java.util.concurrent
Interface BlockingQueue<E>

Type Parameters:
E - the type of elements held in this collection
All Superinterfaces:
Collection sample code for java.util.Collection definition code for java.util.Collection <E>, Iterable sample code for java.lang.Iterable definition code for java.lang.Iterable <E>, Queue sample code for java.util.Queue definition code for java.util.Queue <E>
All Known Implementing Classes:
ArrayBlockingQueue sample code for java.util.concurrent.ArrayBlockingQueue definition code for java.util.concurrent.ArrayBlockingQueue , DelayQueue sample code for java.util.concurrent.DelayQueue definition code for java.util.concurrent.DelayQueue , LinkedBlockingQueue sample code for java.util.concurrent.LinkedBlockingQueue definition code for java.util.concurrent.LinkedBlockingQueue , PriorityBlockingQueue sample code for java.util.concurrent.PriorityBlockingQueue definition code for java.util.concurrent.PriorityBlockingQueue , SynchronousQueue sample code for java.util.concurrent.SynchronousQueue definition code for java.util.concurrent.SynchronousQueue

public interface BlockingQueue<E>
extends Queue sample code for java.util.Queue definition code for java.util.Queue <E>

A Queue sample code for java.util.Queue definition code for java.util.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.

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 sample code for java.util.Collection definition code for 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.

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();
   }
 }
 

This interface is a member of the Java Collections Framework.

Since:
1.5

Method Summary
 boolean add sample code for java.util.concurrent.BlockingQueue.add(E) definition code for java.util.concurrent.BlockingQueue.add(E) (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.
 int drainTo sample code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection) definition code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection) (Collection sample code for java.util.Collection definition code for java.util.Collection <? super E> c)
          Removes all available elements from this queue and adds them into the given collection.
 int drainTo sample code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection, int) definition code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection, int) (Collection sample code for java.util.Collection definition code for java.util.Collection <? super E> c, int maxElements)
          Removes at most the given number of available elements from this queue and adds them into the given collection.
 boolean offer sample code for java.util.concurrent.BlockingQueue.offer(E) definition code for java.util.concurrent.BlockingQueue.offer(E) (E o)
          Inserts the specified element into this queue, if possible.
 boolean offer sample code for java.util.concurrent.BlockingQueue.offer(E, long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.BlockingQueue.offer(E, long, java.util.concurrent.TimeUnit) (E o, long timeout, TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.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 sample code for java.util.concurrent.BlockingQueue.poll(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.BlockingQueue.poll(long, java.util.concurrent.TimeUnit) (long timeout, TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.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 sample code for java.util.concurrent.BlockingQueue.put(E) definition code for java.util.concurrent.BlockingQueue.put(E) (E o)
          Adds the specified element to this queue, waiting if necessary for space to become available.
 int remainingCapacity sample code for java.util.concurrent.BlockingQueue.remainingCapacity() definition code for java.util.concurrent.BlockingQueue.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 sample code for java.util.concurrent.BlockingQueue.take() definition code for java.util.concurrent.BlockingQueue.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 sample code for java.util.Queue definition code for java.util.Queue
element sample code for java.util.Queue.element() definition code for java.util.Queue.element() , peek sample code for java.util.Queue.peek() definition code for java.util.Queue.peek() , poll sample code for java.util.Queue.poll() definition code for java.util.Queue.poll() , remove sample code for java.util.Queue.remove() definition code for java.util.Queue.remove()
 
Methods inherited from interface java.util.Collection sample code for java.util.Collection definition code for java.util.Collection
addAll sample code for java.util.Collection.addAll(java.util.Collection) definition code for java.util.Collection.addAll(java.util.Collection) , clear sample code for java.util.Collection.clear() definition code for java.util.Collection.clear() , contains sample code for java.util.Collection.contains(java.lang.Object) definition code for java.util.Collection.contains(java.lang.Object) , containsAll sample code for java.util.Collection.containsAll(java.util.Collection) definition code for java.util.Collection.containsAll(java.util.Collection) , equals sample code for java.util.Collection.equals(java.lang.Object) definition code for java.util.Collection.equals(java.lang.Object) , hashCode sample code for java.util.Collection.hashCode() definition code for java.util.Collection.hashCode() , isEmpty sample code for java.util.Collection.isEmpty() definition code for java.util.Collection.isEmpty() , iterator sample code for java.util.Collection.iterator() definition code for java.util.Collection.iterator() , remove sample code for java.util.Collection.remove(java.lang.Object) definition code for java.util.Collection.remove(java.lang.Object) , removeAll sample code for java.util.Collection.removeAll(java.util.Collection) definition code for java.util.Collection.removeAll(java.util.Collection) , retainAll sample code for java.util.Collection.retainAll(java.util.Collection) definition code for java.util.Collection.retainAll(java.util.Collection) , size sample code for java.util.Collection.size() definition code for java.util.Collection.size() , toArray sample code for java.util.Collection.toArray() definition code for java.util.Collection.toArray() , toArray sample code for java.util.Collection.toArray(T[]) definition code for java.util.Collection.toArray(T[])
 

