Java, blocked queues

Question

I've been reading about blocking queues and certain questions appeared. All the examples that i've read demonstrated only situations where there are only one consumer and one producer thread. The question is: suppose we have 1 producer and 3 consumers and in the current moment all consumers are called take() method but the queue is empty so they are all waiting for appearing first element. Which of the consumer threads will take the first element when it will appear? The consumer thread which called take() first?

Answer 1

Which of the consumer threads will take the first element when it will appear? The consumer thread which called take() first?

This is tied to the blocking queue implementation as well as the JVM in question but the short answer is most likely yes. Each of the threads will be waiting on a condition and the first thread in the wait queue will be awoken when the condition is signaled.

That said, you should not depend on this functionality since it is very dependent on the particulars of the blocking queue in question as well as the JVM and OS version.

Answer 2

I don't know if you can tell. The real question is: why do you need to know? All listeners should be equivalent. It should not matter which one handles a request. If you have to know, you designed and implemented it incorrectly.

Answer 3

check ArrayBlockingQueue(int capacity, boolean fair) if fair is true,then the queue accesses for threads blocked on insertion or removal, are processed in FIFO order.

Answer 4

I agree with duffymo, the idea of having multiple threads waiting indefinitely for some new elements to pop up in the queue does not sound very well structured.

Also, if you need to know which one of the consumers remove the element, that makes me think that the consumers are actually doing different things, giving life to different ouputs on different scenarios, depending on the order with which the consumers perform the take(). If that is the case you might want to have different queues for the different threads.

If you are not planning to change your code, what about having the threads to perform a poll on regular basis?