Chapter1.Multithreading

Chapter 1. Multithreading

• What Are Threads?

• Interrupting Threads

• Thread States

• Thread Properties

• Synchronization

• Blocking Queues

• Thread-Safe Collections

• Callables and Futures

• Executors

• Synchronizers

• Threads and Swing

You are probably familiar with multitasking in your operating system: the ability to have more than one program working at what seems like the same time. For example, you can print while editing or sending a fax. Of course, unless you have a multiple-processor machine, the operating system is really doling out CPU time to each program, giving the impression of parallel activity. This resource distribution is possible because although you may think you are keeping the computer busy by, for example, entering data, much of the CPU's time will be idle.

Multitasking can be done in two ways, depending on whether the operating system interrupts programs without consulting with them first or whether programs are only interrupted when they are willing to yield control. The former is called preemptive multitasking; the latter is called cooperative (or, simply, nonpreemptive) multitasking. Older operating systems such as Windows 3.x and Mac OS 9 are cooperative multitasking systems, as are the operating systems on simple devices such as cell phones. UNIX/Linux, Windows NT/XP (and Windows 9x for 32-bit programs), and OS X are preemptive. Although harder to implement, preemptive multitasking is much more effective. With cooperative multitasking, a badly behaved program can hog everything.

Multithreaded programs extend the idea of multitasking by taking it one level lower: individual programs will appear to do multiple tasks at the same time. Each task is usually called a threadwhich is short for thread of control. Programs that can run more than one thread at once are said to be multithreaded.

So, what is the difference between multiple processes and multiple threads? The essential difference is that while each process has a complete set of its own variables, threads share the same data. This sounds somewhat risky, and indeed it can be, as you will see later in this chapter. However, shared variables make communication between threads more efficient and easier to program than interprocess communication. Moreover, on some operating systems, threads are more "lightweight" than processesit takes less overhead to create and destroy individual threads than it does to launch new processes.

Multithreading is extremely useful in practice. For example, a browser should be able to simultaneously download multiple images. A web server needs to be able to serve concurrent requests. The Java programming language itself uses a thread to do garbage collection in the backgroundthus saving you the trouble of managing memory! Graphical user interface (GUI) programs have a separate thread for gathering user interface events from the host operating environment. This chapter shows you how to add multithreading capability to your Java applications.

Multithreading changed dramatically in JDK 5.0, with the addition of a large number of classes and interfaces that provide high-quality implementations of the mechanisms that most application programmers will need. In this chapter, we explain the new features of JDK 5.0 as well as the classic synchronization mechanisms, and help you choose between them.

Fair warning: multithreading can get very complex. In this chapter, we cover all the tools that an application programmer is likely to need. However, for more intricate system-level programming, we suggest that you turn to a more advanced reference, such as Concurrent Programming in Java by Doug Lea [Addison-Wesley 1999].