Multithreaded Programming
U
nlike many other computer languages, Java provides built-in support for multithreaded
programming. A multithreaded program contains two or more parts that can run
concurrently. Each part of such a program is called a thread, and each thread defines
a separate path of execution. Thus, multithreading is a specialized form of multitasking.
You are almost certainly acquainted with multitasking, because it is supported by virtually
all modern operating systems. However, there are two distinct types of multitasking: process-
based and thread-based. It is important to understand the difference between the two. For
most readers, process-based multitasking is the more familiar form. A process is, in essence,
a program that is executing. Thus, process-based multitasking is the feature that allows your
computer to run two or more programs concurrently. For example, process-based multitasking
enables you to run the Java compiler at the same time that you are using a text editor. In process-
based multitasking, a program is the smallest unit of code that can be dispatched by the
scheduler.
In a thread-based multitasking environment, the thread is the smallest unit of dispatchable
code. This means that a single program can perform two or more tasks simultaneously. For
instance, a text editor can format text at the same time that it is printing, as long as these
two actions are being performed by two separate threads. Thus, process-based multitasking
deals with the "big picture," and thread-based multitasking handles the details.
Multitasking threads require less overhead than multitasking processes. Processes are
heavyweight tasks that require their own separate address spaces. Interprocess communication
is expensive and limited. Context switching from one process to another is also costly. Threads,
on the other hand, are lightweight. They share the same address space and cooperatively
share the same heavyweight process. Interthread communication is inexpensive, and context
switching from one thread to the next is low cost. While Java programs make use of process-
based multitasking environments, process-based multitasking is not under the control of
Java. However, multithreaded multitasking is.
Multithreading enables you to write very efficient programs that make maximum use of
the CPU, because idle time can be kept to a minimum. This is especially important for the
interactive, networked environment in which Java operates, because idle time is common.
For example, the transmission rate of data over a network is much slower than the rate at
which the computer can process it. Even local file system resources are read and written at a
much slower pace than they can be processed by the CPU. And, of course, user input is much
slower than the computer. In a single-threaded environment, your program has to wait for
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