Question

Suppose I create a thread safe object:

PriorityBlockingQueue<Object> safeQueue = new PriorityBlockingQueue<Object>();

If I synchronize on it:

synchronized (safeQueue) {
   ....
}

Does code that block:

// some non-synchronized block
Object value = safeQueue.poll();

Answer 1

No. The only time you get any blocking is if another thread is also doing a synchronized on the same object. If your code is synchronized (safeQueue) then a call to PriorityBlockingQueue.poll() would only block if poll() was a synchronized method or if the code used synchronized (this) code.

When you call safeQueue.poll() the PriorityBlockingQueue code is actually using an internal ReentrantLock, and not doing a synchronized (this). Here’s the code for poll():

public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return q.poll();
    } finally {
        lock.unlock();
    }
}

Lastly, as you mention, PriorityBlockingQueue is already reentrant so you do not need to synchronize on it to allow multiple threads to access the queue. You could still need to synchronize on it if you needed to solve race conditions in your own code of course.