Java Threads, 3rd Edition

Chapter 10. Thread Pools

For various reasons, thread pools are a very common tool in a multithreaded developer’s toolkit. Most programs that use a lot of threads benefit in some way from using a thread pool.

J2SE 5.0 comes with its own thread pool implementation. Prior to this release, developers were left to write their own thread pool or use any number of commonly available implementations (including one we developed in earlier editions of this book and which is discussed in Appendix A). In this chapter, we discuss the thread pool implementation that comes with J2SE 5.0. If you can’t use that implementation yet, the information in this chapter is still useful: you’ll find out how and when using a thread pool can be advantageous. With that understanding, it’s simple to use any thread pool implementation in your own program.

Why Thread Pools?

The idea behind a thread pool is to set up a number of threads that sit idle, waiting for work that they can perform. As your program has tasks to execute, it encapsulates those tasks into some object (typically a Runnable object) and informs the thread pool that there is a new task. One of the idle threads in the pool takes the task and executes it; when it finishes the task, it goes back and waits for another task.

Thread pools have a maximum number of threads available to run these tasks. Consequently, when you add a task to a thread pool, it might have to wait for an available thread to run it. That may not sound encouraging, but it’s at the ...