Queing of several request threads

[Praveen Balaji]

Hi all,
I need to implement a Request queue with the following problem description:
1. I have a connection pool that will notify when a free connection is available. Concurrent requests wait in a queue for this notification if the pool doesnot have sufficient connections.
2. Requests have a timeout. While waiting for the connection, some of the requests may timeout if they donot receive a free connection in the given time.
Could anyone point me to tips on how I can implement this? If I am able to make the request threads "sleep" for a given time, my problem is 80% solved.
Thanks in advance,
Praveen.

[Peter den Haan]

Let me quote your problem description back at you, with subtle modifications

1. I have a connection pool that will java.lang.Object.notify() when a free connection is available. Concurrent requests wait in the wait set for this notification if the pool does not have sufficient connections.
2. Requests java.lang.Object.wait(timeout) only for a limited period of time. While waiting for the connection, some of the requests may timeout if they do not receive a free connection in the given time.

Hope this helps. If this is too cryptic, please accept my apologies; just ask and I'll try to help.
- Peter

[Praveen Balaji]

Hi, Thanks for replying. I am not exactly using java.lang.Object.notify() because it does not assure which thread it's going to notify. I need to maintain the queue order. I have attached a code snippet that might explain the exact problem I am trying to solve.

  
public class RequestQueue
{
  private ArrayList queue;
  private ConnectionPool;
  private long wait=0;
  private byte[] lock = new byte[0];
  .....

  //Several concurrent request 
  //threads call this method
  void service(){
    synchronized(lock){
      //If there are free connections and 
      //there are no waiting connections,
      if(pool.hasConnection() && queue.size()==0)
        //Service the request and return
        {.... return; }
      //Else, add the requesting thread to 
      //the queue
      queue.add(Thread.currentThread());
    }
    try{
      //CHECK POINT 1
      //Now sleep the thread for the 
      //required time - the max time for
      //which it can wait for service
      //CLEARLY, THIS CHECK POINT CAN
      //HAVE SYNCHRONIZATION ISSUES WITH
      //CHECK POINT 2
      Thread.currentThread().sleep(wait);
    }catch(InterruptedException ie){
      //When a connection is released,
      //this thread is interrupted in the
      //order in which it entered the queue
      synchronized(pool){
        //Service the request and return
        return;
      }
    }
    //Ok, the thread wait is complete and
    //it has not been serviced. So, remove
    //itself from the queue and log an 
    //error and return
    synchronized(queue){
      //LOG: My request has not been serviced
      queue.remove(Thread.currentThread());
      return;
    }
  }
  void releaseConnection(ConnectionWrapper cw){
    //A connection is being released
    synchronized(lock){
      //releasing connection
      pool.releaseConnection(cw);
      //CHECK POINT 2
      //If there are threads waiting,
      //interrupt them so they can get
      //a free connection
      //THIS POINT HAS SYNC PROBLEMS WITH
      //CHECK POINT 1
      if(queue.size()!=0){
        Thread t = queue.remove(1);
        t.interrupt();
      }
    }
  }
}

Thanks in advance,
Praveen

[Praveen Balaji]

Hi,
This is a simple class that explains my problem.

public class InterruptTest implements Runnable
{
  public void run()
  {
    float f1;
    long s_start;
    long s_end;
    for(long i=0; i<1000000L; i++)
    {
      f1 = (float)((i*0.000023/0.0000000000089)%73);
    }
    s_start = System.currentTimeMillis();
    System.out.println("Going to bed at: " + s_start);
    try
    {
      Thread.sleep(5000);
    }
    catch(Exception e)
    {
      System.out.println("Thread Interrupted...");
    }
    s_end = System.currentTimeMillis();
    System.out.println("Got outa bed at: " + s_end);
    System.out.println("Wanted to sleep for 5000 ms. Slept for only (ms): " + (s_end-s_start));
  }
  public static void main(String[] args)
  {
    Thread t1 = new Thread(new InterruptTest());
    t1.start();
    t1.interrupt();
    synchronized(System.out)
    {
      System.out.println("Interrupted....");
    }
  }
}

What happens here is the interrupted flag is set to true even before the thread

t1

goes to sleep. So, when the

sleep()

method of the thread t1 is called, it exits without sleeping.
Now my problem has boiled down to:
How do i set the Thread state back to not interrupted after it has interrupted?
I dont want to mess with client threads.
Thanks in advance for help..
praveen