BlockingQueue

Class Overview

A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.

BlockingQueue methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

A BlockingQueue does not accept null elements. Implementations throw NullPointerException on attempts to add, put or offer a null. A null is used as a sentinel value to indicate failure of poll operations.

A BlockingQueue may be capacity bounded. At any given time it may have a remainingCapacity beyond which no additional elements can be put without blocking. A BlockingQueue without any intrinsic capacity constraints always reports a remaining capacity of Integer.MAX_VALUE.

BlockingQueue implementations are designed to be used primarily for producer-consumer queues, but additionally support the Collection interface. So, for example, it is possible to remove an arbitrary element from a queue using remove(x). However, such operations are in general not performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.

BlockingQueue implementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operations addAll, containsAll, retainAll and removeAll are not necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for addAll(c) to fail (throwing an exception) after adding only some of the elements in c.

A BlockingQueue does not intrinsically support any kind of "close" or "shutdown" operation to indicate that no more items will be added. The needs and usage of such features tend to be implementation-dependent. For example, a common tactic is for producers to insert special end-of-stream or poison objects, that are interpreted accordingly when taken by consumers.

Usage example, based on a typical producer-consumer scenario. Note that a BlockingQueue can safely be used with multiple producers and multiple consumers.

 class Producer implements Runnable {
   private final BlockingQueue queue;
   Producer(BlockingQueue q) { queue = q; }
   public void run() {
     try {
       while (true) { queue.put(produce()); }
     } catch (InterruptedException ex) { ... handle ...}
   }
   Object produce() { ... }
 }

 class Consumer implements Runnable {
   private final BlockingQueue queue;
   Consumer(BlockingQueue q) { queue = q; }
   public void run() {
     try {
       while (true) { consume(queue.take()); }
     } catch (InterruptedException ex) { ... handle ...}
   }
   void consume(Object x) { ... }
 }

 class Setup {
   void main() {
     BlockingQueue q = new SomeQueueImplementation();
     Producer p = new Producer(q);
     Consumer c1 = new Consumer(q);
     Consumer c2 = new Consumer(q);
     new Thread(p).start();
     new Thread(c1).start();
     new Thread(c2).start();
   }
 }
 

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a BlockingQueue happen-before actions subsequent to the access or removal of that element from the BlockingQueue in another thread.

Summary

[Expand] Inherited Methods
From interface java.lang.Iterable abstract Iterator<T> iterator() Returns an Iterator for the elements in this object.
From interface java.util.Collection abstract boolean add(E object) Attempts to add object to the contents of this Collection (optional). abstract boolean addAll(Collection<? extends E> collection) Attempts to add all of the objects contained in Collection to the contents of this Collection (optional). abstract void clear() Removes all elements from this Collection, leaving it empty (optional). abstract boolean contains(Object object) Tests whether this Collection contains the specified object. abstract boolean containsAll(Collection<?> collection) Tests whether this Collection contains all objects contained in the specified Collection. abstract boolean equals(Object object) Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison. abstract int hashCode() Returns an integer hash code for the receiver. abstract boolean isEmpty() Returns if this Collection contains no elements. abstract Iterator<E> iterator() Returns an instance of Iterator that may be used to access the objects contained by this Collection. abstract boolean remove(Object object) Removes one instance of the specified object from this Collection if one is contained (optional). abstract boolean removeAll(Collection<?> collection) Removes all occurrences in this Collection of each object in the specified Collection (optional). abstract boolean retainAll(Collection<?> collection) Removes all objects from this Collection that are not also found in the Collection passed (optional). abstract int size() Returns a count of how many objects this Collection contains. abstract <T> T[] toArray(T[] array) Returns an array containing all elements contained in this Collection. abstract Object[] toArray() Returns a new array containing all elements contained in this Collection.
From interface java.util.Queue abstract boolean add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available. abstract E element() Retrieves, but does not remove, the head of this queue. abstract boolean offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. abstract E peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty. abstract E poll() Retrieves and removes the head of this queue, or returns null if this queue is empty. abstract E remove() Retrieves and removes the head of this queue.