14.2 What Is a Thread?

A thread (or a thread of execution or a thread of control) is a single sequence of executable statements within a program. For Java applications, the flow of control begins at the first statement in main() and continues sequentially through the program statements. For Java applets, the flow of control begins with the first statement in init(). Loops within a program cause a certain block of statements to be repeated. If-else structures cause certain statements to be selected and others to be skipped. Method calls cause the flow of execution to jump to another part of the program, from which it returns after the method’s statements are executed. Thus, within a single thread, you can trace the sequential flow of execution from one statement to the next.

One way to visualize a thread is to imagine that you could make a list of the program’s statements as they are executed by the computer’s central processing unit (CPU). Thus, for a particular execution of a program with loops, method calls, and selection statements, you could list each instruction that was executed, beginning at the first, and continuing until the program stopped, as a single sequence of executed statements. That’s a thread!

Now imagine that we break a program up into two or more independent threads. Each thread will have its own sequence of instructions. Within a single thread, the statements are executed one after the other, as usual. However, by alternately executing the statements from one thread and another, the computer can run several threads concurrently. Even though the CPU executes one instruction at a time, it can run multiple threads concurrently by rapidly alternating among them. The main advantage of concurrency is that it allows the computer to do more than one task at a time. For example, the CPU could alternate between downloading an image from the Internet and running a spreadsheet calculation. This is the same way you ate toast and cereal and drank coffee in our earlier breakfast example. From our perspective, it might look as if the computer had several CPUs working in parallel, but that’s just the illusion created by effectively scheduling threads.