Java Performance Tuning, 2nd Edition

Chapter 10. Threading

Minor Premise: One man can dig a posthole in sixty seconds.

Conclusion: Sixty men can dig a posthole in one second.

—Ambrose Bierce, The Devil’s Dictionary

Multithreading allows an application to do multiple things at the same time. While it is often possible to get the same effect with clever programming in a single thread, Java’s extensive support of threads makes it easier to use multiple threads. In addition, single-threaded applications cannot take advantage of multiprocessor machines.

However, multithreading can be difficult to implement effectively. Multithreading improves performance in many cases, but it also has drawbacks if the default mechanisms for cooperation between threads are used simplistically. In this chapter, we look at the benefits and the disadvantages threads offer to performance. We examine the likely problems that may be encountered and discuss how to minimize the performance downside while still gaining the benefits of multiple threads.

Synchronization and Monitors

Synchronization can be confusing, so I felt it was worth including a short reminder of its subtleties here.

Two or more threads accessing and updating the same data variables have no way of knowing when a particular access or update will occur relative to any other thread accesses. Synchronization ensures that a group of statements (a synchronized block) will execute atomically as far as all synchronized threads are concerned. Synchronization does not address the problem ...