Thread Basics in Java

 The multithreading support in Java revolves around the concept of a thread. The question is, what is a thread? Put simply, a thread is a single stream of execution within a process. OK, maybe that wasn't so simple. It might be better to start off by exploring exactly what a process is.

A process is a program executing within its own address space. Java is a multiprocessing system, meaning that it supports many processes running concurrently in their own address spaces. You may be more familiar with the term multitasking, which describes a scenario very similar to multiprocessing. As an example, consider the variety of applications typically running at once in a graphical environment. As I write this, I am running Microsoft Word along with Internet Explorer, Windows Explorer, Inbox, CD Player, and Volume Control. These applications are all processes executing within the Windows 95 environment. In this way, you can think of processes as being analogous to applications, or stand-alone programs; each process in a system is given its own room to execute.

A thread is a sequence of code executing within the context of a process. As a matter of fact, threads cannot execute on their own; they require the overhead of a parent process to run. Within each of the processes I mentioned running on my machine, there are no doubt a variety of threads executing. For example, Word may have a thread in the background automatically checking the spelling of what I'm writing, while another thread may be automatically saving changes to the document I'm working on. Like Word, each application (process) can be running many threads that are performing any number of tasks. The significance here is that threads are always associated with a particular process.

Note

Threads are sometimes referred to as lightweight processes, implying that they are a limited form of a process. A thread is in fact very similar to a full-blown process, with the major difference being that a thread always runs within the context of another program. Unlike processes, which maintain their own address space and operating environment, threads rely on a parent program for execution resources.

I've described threads and processes using Windows 95 as an example, so you've probably guessed that Java isn't the first system to employ the use of threads. That's true, but Java is the first major programming language to incorporate threads at the heart of the language itself. Typically, threads are implemented at the system level, requiring a platform-specific programming interface separate from the core programming language. This is the case with C/C++ Windows programming, because you have to use the Win32 programming interface to develop multithreaded Windows applications.

Java is presented as both a language and a runtime system, so the Sun architects were able to integrate threads into both. The end result is that you are able to make use of Java threads in a standard, cross-platform fashion. Trust me, this is no small feat, especially considering the fact that some systems, like Windows 3.1, don't have any native support for threads!