while loop in java BlockingQueue implementation

Question

I recently saw the following implementation of enqueue for a BlockingQueue (source)

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);
}

Why is the while loop necessary, and can the while be replaced by if (this.queue.size() == this.limit)

It seems the method enqueue is synchronized so only 1 thread at a time can be executing in the method body and make the call to wait(). Once the thread is notified, can't it just continue onward without checking the this.queue.size() == this.limit condition again?

Thanks. Follow up question: why does enqueue call notifyAll() before the item is added? Wouldn't this potentially cause threads waiting to dequeue to check an empty queue? Would it be better to first add the item and then notifyAll? like so: queue.add(item); if (queue.size()==1){notifyAll();} — morfys
– morfys, Commented Mar 26, 2012 at 8:43
IMHO, the order off the add(item) and notifyAll() doesn't matter as the method enqueue is synchronized. So both the add(item) and notifyAll() calls must complete before another thread can execute. — morfys
– morfys, Commented Mar 26, 2012 at 9:06

mitchnull · Accepted Answer · 2012-03-23 12:59:23Z

5

The documentation on Object.wait() explains it best:

A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:

 synchronized (obj) {
     while (<condition does not hold>)
         obj.wait(timeout);
     ... // Perform action appropriate to condition
 }

answered Mar 23, 2012 at 12:59

mitchnull

6,3712 gold badges34 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Bill Michell · Accepted Answer · 2012-03-26 07:15:07Z

4

No. You need the while because there might be multiple threads waiting for the space in the queue to open up, and the notifyAll() calls will wake all of them up.

The wait() method actually releases the synchronisation monitor so that other threads can make progress. If it didn't, then any thread trying to remove stuff from the queue would also get stuck waiting to enter a synchronised block in the get() method (for example).

Only one of the waiting threads will see a partially empty queue.

Actually, none of them might; the threads might have been woken up with notifyAll for some completely unrelated reason.

edited Mar 26, 2012 at 7:15

answered Mar 23, 2012 at 12:47

Bill Michell

8,3504 gold badges31 silver badges33 bronze badges

Collectives™ on Stack Overflow

while loop in java BlockingQueue implementation

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related