LinkedBlockingQueue put and take functional implementation

Question

I was going through internal implementation of LinkedBlockingQueue put(E e) and take() function.

public E take() throws InterruptedException {
    E x;
    int c = -1;
    final AtomicInteger count = this.count;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lockInterruptibly();
    try {
        while (count.get() == 0) {
            notEmpty.await();
        }
        x = dequeue();
        c = count.getAndDecrement();
        if (c > 1)
            **notEmpty.signal();**
    } finally {
        takeLock.unlock();
    }
    if (c == capacity)
        signalNotFull();
    return x;
}

  public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset local var
        // holding count negative to indicate failure unless set.
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from capacity. Similarly
             * for all other uses of count in other wait guards.
             */
            while (count.get() == capacity) {
                notFull.await();
            }
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                **notFull.signal();**
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

In both methods didn't get why signal() on condition called after a comparison with capacity. If any one can explain it, would be highly appreciable.

Because other threads might still be waiting for that condition, so those have to be able to `take`/`put` if the current `take`/`put` didn't empty/fill the queue. — daniu, Apr 20 '18 at 11:39

xingbin · Accepted Answer · 2018-04-20T14:09:30.030

Here is one situation I can think about using:

if (c > 1)
    notEmpty.signal();

Suppose the queue is empty, and there are 3 threads, thread_1, thread_2, thread_3.

thread_1 call take(), get blocked at notEmpty.await().
thread_2 call take(), get blocked at notEmpty.await().
thread_3 call take(), get blocked at notEmpty.await().

Then, there come other 3 threads, thread_4, thread_5, thread_6.

thread_4 call put(), add an element in the queue, and signal thread_1.
thread_1 wake up, re-aquire the takeLock, and try take the first element.
at the same time, thread_5 call put(), add another element in the queue, and signal thread_2.
thread_2 wake up, try re-aquire the takeLock, but thread_1 is holding the lock now, so it has to wait.
thread_6 interrupts thread_2, thread_2 throws InterruptedException and terminated.
thread_1 takes the first element. But, there is still another element in this queue, so c > 1 in code above occurs. If thread_1 do not call signal, thread_3 can not wake up and take the second element.
thread_1 call signal, thread_3 wake up, and take the second element.

LinkedBlockingQueue put and take functional implementation

1 Answers1