Java BlockingQueue confusion

Question

I am currently reading about Java BlockingQueue and this example is given on many websites for a simple implementation of the BlockingQueue. The code is simple but I am a bit confused. For example lets say we fill up the queue, then we try to enqueue 3 more times. This will make the 3 threads wait. Then when we call dequeue, the code in the dequeue method will enter the second if statement and it will notify all threads. Won't this mean that the 3 threads waiting will all add nodes to the queue? That means we will have 2 more nodes than the limit? I am pretty sure I misunderstood something here so I could use some small explanation.

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();
    }
    this.queue.add(item);
    if(this.queue.size() == 1) {
      notifyAll();
    }
  }


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

    return this.queue.remove(0);
  }

}

akuzminykh · Accepted Answer · 2020-08-07T10:24:53.310

1

No, only one will add a node. Notice that your wait-call in enqueue is inside a loop:

    while(this.queue.size() == this.limit) {
      wait();
    }

All three threads are notified but only one thread can be in the synchronized-block. The first thread to enter the block adds a node, so the queue is full again. The other both threads enter the block (one after another) but see the queue being full again, which will put them right into waiting state again as that's the loop-condition.

You can imagine a wait to be an exit and entrace point of a synchronized-block. When a thread enters wait, then the corresponding lock is released. A thread that has been waiting in a wait and is notified, is trying to acquire the corresponding lock for the critical section again and blocks if it is currently in use. So only one of the notified three threads can enter at a time.

edited Aug 07 '20 at 10:24

answered Aug 07 '20 at 10:04

akuzminykh

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how can only one thread be in the synchronized block? Doesn't a thread let go of a lock when wait is called? – user737163 Aug 07 '20 at 10:07
1

Nevermind, I get what you said now. One thread will run and then set the the length of the queue to the limit again. That means other threads will recheck the loop and wait again. Thanks :D – user737163 Aug 07 '20 at 10:10
Yes, it does, but not forever - until some other thread calls `notify` - then thread will (try) to acquire monitor back. – Vasily Liaskovsky Aug 07 '20 at 10:10

score 1 · Answer 2 · answered Aug 07 '20 at 10:05

1

Note that wait() from .enqueue() is inside loop. Any awoken thread will re-check permit to add an element and since only single thread can execute synchronized method at a time, there will be no problem - one thread gets lucky to insert an element, others continue to wait after failed re-check.

answered Aug 07 '20 at 10:05

Vasily Liaskovsky

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Beautifully explained. Thanks! – user737163 Aug 07 '20 at 10:12

Java BlockingQueue confusion

2 Answers2