Thread-safety of BlockingQueue's drainTo() method

Question

Documentation of BlockingQueue says bulk operations are not thread-safe, though it doesn\'t explicitly mention the method drainTo().

BlockingQueue imp

Answer 1

stumbled upon this question and felt like adding an implementation info.

From Java 8 source of PriorityBlockingQueue :

 /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = Math.min(size, maxElements);
            for (int i = 0; i < n; i++) {
                c.add((E) queue[0]); // In this order, in case add() throws.
                dequeue();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

You can see that a ReentrantLock is used to lock the critical section. The methods poll() and offer() are also using the same lock. So the BlockingQueue implementation in this case of PriorityBlockingQueue is indeed Blocking!