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- Type Parameters:
E- the type of elements held in this queue
- All Superinterfaces:
Collection<E>,Iterable<E>,Queue<E>
- All Known Subinterfaces:
BlockingDeque<E>,TransferQueue<E>
- All Known Implementing Classes:
ArrayBlockingQueue,DelayQueue,LinkedBlockingDeque,LinkedBlockingQueue,LinkedTransferQueue,PriorityBlockingQueue,SynchronousQueue
public interface BlockingQueue<E> extends Queue<E>
AQueuethat 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.BlockingQueuemethods 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 (eithernullorfalse, 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:Summary of BlockingQueue methods Throws exception Special value Blocks Times out Insert add(e)offer(e)put(e)offer(e, time, unit)Remove remove()poll()take()poll(time, unit)Examine element()peek()not applicable not applicable A
BlockingQueuedoes not acceptnullelements. Implementations throwNullPointerExceptionon attempts toadd,putorofferanull. Anullis used as a sentinel value to indicate failure ofpolloperations.A
BlockingQueuemay be capacity bounded. At any given time it may have aremainingCapacitybeyond which no additional elements can beputwithout blocking. ABlockingQueuewithout any intrinsic capacity constraints always reports a remaining capacity ofInteger.MAX_VALUE.BlockingQueueimplementations are designed to be used primarily for producer-consumer queues, but additionally support theCollectioninterface. So, for example, it is possible to remove an arbitrary element from a queue usingremove(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.BlockingQueueimplementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operationsaddAll,containsAll,retainAllandremoveAllare not necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, foraddAll(c)to fail (throwing an exception) after adding only some of the elements inc.A
BlockingQueuedoes 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
BlockingQueuecan 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
BlockingQueuehappen-before actions subsequent to the access or removal of that element from theBlockingQueuein another thread.This interface is a member of the Java Collections Framework.
- Since:
- 1.5
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method Description booleanadd(E e)Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available.booleancontains(Object o)Returnstrueif this queue contains the specified element.intdrainTo(Collection<? super E> c)Removes all available elements from this queue and adds them to the given collection.intdrainTo(Collection<? super E> c, int maxElements)Removes at most the given number of available elements from this queue and adds them to the given collection.booleanoffer(E e)Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success andfalseif no space is currently available.booleanoffer(E e, long timeout, TimeUnit unit)Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.Epoll(long timeout, TimeUnit unit)Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.voidput(E e)Inserts the specified element into this queue, waiting if necessary for space to become available.intremainingCapacity()Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, orInteger.MAX_VALUEif there is no intrinsic limit.booleanremove(Object o)Removes a single instance of the specified element from this queue, if it is present.Etake()Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.-
Methods declared in interface java.util.Collection
addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray
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Method Detail
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add
boolean add(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available. When using a capacity-restricted queue, it is generally preferable to useoffer.- Specified by:
addin interfaceCollection<E>- Specified by:
addin interfaceQueue<E>- Parameters:
e- the element to add- Returns:
true(as specified byCollection.add(E))- Throws:
IllegalStateException- if the element cannot be added at this time due to capacity restrictionsClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is nullIllegalArgumentException- if some property of the specified element prevents it from being added to this queue
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offer
boolean offer(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success andfalseif no space is currently available. When using a capacity-restricted queue, this method is generally preferable toadd(E), which can fail to insert an element only by throwing an exception.- Specified by:
offerin interfaceQueue<E>- Parameters:
e- the element to add- Returns:
trueif the element was added to this queue, elsefalse- Throws:
ClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is nullIllegalArgumentException- if some property of the specified element prevents it from being added to this queue
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put
void put(E e) throws InterruptedException
Inserts the specified element into this queue, waiting if necessary for space to become available.- Parameters:
e- the element to add- Throws:
InterruptedException- if interrupted while waitingClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is nullIllegalArgumentException- if some property of the specified element prevents it from being added to this queue
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offer
boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException
Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.- Parameters:
e- the element to addtimeout- how long to wait before giving up, in units ofunitunit- aTimeUnitdetermining how to interpret thetimeoutparameter- Returns:
trueif successful, orfalseif the specified waiting time elapses before space is available- Throws:
InterruptedException- if interrupted while waitingClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is nullIllegalArgumentException- if some property of the specified element prevents it from being added to this queue
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take
E take() throws InterruptedException
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.- Returns:
- the head of this queue
- Throws:
InterruptedException- if interrupted while waiting
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poll
E poll(long timeout, TimeUnit unit) throws InterruptedException
Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.- Parameters:
timeout- how long to wait before giving up, in units ofunitunit- aTimeUnitdetermining how to interpret thetimeoutparameter- Returns:
- the head of this queue, or
nullif the specified waiting time elapses before an element is available - Throws:
InterruptedException- if interrupted while waiting
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remainingCapacity
int remainingCapacity()
Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, orInteger.MAX_VALUEif there is no intrinsic limit.Note that you cannot always tell if an attempt to insert an element will succeed by inspecting
remainingCapacitybecause it may be the case that another thread is about to insert or remove an element.- Returns:
- the remaining capacity
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remove
boolean remove(Object o)
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an elementesuch thato.equals(e), if this queue contains one or more such elements. Returnstrueif this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
removein interfaceCollection<E>- Parameters:
o- element to be removed from this queue, if present- Returns:
trueif this queue changed as a result of the call- Throws:
ClassCastException- if the class of the specified element is incompatible with this queue (optional)NullPointerException- if the specified element is null (optional)
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contains
boolean contains(Object o)
Returnstrueif this queue contains the specified element. More formally, returnstrueif and only if this queue contains at least one elementesuch thato.equals(e).- Specified by:
containsin interfaceCollection<E>- Parameters:
o- object to be checked for containment in this queue- Returns:
trueif this queue contains the specified element- Throws:
ClassCastException- if the class of the specified element is incompatible with this queue (optional)NullPointerException- if the specified element is null (optional)
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drainTo
int drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collectioncmay result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result inIllegalArgumentException. 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:
UnsupportedOperationException- if addition of elements is not supported by the specified collectionClassCastException- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException- if the specified collection is nullIllegalArgumentException- if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection
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drainTo
int drainTo(Collection<? super E> c, int maxElements)
Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collectioncmay result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result inIllegalArgumentException. 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 intomaxElements- the maximum number of elements to transfer- Returns:
- the number of elements transferred
- Throws:
UnsupportedOperationException- if addition of elements is not supported by the specified collectionClassCastException- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException- if the specified collection is nullIllegalArgumentException- if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection
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