When do I need to call this method Runtime.getRuntime().addShutdownHook()

Question

When do I actually need call this method Runtime.getRuntime().addShutdownHook() and when or why I need to shutdown my application. Could anyone please explain me this by giving some example.

Thanks

Answer 1

As far as I know, I will explain this below. You can google it and find lot of information too.

addShutdownHook() will register some actions which is to be performed on a Program's termination. The program that you start ends in two ways:

1. the main thread (Root) ends its running context;
2. the program meets some unexpected situation, so it cannot proceed further.

If you add a ShutdownHook, the hook will start a thread that will start running at time of termination only. For example:

 Runtime.getRuntime().addShutdownHook(new Thread() {       public void run() {         System.out.println("Running Shutdown Hook");       }     }); 

will print a Running Shutdown Hook at the time of program termination at any point. You might even call a System.exit(0).

For examples, you can google, there are enough of them. And the question 'When should you use this' is like asking 'What does catch do in a try-catch statement'.

You might have many situations like:

• your program had created many temporary files in filesystem you want to delete it;
• you need to send a distress signal to another process/machine before terminating;
• execute any clean-up actions, logging or after-error actions on unexpected behaviours.

All this will be needed for some point of time.

For examples you can go in here Example 1 or Example 2

Answer 2

You only worry about shutdown hooks when you want something to happen when a shutdown occurs on the virtual machine.

From Javadoc:

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 ^C, or a system-wide event, such as user logoff or system shutdown.

Thus, a shutdown hook is a initialized and unstarted thread that gets executed when a JVM shutdown occurs.

Popular examples of shutdown hooks exists in application servers (such as JBoss AS). When you press Ctrl+C, the JVM calls all the Runtime shutdown hooks registered (such as JBoss shutdown hooks) before exiting.