Unraveling threads

Multi-threaded programming offers developers substantial advantages, and users increased performance. Despite the obvious benefits, novice programmers are reluctant to write multi-threaded applications because of their perceived complexity. This article will offer a gentle introduction to multi-threaded programming. -- David Reilly

So, what exactly are threads?

Threads are multiple instances of an application or applet that execute concurrently.

Woah! That's a mouthful isn't it. Let's break it down a little.

When we start an application, it executes in its own process - it has its own chunk of memory for data storage, and it runs alongside other processes. Modern operating systems (such as Windows, and Unix) create the illusion that these processes are running concurrently. A process can execute at the same time as another process, and the operating shares time between all running processes (though not always evenly). Each process represents a single thread of execution.

Now a multi-threaded application is a special kind of application. It uses only a single process, but that process has multiple threads of execution. The same code and data is used, but we have more than one instance of the application running. Each thread runs concurrently, so when one thread is idle (perhaps waiting for data input), another thread can be performing some useful task. Easy huh?

What can threads do?

Threads are extremely useful, because they allow developers to write applications that are more responsive to the user. When your application is processing data, its poor design for the user-interface to be disabled until the task is complete. If your application is single-threaded, the user-interface can't respond to user commands if the application is performing another task. Furthermore, applications can their idle time to process things behind the scenes. For example, an applet that displays some animation can be preparing the next frame.

So how do I use threads?

Threads are actually quite easy to use, provided you do a little planning. First you have to identify the tasks a thread will perform. Usually your thread(s) will perform processing, allowing your primary thread to handle user-interface events. Once you've identified a task, you have to write the code for your thread.

Our example will be an applet that displays a simple text scrolling animation. We'll create a new thread to create the animation, leaving our application free to respond to the user interface. Our applet will respond to two user-interface events : mouseDown and mouseUp.

This example requires a Java enabled browser

Example 1.0 - Animated applet with multiple threads

When a user clicks on the applet, animation stops. When the mouse button is released, the animation resumes. Other than these two actions, all our applet does is launch a second thread to perform the animation. Let's look at the thread specific code.

Creating a new thread

Creating a new thread is pretty simple, and takes only a few lines of code. In our example, the new thread is created in the applet's start() method. We first check to see if an animation thread is already running, and if not, create one.

    // Pass our thread an instance of java.lang.Runnable (us)
    AnimationThread = new Thread(this);

Notice the parameter we use when creating a new Thread object. This form of constructor accepts an instance of java.lang.Runnable, which is an interface our applet implements. We'll see how this works later, but for those who are curious, it is an interface that indicates our applet has work a thread can perform. We use the this keyword to indicate that the current class/object is being referred to.

Next, we call that thread's start method. Once started, a thread will execute independently, until it completes its task, or the thread is suspended or stopped. Here's the full code for our start method.

// Check to see if thread is active
if (AnimationThread == null)
{
    // Pass our thread an instance of java.lang.Runnable (us)
    AnimationThread = new Thread(this);

    // Start the thread (new thread executes public void run() method)
    AnimationThread.start();
}

Putting threads to work

When a thread is started, it calls the run() method of the class implementing the java.lang.Runnable interface. That means that we need to write a run() method to get our thread to perform useful work, as well as indicating in our class declaration that our applet will implement the interface.

Here's the change we need to make to a class declaration (change in bold). We simple add the implements keyword, and indicate the interface that is implemented.

public class AnimationApplet extends Applet implements Runnable

Finally, we need to write a run() method for our thread runnable class. Your run method must be public, and should contain the code you want the thread to execute. When the run method completes, the thread will stop - so make sure your run() method contains a loop of some kind so that it can continue to perform useful work.

// Thread code execution starts here
public void run()
{
    // Initialization code
    ................

    while (true)
    {
        try
        {                
            // Create the next frame of animation
            ................

            repaint();
            Thread.sleep(10);

        }
        catch (InterruptedException e)
        {
            stop();
        }
    }
}

Most of the work performed in this particular run() method is not related to the topic of threads, but feel free to examine the source in more detail if you're interested. One important point that must be made is that our thread tries not to consume all available CPU resources. Our thread, when it has created the next animation frame, goes to sleep. The JVM will, most likely, allocate a fair share of CPU time to each thread, but in the event it does not, our thread will sleep. Unless your thread is extremely important, its probably a good idea to be considerate of other application threads.

Summary

Writing multi-threaded applications need not be a difficult or mysterious task. Simply create a new class that implements the java.lang.Runnable class, or implement it in your application/applets class. Then, pass it as a parameter to a thread constructor, and start the thread. Voila! Instant threads.

Further Reading

Java Threads, by Scott Oak. Published by O'Reilly & Associates. ISBN : 1565924185

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