Use JVM Shutdown Hooks to Optimize Resources-2 : Page 2

A Simple Example: the SimpleHook Class
You implement JVM shutdown hooks using a separate class that extends the Thread class (click here to download the accompanying source code for this Solution). When you register an instance of this separate class with a JVM (you will learn how to do this later), the shutdown thread class starts when the JVM terminates.

The non-public class MyShutdown is defined at the bottom of the file SimpleHook.java (included in the source code download):

class MyShutdown extends Thread {
    public MyShutdown(SimpleHook managedClass) {
        super();
        this.managedClass = managedClass;
    }
    private SimpleHook managedClass;
    public void run() {
        System.out.println("MyShutDown thread started");
        try {
            managedClass.freeResources();
        } catch (Exception ee) {
            ee.printStackTrace();
        }
    }
}

Notice that the constructor requires a reference to the managed application class. In the design pattern this Solution (and I) use, the managed parent class is expected to define a method freeResources(). To be tidy, you could define a separate interface that defines the freeResources() method signature and have managed classes implement this interface. However, to keep this example simple, I just concentrate on using JVM shutdown hooks.

The constructor for the managed class SimpleHook creates an instance of the MyShutdown class and registers this instance with the JVM:

    public SimpleHook() {
        // set up service termination hook (gets called
        // when the JVM terminates from a signal):
        MyShutdown sh = new MyShutdown(this);
        Runtime.getRuntime().addShutdownHook(sh);
    }

See the SimpleHook.java source file for the complete code.

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