Method Detail

offer sample code for java.util.concurrent.BlockingQueue.offer(E) definition code for java.util.concurrent.BlockingQueue.offer(E)

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) sample code for java.util.Collection.add(E) definition code for java.util.Collection.add(E) , which can fail to insert an element only by throwing an exception.

Specified by:
offer sample code for java.util.Queue.offer(E) definition code for java.util.Queue.offer(E) in interface Queue sample code for java.util.Queue definition code for java.util.Queue <E>
Parameters:
o - the element to add.
Returns:
true if it was possible to add the element to this queue, else false
Throws:
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if the specified element is null

offer sample code for java.util.concurrent.BlockingQueue.offer(E, long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.BlockingQueue.offer(E, long, java.util.concurrent.TimeUnit)

boolean offer(E o,
              long timeout,
              TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
              throws InterruptedException sample code for java.lang.InterruptedException definition code for 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:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if interrupted while waiting.
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if the specified element is null.

poll sample code for java.util.concurrent.BlockingQueue.poll(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.BlockingQueue.poll(long, java.util.concurrent.TimeUnit)

E poll(long timeout,
       TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
       throws InterruptedException sample code for java.lang.InterruptedException definition code for 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:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if interrupted while waiting.

take sample code for java.util.concurrent.BlockingQueue.take() definition code for java.util.concurrent.BlockingQueue.take()

E take()
       throws InterruptedException sample code for java.lang.InterruptedException definition code for 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:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if interrupted while waiting.

put sample code for java.util.concurrent.BlockingQueue.put(E) definition code for java.util.concurrent.BlockingQueue.put(E)

void put(E o)
         throws InterruptedException sample code for java.lang.InterruptedException definition code for 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:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if interrupted while waiting.
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if the specified element is null.

remainingCapacity sample code for java.util.concurrent.BlockingQueue.remainingCapacity() definition code for java.util.concurrent.BlockingQueue.remainingCapacity()

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 another thread is about to put or take an element.

Returns:
the remaining capacity

add sample code for java.util.concurrent.BlockingQueue.add(E) definition code for java.util.concurrent.BlockingQueue.add(E)

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 sample code for java.util.Collection.add(E) definition code for java.util.Collection.add(E) in interface Collection sample code for java.util.Collection definition code for java.util.Collection <E>
Parameters:
o - the element
Returns:
true (as per the general contract of Collection.add).
Throws:
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if the specified element is null
IllegalStateException sample code for java.lang.IllegalStateException definition code for java.lang.IllegalStateException - if element cannot be added

drainTo sample code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection<? super E>) definition code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection<? super E>)

int drainTo(Collection sample code for java.util.Collection definition code for java.util.Collection <? super E> c)
Removes all available elements from this queue and adds them into the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Parameters:
c - the collection to transfer elements into
Returns:
the number of elements transferred.
Throws:
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if c is null
IllegalArgumentException sample code for java.lang.IllegalArgumentException definition code for java.lang.IllegalArgumentException - if c is this queue

drainTo sample code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection<? super E>, int) definition code for java.util.concurrent.BlockingQueue.drainTo(java.util.Collection<? super E>, int)

int drainTo(Collection sample code for java.util.Collection definition code for java.util.Collection <? super E> c,
            int maxElements)
Removes at most the given number of available elements from this queue and adds them into the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Parameters:
c - the collection to transfer elements into
maxElements - the maximum number of elements to transfer
Returns:
the number of elements transferred.
Throws:
NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException - if c is null
IllegalArgumentException sample code for java.lang.IllegalArgumentException definition code for java.lang.IllegalArgumentException - if c is this queue