Learning Java, 4th Edition

Introducing Threads

Conceptually, a thread is a flow of control within a program. A thread is similar to the more familiar notion of a process, except that threads within the same application are much more closely related and share much of the same state. It’s kind of like a golf course, which many golfers use at the same time. The threads cooperate to share a working area. They have access to the same objects, including static and instance variables, within their application. However, threads have their own copies of local variables, just as players share the golf course but do not share some personal items like clubs and balls.

Multiple threads in an application have the same problems as the golfers—in a word, synchronization. Just as you can’t have two sets of players blindly playing the same green at the same time, you can’t have several threads trying to access the same variables without some kind of coordination. Someone is bound to get hurt. A thread can reserve the right to use an object until it’s finished with its task, just as a golf party gets exclusive rights to the green until it’s done. And a thread that is more important can raise its priority, asserting its right to play through.

The devil is in the details, of course, and those details have historically made threads difficult to use. Fortunately, Java makes creating, controlling, and coordinating threads simpler by integrating some of these concepts directly into the language.

It is common to stumble over threads ...