Wednesday, June 15, 2011

Chapter 1: Introduction to Threads

Threads are an important and interesting aspect of the Java Programming Language. If you are an experienced Java Programmer, you are invariably expected to know the intricacies of multi-threading. Well, this series of chapters is aimed at helping you understand this interesting and complicated concept.

So, lets get started!!!

Why Use Threads?

The notion of threading is so ingrained in Java that it's almost impossible to write even the simplest programs in Java without creating and using threads. And many of the classes in the Java API are already threaded, so often you are using multiple threads without realizing it.

The biggest question you might have is “Why use Threads?”

Well, my friend, it is not so simple to explain the reason in a couple of lines. Maybe a 100 page thesis would sound more appropriate. But still, am gonna try explaining in a short and concise manner.

First and foremost – threads increase performance. Given the processing power of the modern day processors, irrespective of how much code you write, the processor is going to finish running them in a jiffy and will remain underutilized almost 99% or more times. By creating multiple threads in your program, you are trying to utilize the processor effectively trying to keep it as little time as possible “idle”

Some of the significant reasons for using Threads are explained in the subsequent sections of this article.

Nonblocking I/O

In Java, as in most programming languages, when you try to get input from the user, you execute a read() method specifying the user's terminal (System.in in Java). When the program executes the read() method, the program typically waits until the user types at least one character before it continues and executes the next statement. This type of I/O is called blocking I/O : the program blocks until some data is available to satisfy the read() method.

This type of behavior is often undesirable. If you're reading data from a network socket, that data is often not available when you want to read it: the data may have been delayed in transit over the network, or you may be reading from a network server that sends data only periodically. If the program blocks when it tries to read from the socket, it's unable to do anything else until the data is actually available. If the program has a user interface that contains a button and the user presses the button while the program is executing the read() method, nothing happens: the program is unable to handle the mouse events and execute the event processing method associated with the button. This can be very frustrating for the user, who thinks the program has hung.

This kind of situation is where Threads in Java look the most promising solution.

Independent Tasks

A Java program is often called on to perform independent tasks. In the simplest case, a single applet may perform two independent animations for a web page. A more complex program would be a calculation server that performs calculations on behalf of several clients simultaneously. In either case, while it is possible to write a single-threaded program to perform multiple tasks, it's easier and more elegant to place each task in its own thread.

Parallelizable Algorithms

With the advent of virtual machines that can use multiple CPUs simultaneously, Java has become a useful platform for developing programs that use algorithms that can be parallelized; that is, running one iteration of the loop on one CPU while another iteration of the loop is simultaneously running on another CPU. Dependencies between the data that each iteration of the loop needs may prohibit a particular loop from being parallelized, and there may be other reasons why a loop should not be parallelized. But for many programs with CPU-intensive loops, parallelizing the loop greatly speeds up the execution of the program when it is run on a machine with multiple processors.

Now that we know why and how threads are useful for us as programmers, lets dig deep into the magical world of Threads!!!

Next: What is a Thread

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