Java shutdown hooks

Java allows you to add shutdown hooks to your code. A shutdown hook is simply a thread that has been left in the initialized state. When your JVM is about to shutdown, the shutdown hook thread kicks in. The finalization processes of java objects run after the shutdown hooks complete. The JVM allows you to register more than one shutdown hook.

public class Task
{
    public static void main(String[] args)
    {
        Runtime runtime = Runtime.getRuntime();
        Thread thread = new Thread(new ShutDownListener());
        runtime.addShutdownHook(thread);
        someProcess();
    }
 
    private static void someProcess()
    {
        try
        {
            System.out.println("I am busy");
            Thread.sleep(2000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}
 
class ShutDownListener implements Runnable
{
    @Override
    public void run()
    {
        System.out.println("I am shutting down");
    }
}

The Task class is a simple class that runs a process. Once this process finishes up, we want the JVM to run a shutdown hook to notify us that the JVM is shutting down. When this program is run, the output is

I am busy
I am shutting down

The shutdown hook still executes. So the hook will execute even if there are errors / exceptions in the main program. What happens when the hook itself throws an error / exception ?. The uncaught exception will by default be printed to System.err and propagated to the VM. The behavior is comparable to a thread encountering an uncaught exception.

It seems pretty robust. However the shutdown hook is not guaranteed to execute. If a user closes the app abruptly or the VM crashes, or you click on the little red button on the eclipse console view, the shutdown hook will not run. It is not a good idea to use a shutdown hook to release critical resources (I have seen some code snippets that do this). It might end up not running and cause damage. The shutdown hook will execute only on normal termination or orderly shutdown.

Use the shutdown hook if you would like to do trivial operations with it. You can write code inside the hook that can clean up after the program (say delete a temporary file). Or write a bye bye message. The code inside the hook should not do critical things like release DB connections that your program acquired. Doing something like that is asking for trouble, since this code may never run.

Also keep in mind that hooks run concurrently. Each hook is registered as a Thread with the VM and each Thread will run in parallel with the other hooks. If the hooks synchronize over resources incorrectly you will end up dead locking the application.

The hooks also need to finish up quickly. If they do not, that poses a problem. The application will wait for ever to exit gracefully.

So now you know what shutdown hooks are and how to use them, if you ever need to.

In Summary

• Do not write shutdown hooks to do critical tasks
  
• Make sure your shut down hooks complete quickly
  
• Consider using one shutdown hook instead of several. If you decide to use several shutdown hooks, make their activities thread safe.
  
• Hook code flow should not depend on the method in which the application was shutdown. How an application terminates is never guaranteed.