[Java] Shutdown Hook

Shutdown Hook

- Shutdown hook can be used to perform cleanup resource or save the state when JVM shuts down normally or abruptly

- clean resource means closing log file, sending some alerts...]

- If you want to execute some code before JVM shuts down, use shutdown hook

1. When does the JVM shut down?

- user presses ctrl+c on the command prompt

- System.exit(int) method is invoked

- user logoff

- user shutdown etc..

2. The addShutdownHook(Thread hook) mehtod

- addShutdownHook() method of Runtime class is used to register the thread with the Virtual Machine

public void addShutdownHook(Thread hook){}

3. Simple Example

class MyThread extends Thread{ public void run(){ System.out.println("Shut down task completed"); } } public class Shutdown { public static void main(String[]args){ Runtime r = Runtime.getRuntime(); r.addShutdownHook(new MyThread()); System.out.println("Now main sleeping...press ctrl+c"); try{ Thread.sleep(3000); }catch(Exception e){ } } }