java.util.concurrent
Interface BlockingQueue<E>

All Superinterfaces:

Collection, java.lang.Iterable, Queue

All Known Implementing Classes:

DelayQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue, ArrayBlockingQueue

public interface BlockingQueue<E>

extends Queue<E>

A Queue that additionally supports operations that wait for elements to exist when retrieving them, and wait for space to exist when storing them.

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 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 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();
   }
 }
 

Since:

1.5

Author:

Doug Lea

Method Summary

boolean	add(E o) Adds the specified element to this queue if it is possible to do so immediately, returning true upon success, else throwing an IllegalStateException.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this queue if it is possible to do so.
boolean	offer(E o) Inserts the specified element into this queue, if possible.
boolean	offer(E o, long timeout, TimeUnit unit) Inserts the specified element into this queue, waiting if necessary up to the specified wait time for space to become available.
E	poll(long timeout, TimeUnit unit) Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue.
void	put(E o) Adds the specified element to this queue, waiting if necessary for space to become available.
int	remainingCapacity() Returns the number of elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.
E	take() Retrieves and removes the head of this queue, waiting if no elements are present on this queue.

Methods inherited from interface java.util.Queue

element, peek, poll, remove

Methods inherited from interface java.util.Collection

clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray

Method Detail

offer

public boolean offer(E o)

Inserts the specified element into this queue, if possible. When using queues that may impose insertion restrictions (for example capacity bounds), method offer is generally preferable to method Collection.add(E), which can fail to insert an element only by throwing an exception.

Specified by:

offer in interface Queue

Parameters:

o - the element to add.

Returns:

true if it was possible to add the element to this queue, else false

Throws:

java.lang.NullPointerException - if the specified element is null

offer

public boolean offer(E o,
                     long timeout,
                     TimeUnit unit)
              throws java.lang.InterruptedException

Inserts the specified element into this queue, waiting if necessary up to the specified wait time for space to become available.

Parameters:

o - the element to add

timeout - how long to wait before giving up, in units of unit

unit - a TimeUnit determining how to interpret the timeout parameter

Returns:

true if successful, or false if the specified waiting time elapses before space is available.

Throws:

java.lang.InterruptedException - if interrupted while waiting.

java.lang.NullPointerException - if the specified element is null.

poll

public E poll(long timeout,
              TimeUnit unit)
       throws java.lang.InterruptedException

Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue.

Parameters:

timeout - how long to wait before giving up, in units of unit

unit - a TimeUnit determining how to interpret the timeout parameter

Returns:

the head of this queue, or null if the specified waiting time elapses before an element is present.

Throws:

java.lang.InterruptedException - if interrupted while waiting.

take

public E take()
       throws java.lang.InterruptedException

Retrieves and removes the head of this queue, waiting if no elements are present on this queue.

Returns:

the head of this queue

Throws:

java.lang.InterruptedException - if interrupted while waiting.

put

public void put(E o)
         throws java.lang.InterruptedException

Adds the specified element to this queue, waiting if necessary for space to become available.

Parameters:

o - the element to add

Throws:

java.lang.InterruptedException - if interrupted while waiting.

java.lang.NullPointerException - if the specified element is null.

remainingCapacity

public int remainingCapacity()

Returns the number of elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.

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.

Returns:

the remaining capacity

add

public boolean add(E o)

Adds the specified element to this queue if it is possible to do so immediately, returning true upon success, else throwing an IllegalStateException.

Specified by:

add in interface Collection

Parameters:

o - the element

Returns:

true (as per the general contract of Collection.add).

Throws:

java.lang.NullPointerException - if the specified element is null

java.lang.IllegalStateException - if element cannot be added

addAll

public boolean addAll(Collection<? extends E> c)

Adds all of the elements in the specified collection to this queue if it is possible to do so. The behavior of this operation need not be atomic; a failure may occur after adding only some elements.

Specified by:

addAll in interface Collection

Parameters:

c - collection whose elements are to be added to this queue

Returns:

true if this queue changed as a result of the call.

Throws:

java.lang.NullPointerException - if c or any element in c is null

java.lang.IllegalStateException - if any element cannot be added.

See Also:

Collection.add(Object)