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Unlike most 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.
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
traditional, single-threaded environment, your program has to wait for each of these tasks to
finish before it can proceed to the next one—even though the CPU is sitting idle
most of the time.
Multithreading lets you gain access to this idle time and put it to good use.
If you have programmed for operating systems such as Windows 98 or Windows NT, then you are already familiar with multithreaded programming. However, the fact that Java manages threads makes multithreading especially convenient, because many of the details are handled for you.
View Tutorial By: Vineeth at 2012-02-01 03:50:21
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