Using a Thread to Perform Repeated Tasks

[sukishan]

Using a Thread to Perform Repeated Tasks
An applet that performs the same task over and over again typically should have a thread with a while (or do...while) loop that performs the task. A typical example is an applet that performs timed animation, such as a movie player or a game. Animation applets need a thread that requests repaints at regular intervals. Another example is an applet that reads data supplied by a server-side application. (See Using a Server to Work Around Security Restrictions for such an example.)

Applets typically create threads for repetitive tasks in the applet start method. Creating the thread there makes it easy for the applet to stop the thread when the user leaves the page. All you need to do is implement the stop method so that it stops the applet's thread. When the user returns to the applet's page, the start method is called again, and the applet can again create a thread to perform the repetitive task.

Below is AnimatorApplet's implementation of the start and stop methods.

public void start() {
    if (frozen) {
        //Do nothing.  The user has requested that we
        //stop changing the image.
    } else {
        //Start animating!
        if (animatorThread == null) {
            animatorThread = new Thread(this);
        }
        animatorThread.start();
    }
}

public void stop() {
    animatorThread = null;
}

The this in new Thread(this) indicates that the applet provides the body of the thread. It does so by implementing the java.lang.Runnable interface, which requires the applet to provide a run method that forms the body of the thread. We'll discuss AnimatorApplet's run method more a little later.

Notice that nowhere in the AnimatorApplet class is the Thread stop method called. This is because calling the Thread stop method is like clubbing the thread over the head. It's a drastic way to get the thread to stop what it's doing. Instead, you can write the thread's run method in such a way that the thread will gracefully exit when you tap it on the shoulder. This shoulder tap comes in the form of setting to null an instance variable of type Thread.

In AnimatorApplet, this instance variable is called animatorThread. The start method sets it to refer to the newly created Thread object. When the applet needs to kill the thread, it sets animatorThread to null. This kills the thread not by making it be garbage collected — it can't be garbage collected while it's runnable — but because at the top of its loop, the thread checks animatorThread, continuing or exiting depending on the value of animatorThread. Here's the relevant code:

public void run() {
    . . .
    while (Thread.currentThread() == animatorThread) {
        ...//Display a frame of animation and then sleep.
    }
}

If animatorThread refers to the same thread as the currently executing thread, the thread continues executing. If, on the other hand, animatorThread is null, the thread exits. If animatorThread refers to another thread, then a race condition has occurred: start has been called so soon after stop (or this thread has taken such a long time in its loop) that start has created another thread before this thread reached the top of its while loop. Whatever the cause of the race condition, this thread should exit.