|
Java™ Platform Standard Ed. 6 |
|||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
E
- the type of elements held in this collectionpublic interface BlockingQueue<E>
A 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 |
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
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 Java Collections Framework.
Method Summary | |
---|---|
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. |
boolean |
contains(Object o)
Returns true if this queue contains the specified element. |
int |
drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them to the given collection. |
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. |
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. |
boolean |
offer(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. |
E |
poll(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. |
void |
put(E e)
Inserts the specified element into this queue, waiting if necessary for space to become available. |
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. |
boolean |
remove(Object o)
Removes a single instance of the specified element from this queue, if it is present. |
E |
take()
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available. |
Methods inherited from interface java.util.Queue |
---|
element, peek, poll, remove |
Methods inherited from interface java.util.Collection |
---|
addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, removeAll, retainAll, size, toArray, toArray |
Method Detail |
---|
boolean add(E e)
offer
.
e
- the element to add
Collection.add(E)
)
IllegalStateException
- if the element cannot be added at this
time due to capacity restrictions
ClassCastException
- if the class of the specified element
prevents it from being added to this queue
NullPointerException
- if the specified element is null
IllegalArgumentException
- if some property of the specified
element prevents it from being added to this queueboolean offer(E e)
add(E)
, which can fail to insert an
element only by throwing an exception.
e
- the element to add
ClassCastException
- if the class of the specified element
prevents it from being added to this queue
NullPointerException
- if the specified element is null
IllegalArgumentException
- if some property of the specified
element prevents it from being added to this queuevoid put(E e) throws InterruptedException
e
- the element to add
InterruptedException
- if interrupted while waiting
ClassCastException
- if the class of the specified element
prevents it from being added to this queue
NullPointerException
- if the specified element is null
IllegalArgumentException
- if some property of the specified
element prevents it from being added to this queueboolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException
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
InterruptedException
- if interrupted while waiting
ClassCastException
- if the class of the specified element
prevents it from being added to this queue
NullPointerException
- if the specified element is null
IllegalArgumentException
- if some property of the specified
element prevents it from being added to this queueE take() throws InterruptedException
InterruptedException
- if interrupted while waitingE poll(long timeout, TimeUnit unit) throws InterruptedException
timeout
- how long to wait before giving up, in units of
unitunit
- a TimeUnit determining how to interpret the
timeout parameter
InterruptedException
- if interrupted while waitingint remainingCapacity()
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.
boolean remove(Object o)
remove
in interface Collection<E>
o
- element to be removed from this queue, if present
ClassCastException
- if the class of the specified element
is incompatible with this queue (optional)
NullPointerException
- if the specified element is null (optional)boolean contains(Object o)
contains
in interface Collection<E>
o
- object to be checked for containment in this queue
ClassCastException
- if the class of the specified element
is incompatible with this queue (optional)
NullPointerException
- if the specified element is null (optional)int drainTo(Collection<? super E> c)
c
- the collection to transfer elements into
UnsupportedOperationException
- if addition of elements
is not supported by the specified collection
ClassCastException
- if the class of an element of this queue
prevents it from being added to the specified collection
NullPointerException
- if the specified collection is null
IllegalArgumentException
- if the specified collection is this
queue, or some property of an element of this queue prevents
it from being added to the specified collectionint drainTo(Collection<? super E> c, int maxElements)
c
- the collection to transfer elements intomaxElements
- the maximum number of elements to transfer
UnsupportedOperationException
- if addition of elements
is not supported by the specified collection
ClassCastException
- if the class of an element of this queue
prevents it from being added to the specified collection
NullPointerException
- if the specified collection is null
IllegalArgumentException
- 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
|
Java™ Platform Standard Ed. 6 |
|||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
Copyright 2006 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.