Surviving Abrupt Shutdown

In many circumstances, you need a chance to do some clean-up when the user shuts down your application. The problem is, the user does not always follow the recommended procedure to exit. Java provides an elegant way for programmers to execute code in the middle of the shutdown process, thus making sure your clean-up code is always executed. This article shows how to use a shutdown hook to guarantee that clean-up code is always run, regardless of how the user terminates the application.

You may have code that must run just before an application completely exits. For example, if you are writing a text editor with Swing and your application creates a temporary edit file when it starts, this temporary file must be deleted when the user closes your application. If you are writing a servlet container such as Tomcat or Jetty, you must call the destroy method of all loaded servlets before the application shuts down.

In many cases, you rely on the user to close the application as prescribed. For instance, in the first example, you may provide a JButton that, when clicked, runs the clean up code before exiting. Alternatively, you may use a Window listener that listens to the windowClosing event. Tomcat uses a batch file that can be executed for a proper shutdown. However, you know that the user is the king; he or she can do whatever they want with the application. He or she might be nice enough to follow your instruction, but could just close the console or log off of the system without first closing your application.

In Java, the virtual machine shuts down itself in response to two types of events: first, when the application exits normally, by calling the System.exit method or when the last non-daemon thread exits. Second, when the user abruptly forces the virtual machine to terminate; for example, by typing Ctrl+C or logging off from the system before closing a running Java program.

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Fortunately, the virtual machine follows this two-phase sequence when shutting down:

1. The virtual machine starts all registered shutdown hooks, if any. Shutdown hooks are threads registered with the Runtime. All shutdown hooks are run concurrently until they finish.
2. The virtual machine calls all uninvoked finalizers, if appropriate.

In this article, we are interested in the first phase, because it allows the programmer to ask the virtual machine to execute some clean-up code in the program. A shutdown hook is simply an instance of a subclass of the Thread class. Creating a shutdown hook is simple:

1. Write a class extending the Thread class.
2. Provide the implementation of your class' run method. This method is the code that needs to be run when the application is shut down, either normally or abruptly.
3. In your application, instantiate your shutdown hook class.
4. Register the shutdown hook with the current runtime's addShutdownHook method.

As you may have noticed, you don't start the shutdown hook as you would other threads. The virtual machine will start and run your shutdown hook when it runs its shutdown sequence.

The code in Listing 1 provides a simple class called ShutdownHookDemo and a subclass of Thread named ShutdownHook. Note that the run method of the ShutdownHook class simply prints the string "Shutting down" to the console. Of course, you can insert any code that needs to be run before the shutdown.

After instantiation of the public class, its start method is called. The start method creates a shutdown hook and registers it with the current runtime.

ShutdownHook shutdownHook = new ShutdownHook();
Runtime.getRuntime().addShutdownHook(shutdownHook);

Then, the program waits for the user to press Enter.

System.in.read();

When the user does press Enter, the program exits. However, the virtual machine will run the shutdown hook, printing the words "Shutting down."

package test;

public class ShutdownHookDemo {
    public void start() {
        System.out.println("Demo");
        ShutdownHook shutdownHook = new ShutdownHook();
        Runtime.getRuntime().addShutdownHook(shutdownHook);
    }

    public static void main(String[] args) {
        ShutdownHookDemo demo = new ShutdownHookDemo();
        demo.start();
        try {
            System.in.read();
        }
        catch(Exception e) {
        }
    }
}

class ShutdownHook extends Thread {
    public void run() {
        System.out.println("Shutting down");
    }
}

As another example, consider a simple Swing application whose class is called MySwingApp (see Figure 1). This application creates a temporary file when it is launched. When closed, the temporary file must be deleted. The code for this class is given in Listing 2 on the following page.

Figure 1: A simple Swing application

Pages: 1, 2

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