Thread Pools are useful when you need to limit the number of threads running in your application at the same time. There is a performance overhead associated with starting a new thread, and each thread is also allocated some memory for its stack etc.

Instead of starting a new thread for every task to execute concurrently, the task can be passed to a thread pool. As soon as the pool has any idle threads the task is assigned to one of them and executed. Internally the tasks are inserted into a Blocking Queue which the threads in the pool are dequeuing from. When a new task is inserted into the queue one of the idle threads will dequeue it successfully and execute it. The rest of the idle threads in the pool will be blocked waiting to dequeue tasks.

Thread pools are often used in multi threaded servers. Each connection arriving at the server via the network is wrapped as a task and passed on to a thread pool. The threads in the thread pool will process the requests on the connections concurrently. A later trail will get into detail about implementing multithreaded servers in Java.

Java 5 comes with built in thread pools in the java.util.concurrent package, so you don't have to implement your own thread pool. Still it can be useful to know a bit about the implementation of a thread pool anyways.

Here is a simple thread pool implementation:

public class ThreadPool {  private BlockingQueue taskQueue = null;  private List<PoolThread> threads = new ArrayList<PoolThread>();  private boolean isStopped = false;  public ThreadPool(int noOfThreads, int maxNoOfTasks){    taskQueue = new BlockingQueue(maxNoOfTasks);    for(int i=0; i<noOfThreads; i++){      threads.add(new PoolThread(taskQueue));    }    for(PoolThread thread : threads){      thread.start();    }  }  public void synchronized execute(Runnable task){    if(this.isStopped) throw      new IllegalStateException("ThreadPool is stopped");    this.taskQueue.enqueue(task);  }  public synchronized void stop(){    this.isStopped = true;    for(PoolThread thread : threads){      thread.stop();    }  }}

public class PoolThread extends Thread {  private BlockingQueue taskQueue = null;  private boolean       isStopped = false;  public PoolThread(BlockingQueue queue){    taskQueue = queue;  }  public void run(){    while(!isStopped()){      try{        Runnable runnable = (Runnable) taskQueue.dequeue();        runnable.run();      } catch(Exception e){        //log or otherwise report exception,        //but keep pool thread alive.      }    }  }  public synchronized void stop(){    isStopped = true;    this.interrupt(); //break pool thread out of dequeue() call.  }  public synchronized void isStopped(){    return isStopped;  }}

The thread pool implementation consists of two parts. A ThreadPool class which is the public interface to the thread pool, and a PoolThread class which implements the threads that execute the tasks.

To execute a task the method ThreadPool.execute(Runnable r) is called with a Runnable implementation as parameter. The Runnable is enqueued in the blocking queue internally, waiting to be dequeued.

The Runnable will be dequeued by an idle PoolThread and executed. You can see this in the PoolThread.run() method. After execution the PoolThread loops and tries to dequeue a task again, until stopped.

To stop the ThreadPool the method ThreadPool.stop() is called. The stop called is noted internally in the isStopped member. Then each thread in the pool is stopped by calling PoolThread.stop(). Notice how the execute() method will throw an IllegalStateException if execute() is called after stop() has been called.

The threads will stop after finishing any task they are currently executing. Notice the this.interrupt() call in PoolThread.stop(). This makes sure that a thread blocked in a wait() call inside the taskQueue.dequeue() call breaks out of the wait() call, and leaves the dequeue() method call with an InterruptedException thrown. This exception is caught in the PoolThread.run() method, reported, and then the isStopped variable is checked. Since isStopped is now true, the PoolThread.run() will exit and the thread dies.