LinkedBlockingQueue vs ConcurrentLinkedQueue

1. Introduction

LinkedBlockingQueue and ConcurrentLinkedQueue are the two most frequently used concurrent queues in Java. Although both queues are often used as a concurrent data structure, there are subtle characteristics and behavioral differences between them.

In this short tutorial, we’ll discuss both of these queues and explain their similarities and differences.

2. LinkedBlockingQueue

The LinkedBlockingQueue is an optionally-bounded blocking queue implementation, meaning that the queue size can be specified if needed.

Let’s create a LinkedBlockingQueue which can contain up to 100 elements:

BlockingQueue<Integer> boundedQueue = new LinkedBlockingQueue<>(100);

We can also create an unbounded LinkedBlockingQueue just by not specifying the size:

BlockingQueue<Integer> unboundedQueue = new LinkedBlockingQueue<>();

An unbounded queue implies that the size of the queue is not specified while creating. Therefore, the queue can grow dynamically as elements are added to it. However, if there is no memory left, then the queue throws a java.lang.OutOfMemoryError.

We can create a LinkedBlockingQueue from an existing collection as well:

Collection<Integer> listOfNumbers = Arrays.asList(1,2,3,4,5);
BlockingQueue<Integer> queue = new LinkedBlockingQueue<>(listOfNumbers);

The LinkedBlockingQueue class implements the BlockingQueue interface, which provides the blocking nature to it.

A blocking queue indicates that the queue blocks the accessing thread if it is full (when the queue is bounded) or becomes empty. If the queue is full, then adding a new element will block the accessing thread unless there is space available for the new element. Similarly, if the queue is empty, then accessing an element blocks the calling thread:

ExecutorService executorService = Executors.newFixedThreadPool(1);
LinkedBlockingQueue<Integer> queue = new LinkedBlockingQueue<>();
executorService.submit(() -> {
  try {
    queue.take();
  } 
  catch (InterruptedException e) {
    // exception handling
  }
});

In the above code snippet, we are accessing an empty queue. Therefore, the take method blocks the calling thread.

The blocking feature of the LinkedBlockingQueue is associated with some cost. This cost is because every put or the take operation is lock contended between the producer or the consumer threads. Therefore, in scenarios with many producers and consumers, put and take actions could be slower.

3. ConcurrentLinkedQueue

A ConcurrentLinkedQueue is an unbounded, thread-safe, and non-blocking queue.

Let’s create an empty ConcurrentLinkedQueue:

ConcurrentLinkedQueue queue = new ConcurrentLinkedQueue();

We can create a ConcurrentLinkedQueue from an existing collection as well:

Collection<Integer> listOfNumbers = Arrays.asList(1,2,3,4,5);
ConcurrentLinkedQueue<Integer> queue = new ConcurrentLinkedQueue<>(listOfNumbers);

Unlike a LinkedBlockingQueue, a ConcurrentLinkedQueue is a non-blocking queue. Thus, it does not block a thread once the queue is empty. Instead, it returns null. Since its unbounded, it’ll throw a java.lang.OutOfMemoryError if there’s no extra memory to add new elements.

Apart from being non-blocking, a ConcurrentLinkedQueue has additional functionality.

In any producer-consumer scenario, consumers will not contend with producers; however, multiple producers will contend with one another:

int element = 1;
ExecutorService executorService = Executors.newFixedThreadPool(2);
ConcurrentLinkedQueue<Integer> queue = new ConcurrentLinkedQueue<>();

Runnable offerTask = () -> queue.offer(element);

Callable<Integer> pollTask = () -> {
  while (queue.peek() != null) {
    return queue.poll().intValue();
  }
  return null;
};

executorService.submit(offerTask);
Future<Integer> returnedElement = executorService.submit(pollTask);
assertThat(returnedElement.get().intValue(), is(equalTo(element)));

The first task, offerTask, adds an element to the queue, and the second task, pollTask, retrieve an element from the queue. The poll task additionally checks the queue for an element first as ConcurrentLinkedQueue is non-blocking and can return a null value.

4. Similarities

Both LinkedBlockingQueue and the ConcurrentLinkedQueue are queue implementations and share some common characteristics. Let’s discuss the similarities between these two queues:

1. Both implement the Queue Interface
2. They both use linked nodes to store their elements
3. Both are suitable for concurrent access scenarios

5. Differences

Although both of these queues have certain similarities, there are substantial characteristics differences, too:

These queues use the Queue as the base interface and implement the poll and offer methods. However, because of different internal implementations, they behave differently.

While ConcurrentLinkedQueue poll and offer as completely lock-free. The LinkedBlockingQueue will block these operations. The only case when the LinkedBlockingQueue methods will return immediately is if we use a bounded queue and this queue is empty or full.

Thus it’s important to check the implementation before using these methods, as it might result in unexpected locking, affecting the application’s performance.

6. Conclusion

In this article, we learned about LinkedBlockingQueue and ConcurrentLinkedQueue.

First, we individually discussed these two queue implementations and some of their characteristics. Then, we saw the similarities between these two queue implementations. Finally, we explored the differences between these two queue implementations.