How to code run method in Thread pooling

Question

I got very confused by reading Thread Pooling. I learnt the concept, how they actually works. But I confused in the part , how to code this.

I searched a lot on the net. Finally I got a blog, that have codes , given below,

CONDITION IS, NOT TO USE IN-BUILT CLASS

Code 1

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();
    }
  }

}

Code 2

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;
  }
}

Code 3:-

public class BlockingQueue {

  private List queue = new LinkedList();
  private int  limit = 10;

  public BlockingQueue(int limit){
    this.limit = limit;
  }

  public synchronized void enqueue(Object item)
  throws InterruptedException  {
    while(this.queue.size() == this.limit) {
      wait();
    }
    if(this.queue.size() == 0) {
      notifyAll();
    }
    this.queue.add(item);
  }

  public synchronized Object dequeue()
  throws InterruptedException{
    while(this.queue.size() == 0){
      wait();
    }
    if(this.queue.size() == this.limit){
      notifyAll();
    }

    return this.queue.remove(0);
  }    
}

I tried to understand , what this code do. But I dont get the flow of this code. Can you help me to understand this code.

Mainly I have problems in **Code 2 :- run method**

Why execute method's argument are of Runnable type?

How input array given to this code??

Help me.

Thanks in advance.

Answer 1

  public void run(){
    while(!isStopped()){

Loop until the thread pool is stopped.

      try{
        Runnable runnable = (Runnable) taskQueue.dequeue();

Pull the head task off the task queue.

        runnable.run();

Run the task.

      } catch(Exception e){
        //log or otherwise report exception,
        //but keep pool thread alive.

Do nothing special if the task throws an exception, just don't pass it on.

      }
    }
  }

Answer 2

Edit:

I now understand that this is a class project but I'll leave my answer for posterity.

If you are trying to use thread-pools under Java then all of this has been already implemented for you by the java.util.concurrent.* classes. Other answers address and explain your specific code.

For example, this is what you need to setup a thread pool using the ExecutorService code. Underneath the covers the ExecutorService handles the threads and uses a LinkedBlockingQueue . You define the MyJob class which implements 'Runnable' and does the work that is run by the threads in the pool. It can be a short or a long running task depending on what you need.

// create a thread pool with 10 workers
ExecutorService threadPool = Executors.newFixedThreadPool(10);
// or you can create an open-ended thread pool
// ExecutorService threadPool = Executors.newCachedThreadPool();
// define your jobs somehow
for (MyJob job : jobsToDo) {
    threadPool.submit(job);
}
// once we have submitted all jobs to the thread pool, it should be shutdown
threadPool.shutdown();
...
public class MyJob implements Runnable {
    // you can construct your jobs and pass in context for them if necessary
    public MyJob(String someContext) {
        ...
    }
    public void run() {
        // process the job
    }
}