Puzzle 39: Hello, Goodbye

This program adds an unusual twist to the usual Hello world program. What does it print?

public class HelloGoodbye {

    public static void main(String[] args) {

        try {

            System.out.println("Hello world");

            System.exit(0);

        } finally {

            System.out.println("Goodbye world");

        }

    }

}

Solution 39: Hello, Goodbye

The program contains two println statements: one in a try block and the other in the corresponding finally block. The TRy block executes its println and finishes execution prematurely by calling System.exit. At this point, you might expect control to transfer to the finally block. If you tried the program, though, you found that it never can say goodbye: It prints only Hello world. Doesn't this violate the principle explained in Puzzle 36?

It is true that a finally block is executed when a try block completes execution whether normally or abruptly. In this program, however, the try block does not complete execution at all. The System.exit method halts the execution of the current thread and all others dead in their tracks. The presence of a finally clause does not give a thread special permission to continue executing.

When System.exit is called, the virtual machine performs two cleanup tasks before shutting down. First, it executes all shutdown hooks that have been registered with Runtime.addShutdownHook. This is useful to release resources external to the VM. Use shutdown hooks for behavior that must occur before the VM exits. The following version of the program demonstrates this technique, printing both Hello world and Goodbye world, as expected:

public class HelloGoodbye {

    public static void main(String[] args) {

        System.out.println("Hello world");

        Runtime.getRuntime().addShutdownHook(

            new Thread() {

                public void run() {

                    System.out.println("Goodbye world");

                }

            });

        System.exit(0);

    }

}

The second cleanup task performed by the VM when System.exit is called concerns finalizers. If either System.runFinalizersOnExit or its evil twin Runtime.runFinalizersOnExit has been called, the VM runs the finalizers on all objects that have not yet been finalized. These methods were deprecated a long time ago and with good reason. Never call System.runFinalizersOnExit or Runtime.runFinalizersOnExit for any reason: They are among the most dangerous methods in the Java libraries [ThreadStop]. Calling these methods can result in finalizers being run on live objects while other threads are concurrently manipulating them, resulting in erratic behavior or deadlock.

In summary, System.exit stops all program threads immediately; it does not cause finally blocks to execute, but it does run shutdown hooks before halting the VM. Use shutdown hooks to terminate external resources when the VM shuts down. It is possible to halt the VM without executing shutdown hooks by calling System.halt, but this method is rarely used.