Threads are a very important Part of Java, but creating large numbers of threads can degrade the performance of application.By thread pools, which allow you to maintain fixed number of threads running assigned tasks to each of the threads.
Suppose A thing may have to visit tens of thousands of pages, and you will not create tens of thousands of thread because it is an additional problem. For overcome this problem we need a thread pool. Thread pool give some fixed number of thread and these thread will do task. Once a thread finish one task, then it is assigned to new task. We will not create a new thread always. This is way thread pool will work. Developer has to implement this feature.
In Java:
• A thread pool implementation is provided in the form of the ThreadPoolExecutor class, introduced in Java 1.5;
• you can put in different implementations of BlockingQueue to specify different queue behavior such as queue bounds or priority ordering
Implementation of Thread pool
This can be done by like this
public class ThreadPool extends java.lang.Object implements ThreadPoolInt
This class is generic implementation of thread pool which have following input
1) Size of the pool to be constructed
2) Name of the class which implements Runnable (which has a visible default constructor)
It constructs a thread pool with active threads that are waiting for activation. Once the threads have finished their task they will come back and wait for assigning a new task.
Benefits of Thread Pooling
• It saves the machine work of creating a new thread.
• Single thread can recycle again and again.
• The size of thread pool is given in starting .We can expand on the designs presented in this chapter to include a method to support growing the size of the pool at runtime if you need this kind of dynamic tuning.
• Response time can be quick.
Risks of using thread pools–
The disadvantage of thread pool is that. Suppose if an task is rejected because the thread pool is empty. There will be high rejection rate.Or if task on wait state then waiting time would be too long. Sometime there is problem of deadlock also.
References
http://www.japisoft.com/jservices/javadoc/jason/service/pool/ThreadPool.html
http://java.sun.com/developer/Books/javaprogramming/threads/chap13.pdf