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public interface BlockingQueue<E>
extends Queue<E>
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:
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 |
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.
This interface is a member of the
../../../../technotes/guides/collections/index.html">
Java Collections Framework.
Method Summary | |
boolean |
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boolean | |
int |
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int |
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boolean |
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E | |
void |
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int |
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boolean | |
E |
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Methods inherited from interface java.util.Collection<E> | |
T[] toArray , add , addAll , clear , contains , containsAll , equals , hashCode , isEmpty , iterator , remove , removeAll , retainAll , size , toArray |
Methods inherited from interface java.lang.Iterable<E> | |
iterator |
public 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. When using a capacity-restricted queue, it is generally preferable to useoffer
.
- Specified by:
- add in interface Queue<E>
- add in interface Collection<E>
- Parameters:
e
- the element to add
- Returns:
- true (as specified by
Collection.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
public boolean contains(Object o)
Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).
- Specified by:
- contains in interface Collection<E>
- Parameters:
o
- object to be checked for containment in this queue
- Returns:
- true if 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)
public int drainTo(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 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:
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
public int drainTo(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 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 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
public boolean offer(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 false if 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.
- Parameters:
e
- the element to add
- Returns:
- true if the element was added to this queue, else false
- 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
public 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 of unitunit
- 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
- 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
public 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 of unitunit
- 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 available
- Throws:
InterruptedException
- if interrupted while waiting
public 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
public int remainingCapacity()
Returns the number of additional 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 insert an element will succeed by inspecting remainingCapacity because it may be the case that another thread is about to insert or remove an element.
- Returns:
- the remaining capacity
public boolean remove(Object o)
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element e such that o.equals(e), if this queue contains one or more such elements. Returns true if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).
- Specified by:
- remove in interface Collection<E>
- Parameters:
o
- element to be removed from this queue, if present
- Returns:
- true if 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)
public 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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