BlockingQueue

BlockingQueue is an interface in the java.util.concurrent package that represents a queue which supports blocking operations. It extends the standard Queue interface and provides methods that wait for the queue to become non-empty when retrieving and for space to become available when storing. This behavior is useful for coordinating producer and consumer threads without explicit locking.

BlockingQueue supports both indefinite and time-bounded blocking. For example, put inserts an element and blocks if necessary until space becomes available, while take retrieves and removes the head of the queue, blocking if the queue is empty. Non-blocking alternatives include poll and offer, which return null or false when they cannot immediately perform the operation. Time-bounded variants include poll with a timeout and offer with a timeout, which wait for a specified period for the operation to succeed.

Key methods include put(E e), take(), poll(), offer(E e), offer(E e, long timeout, TimeUnit unit), poll(long timeout, TimeUnit unit), and remainingCapacity(), which reports available space for bounded queues. Some implementations also provide drainTo methods to transfer elements to a collection efficiently.

Common implementations include ArrayBlockingQueue (bounded with fixed capacity), LinkedBlockingQueue (bounded or unbounded depending on construction), PriorityBlockingQueue (typically unbounded and orders elements by priority), DelayQueue (unbounded and uses Delayed elements), and SynchronousQueue (zero-capacity handoff between producers and consumers). The BlockingQueue interface is widely used in concurrent programming, including as a work queue for executors and as a communication channel in producer-consumer patterns. It is designed to be thread-safe, providing proper memory visibility and coordination across multiple producers and consumers.