阻塞队列元素对其他线程不可见，有时导致没有元素检索

Question

I am using LinkedBlokingQueue and it is shared between 2 threads. From the 1st thread constructor I have passed Queue object to 2nd thread.

I use likedBlockingQueue.put in 1st thread.

Second thread uses likedBlockingQueue.size()>0 and likedBlockingQueue.peek() to get the element.

The issue is in 1 of my environment sometimes the second thread is giving empty, though I can see the first thread is at blocking call at put (as I have initialized queue size as 1). This issue is not happening always but only replicating sometimes. Couldn't able to understand what causes this issue as per the java documents this Queue seems to be thread safe. Could anyone share any points on this?

I tried replicating this but only happens few times, other times I could able to get the element from the Queue.

1 Thread :

private final Queue<T> linkedQ = new LinkedBlokingQueue(1);
linkedQ.put(element);

2 Thread :

while(condition)
{
    if(this.linkedQ.size>0)
    {
        Object a = this.linkedQ.peek();
        linkedQ.remove();
        break;
    }
}

No error, when I debug when this issue happened, the Q in the second thread showed empty.

Answer 1

In this case you should be using neither size() nor peek() nor remove() . The LinkedBlockingQueue is a queue that blocks (hence the name). Both size() and peek() are non-blocking (well, technically they block for a second, but they don't act in a producer-consumer fashion).

You want an operation that tells the current thread "Wait here until an element appears on the queue". Looking at the API, for peek() it reads

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Returning null if the queue is empty explains that no waiting is performed. The operation you're looking for is take() .

public E take()

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Your usage here would then be:

while(condition) {
    Object a = linkedQ.take(); // leaving out the interrupted exception for brevities sake
}