Java Doc for BlockingQueue.java in  » Apache-Harmony-Java-SE » java-package » java » util » concurrent » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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.

While BlockingQueue is designed to be used primarily for producer-consumer queues, it additionally supports the java.util.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. Also, the bulk Collection operations, most notably addAll, are not necessarily performed atomically, so it is possible 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();
 }
 }
 

This interface is a member of the Java Collections Framework.
since:
   1.5
author:
   Doug Lea<
Parameters:
  E - > the type of elements held in this collection

Note that you cannot always tell if an attempt to add an element will succeed by inspecting remainingCapacity because it may be the case that a waiting consumer is ready to take an element out of an otherwise full queue. the remaining capacity