Threads of Control

The Clock Applet (notes)

The Clock applet shown below displays the current time and updates its display every second. You can scroll this page and perform other tasks while the clock continues to update because the code that updates the clock's display runs within its own thread.

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Your browser doesn't understand the APPLET tag. Here's a screenshot of the clock applet that you would see running here if it did:

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This section highlights and explains the source code for the clock applet in detail. In particular, this page describes the code segments that implement the clock's threaded behavior; it does not describe the code segments that are related to the life cycle of the applet. If you have not written your own applets before or are not familiar with the life cycle of any applet, you may want to take this time to familiarize yourself with the material in The Life Cycle of an Applet before proceeding with this page.

Deciding to Use the Runnable Interface

The Clock applet uses the Runnable interface to provide the run method for its thread. To run within a Java-compatible browser, the Clock class has to derive from the Applet class. However, the Clock applet also needs to use a thread so that it can continuously update its display without taking over the process in which it is running. (Some browsers might create a new thread for every applet to prevent a misbehaved applet from taking over the main browser thread. However, you should not count on this when writing your applets; your applets should create their own threads when doing compute-intensive work.) But since the Java language does not support multiple inheritance, the Clock class can not inherit from Thread as well as from Applet. Thus, the Clock class must use the Runnable interface to provide its threaded behavior.

Applets are not threads, nor do any existing Java-compatible browsers or applet viewers automatically create threads in which to run applets. Therefore, if an applet needs any threads, it must create its own. The Clock applet needs one thread in which to perform its display updates because it updates its display frequently and the user needs to be able to perform other tasks (such as going to another page, or scrolling this one) at the same time the clock is running.

The Runnable Interface

The Clock applet provides a run method for its thread via the Runnable interface. The class definition for the Clock class indicates that it is a subclass of Applet and implements the Runnable interface. If you are not familiar with interfaces review the information in the Objects, Classes, and Interfaces lesson.

class Clock extends Applet implements Runnable {

The Runnable interface defines a single method called run that doesn't accept any arguments and doesn't return a value. Because the Clock class implements the Runnable interface, it must provide an implementation for the run method as defined in the interface. However, before explaining the Clock's run method, let's to look at some of the other elements of the Clock applet's code.

Creating the Thread

The application in which an applet is running calls the applet's start method when the user visits the applet's page. The Clock applet creates a Thread, clockThread, in its start method and starts the thread.

public void start() {
    if (clockThread == null) {
        clockThread = new Thread(this, "Clock");
        clockThread.start();
    }
}    

First, the start method checks to see if clockThread is null. If clockThread is null, then the applet has just been loaded or has been previously stopped and a new thread must be created. Otherwise, the applet is already running. The applet creates a new thread with this invocation:

clockThread = new Thread(this, "Clock");

Notice that this--the Clock applet--is the first argument to the thread constructor. The first argument to this Thread constructor must implement the Runnable interface and becomes the thread's target. When constructed in this way, the clock thread gets its run method from its target Runnable object--in this case, the Clock applet.

The second argument is just a name for the thread.

Stopping the Thread

When you leave the page that displays the Clock applet, the application in which the applet is running calls the applet's stop method. The Clock's stop method sets the clockThread to null. This tells the main loop in the run method to terminate (see the next section), eventually resulting in the thread stopping and being garbage collected. the continual updating of the clock.

public void stop() {
    clockThread = null;
}

You could use clockThread.stop instead, which would immediately stop the clock thread. However, the Thread class's stop method has a sudden effect, which means that the run method might be in the middle of some critical operation when the thread is stopped. For more complex run methods, using Thread's stop method might leave the program in an inconsistent or awkward state. For this reason, it's best to avoid using the Thread class's stop method when possible.

If you revisit the page, the start method is called again, and the clock starts up again with a new thread.

The Run Method

And finally the Clock's run method implements the heart of the Clock applet and looks like this:

public void run() {
        // loop terminates when clockThread
	// is set to null in stop()
    while (Thread.currentThread() == clockThread) {
        repaint();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e){
        }
    }
}   

As you saw in the previous section, when the applet is asked to stop, the applet sets the clockThread to null; this lets the run method know when to stop. Thus the first line of the run method loops until clockThread is null. Within the loop, the applet repaints itself and then tells the thread to sleep for 1 second (1000 milliseconds). An applet's repaint method ultimately calls the applet's paint method, which does the actual update of the applet's display area. The Clock applet's paint method gets the current time and draws it to the screen.

public void paint(Graphics g) {
    Date now = new Date();
    g.drawString(now.getHours() + ":" + now.getMinutes() + ":" + now.getSeconds(), 5, 10);
}

Threads of Control