Java Runtime addShutdownHook(Thread hook) Method

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Description

The Java Runtime addShutdownHook(Thread hook) method registers a new virtual-machine shutdown hook.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.

A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.

Declaration

Following is the declaration for java.lang.Runtime.addShutdownHook() method

public void addShutdownHook(Thread hook)

Parameters

hook − An initialized but unstarted Thread object

Return Value

This method does not return a value.

Exception

• IllegalArgumentException − If the specified hook has already been registered, or if it can be determined that the hook is already running or has already been run

• IllegalStateException − If the virtual machine is already in the process of shutting down

• SecurityException − If a security manager is present and it denies RuntimePermission("shutdownHooks")

Example: Adding Shutdown Hook to a Thread Object

The following example shows the usage of Java Runtime addShutdownHook() method. In this program, we've created one static inner class Message which is extending Thread. In main method, we've registered a shutdown hook with a new Message object using addShutdownHook() method. Then we kept system to sleep for 3 seconds and then printed a closing message. As shutdown hook is registered with Message object, its run method is called when program exits.

package com.tutorialspoint;

public class RuntimeDemo {

   // a class that extends thread that is to be called when program is exiting
   static class Message extends Thread {

      public void run() {
         System.out.println("Bye.");
      }
   }

   public static void main(String[] args) {
      try {

         // register Message as shutdown hook
         Runtime.getRuntime().addShutdownHook(new Message());

         // print the state of the program
         System.out.println("Program is starting...");

         // cause thread to sleep for 3 seconds
         System.out.println("Waiting for 3 seconds...");
         Thread.sleep(3000);

         // print that the program is closing 
         System.out.println("Program is closing...");
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Program is starting...
Waiting for 3 seconds...
Program is closing...
Bye.