using a queue with threads in simple java program

Question

i was just wondering if i could get some advice on some code

OK so i have a threaded class (very basic) basically calls this object whose code i will show just now... this code is giving me a infinite wait() and i don't know why.

  public  void  play()
  {

     if(!queue.isEmpty()){
       synchronized(this)
       {
         if(queue.peek().ballCount <=AvailableGolfBalls)
         {
            //check if there all people in queue, if yes, give them preferance
             queue.poll().notify();
         }
     }
  }




hasBalls=false;

try
{
    while (!hasBalls)
    {
            if(AvailableGolfBalls >= count)
           {

                    AvailableGolfBalls -=count;
                    synchronized(this){
                      //the main code for thread
                     }

                    hasBalls=true;


            }
           else
            {
                //there isnt enough balls,wait
                queue.add(this);

                Thread.sleep(500);
                 System.out.println(ThreadID.get() +" -no balls availiable ");

                synchronized(this)
                {

                    this.wait();
                }

            }
    }


}
catch (InterruptedException exception)
{

}

AvailableGolfBalls +=count;

}

i simplified my code as much as i could, its a ridiculously simple program though, but i just started with multi threading a week ago and a lot of the concepts still confuse me. what this program does is essentially is every thread needs a certain amount of balls before it can run, if it doesn't have the required balls, stand in queue until its available.

I found this code hilarious. if(!hasBalls) aka if(woman) — user2638922
– user2638922, Commented Aug 11, 2013 at 17:16
im using a private Queue<stuff> queue = new ConcurrentLinkedQueue<stuff>();//* for a queue — jambuls
– jambuls, Commented Aug 11, 2013 at 17:24
If you want a Queue with Threads I suggest using an ExecutorService as it is designed to do both, and provide manageability and task based control and results. — Peter Lawrey
– Peter Lawrey, Commented Aug 11, 2013 at 18:16

William Morrison · Accepted Answer · 2013-08-11 23:38:23Z

1

You are not calling notify while synchronizing on the correct object.

You are synchronizing on this and calling notify on an object stored in your queue. You must synchronize on objects stored in your queue to correctly call notify on them.

Object obj = null;
synchronized(this)
{
     if(queue.peek().ballCount <=AvailableGolfBalls)
     {
         //check if there all people in queue, if yes, give them preferance
         obj = queue.poll();
     }
}
if(obj!=null){
    synchronized(obj){
        obj.notify();
    }
}

This is what I think is wrong. Your code is very confusing because we don't know what type this is.

edited Aug 11, 2013 at 23:38

answered Aug 11, 2013 at 17:44

William Morrison

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3 Comments

docDevil Over a year ago

I think you should put obj in synchronized otherwise you will get a monitoring exception. Because this is not pointing to the same object as obj that will point to the object you polled from the queue.

jambuls Over a year ago

the object is a object that each one of the thread calls, it is the class in which that method is saved. is my logic right though? roughly?

William Morrison Over a year ago

Based on the code posted I can't comment whether your logic is correct as there's not enough information. Post more code related to synchronization and threading and I can help more.

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