Java Programing

can be downloading an image. Multithreading is especially useful in GUI-based applications, as it improves the interactive performance of these applications. Unfortunately for us, programming with multiple threads can be quite a headache. The difficulty lies in making sure routines are implemented so they can be run by multiple concurrent threads. If a routine changes the value of a state variable, for example, then only one thread should be executing the routine at a time. Later in this section, we’ll examine briefly the issue of coordinating multiple threads’ access to shared data. In other languages, synchronization of threads can be extremely complex and error-prone. You’ll see that Java gives you a few simple tools that help you deal with many of these problems. Java threads can be started, stopped, suspended, and prioritized. Threads are preemptive, so a higher priority thread can interrupt a lower priority thread when vying for processor time. See Chapter 8, for a complete discussion of threads. The Java runtime system creates and manages a number of threads. (Exactly how varies with the implementation.) We’ve already mentioned the repaint thread, which manages repaint( ) requests and event processing for GUI components that belong to the java.awt and javax.swing packages. Our example applications have done most of their work in one thread. Methods like mouseDragged( ) and actionPerformed( ) are invoked by the windowing thread and run on its time. Similarly, our constructor runs as part of the main application thread. This means we are somewhat limited in the amount of processing we do within these methods. If we were, for instance, to go into an endless loop in our constructor, our application would never appear, as it would never finish initializing. If we want an application to perform any extensive processing, such as animation, a lengthy calculation, or communication, we should create separate threads for these tasks. 2.4.2 The Thread Class As you might have guessed, threads are created and controlled as Thread objects. An instance of the Thread class corresponds to a single thread. It contains methods to start, control, and stop the thread’s execution. Our basic plan is to create a Thread object to handle our blinking code. We call the Thread’s start( ) method to begin execution. Once the thread starts, it continues to run until we call the Thread’s interrupt( ) method to terminate it. So how do we tell the thread which method to run? Well, the Thread object is rather picky; it always expects to execute a method called run( ) to perform the action of the thread. The run( ) method can, however, with a little persuasion, be located in any class we desire. We specify the location of the run( ) method in one of two ways. First, the Thread class itself has a method called run( ). One way to execute some Java code in a separate thread is to subclass Thread and override its run( ) method to do our bidding. Invoking the start( ) method of the subclass object causes its run( ) method to execute in a separate thread. It’s not always desirable or possible to create a subclass of Thread to contain our run( ) method. The Thread class has a constructor that takes an object reference as its argument. If we create a Thread object using this constructor and call its start( ) method, the Thread executes the run( ) method of the argument object, rather than its own. In order to accomplish this, Java needs a guarantee that the object we are passing it does indeed contain a compatible run( ) method. We already know how to make such a guarantee: we use an interface. Java provides an interface named Runnable that must be implemented by any class that wants to become a Thread. - 50

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