Learning Java, 5th Edition

Chapter 9. Threads

We take for granted that modern computer systems can manage many applications and operating system (OS) tasks running concurrently and make it appear that all the software is running simultaneously. Most systems today have multiple processors and or at least multiple cores and can achieve an impressive degree of parallelism. The OS still juggles applications at a higher level but turns its attention from one to the next so quickly that they also appear to run at once.

In the old days, the unit of concurrency for such systems was the application or process. To the OS, a process was more or less a black box that decided what to do on its own. If an application required greater concurrency, it could get it only by running multiple processes and communicating between them, but this was a heavyweight approach and not very elegant. Later, the concept of threads was introduced. Threads provide fine-grained concurrency within a process under the application’s own control. Threads have existed for a long time, but have historically been tricky to use. In Java, support for threading is built into the language, making it easier to work with threads. The Java concurrency utilities address common patterns and practices in multithreaded applications and raise them to the level of tangible Java APIs. Collectively, this means that Java is a language that supports threading both natively and at a high level. It also means that Java’s APIs take full advantage of threading, so ...