

         

          Runtime.getRuntime().addShutdownHook(new Thread(){

       public void run() {

          try{

                System.out.println("executing shutdown hook");

          }catch (Exception e){

                e.printStackTrace();

          }

          System.out.println("shutdown hook executed successfully");

       }

     });

         

          Thread.sleep(4000); //Optional delay

          System.out.println("main thread ended");

   }

}

/*OUTPUT

main thread started

main thread ended

executing shutdown hook

shutdown hook executed successfully

*/