BlockingQueue (Java Platform SE 6) (original) (raw)

java.util.concurrent

Interface BlockingQueue

Type Parameters:

E - the type of elements held in this collection

All Superinterfaces:

Collection, Iterable, Queue

All Known Subinterfaces:

BlockingDeque

All Known Implementing Classes:

ArrayBlockingQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue

public interface BlockingQueue

extends Queue

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 (eithernull 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)](../../../java/util/concurrent/BlockingQueue.html#offer%28E, long, java.util.concurrent.TimeUnit%29)
Remove	remove()	[poll()](../../../java/util/concurrent/BlockingQueue.html#poll%28long, java.util.concurrent.TimeUnit%29)	take()	[poll(time, unit)](../../../java/util/concurrent/BlockingQueue.html#poll%28long, java.util.concurrent.TimeUnit%29)
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. Anull is used as a sentinel value to indicate failure ofpoll 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 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.

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, foraddAll(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 aBlockingQueue 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.

Since:

1.5

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, returningtrue upon success and throwing anIllegalStateException 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](../../../java/util/concurrent/BlockingQueue.html#drainTo%28java.util.Collection, int%29)(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, returningtrue upon success and false if no space is currently available.
boolean	[offer](../../../java/util/concurrent/BlockingQueue.html#offer%28E, long, java.util.concurrent.TimeUnit%29)(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](../../../java/util/concurrent/BlockingQueue.html#poll%28long, java.util.concurrent.TimeUnit%29)(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

add

boolean add(E e)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue upon success and throwing anIllegalStateException if no space is currently available. When using a capacity-restricted queue, it is generally preferable to use offer.

Specified by:

[add](../../../java/util/Collection.html#add%28E%29) in interface [Collection](../../../java/util/Collection.html "interface in java.util")<[E](../../../java/util/concurrent/BlockingQueue.html "type parameter in BlockingQueue")>

Specified by:

[add](../../../java/util/Queue.html#add%28E%29) in interface [Queue](../../../java/util/Queue.html "interface in java.util")<[E](../../../java/util/concurrent/BlockingQueue.html "type parameter in BlockingQueue")>

Parameters:

e - the element to add

Returns:

true (as specified by Collection.add(E))

Throws:

[IllegalStateException](../../../java/lang/IllegalStateException.html "class in java.lang") - if the element cannot be added at this time due to capacity restrictions

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this queue

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null

[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of the specified element prevents it from being added to this queue

offer

boolean offer(E e)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue upon success and false if no space is currently available. When using a capacity-restricted queue, this method is generally preferable to add(E), which can fail to insert an element only by throwing an exception.

Specified by:

[offer](../../../java/util/Queue.html#offer%28E%29) in interface [Queue](../../../java/util/Queue.html "interface in java.util")<[E](../../../java/util/concurrent/BlockingQueue.html "type parameter in BlockingQueue")>

Parameters:

e - the element to add

Returns:

true if the element was added to this queue, elsefalse

Throws:

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this queue

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null

[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of the specified element prevents it from being added to this queue

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](../../../java/lang/InterruptedException.html "class in java.lang") - if interrupted while waiting

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this queue

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null

[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of the specified element prevents it from being added to this queue

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 add

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

unit - a TimeUnit determining how to interpret thetimeout parameter

Returns:

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

Throws:

[InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang") - if interrupted while waiting

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this queue

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null

[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of the specified element prevents it from being added to this queue

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](../../../java/lang/InterruptedException.html "class in java.lang") - if interrupted while waiting

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 ofunit

unit - a TimeUnit determining how to interpret thetimeout parameter

Returns:

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

Throws:

[InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang") - if interrupted while waiting

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, 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

remove

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](../../../java/util/Collection.html#remove%28java.lang.Object%29) in interface [Collection](../../../java/util/Collection.html "interface in java.util")<[E](../../../java/util/concurrent/BlockingQueue.html "type parameter in BlockingQueue")>

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](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element is incompatible with this queue (optional)

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null (optional)

contains

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](../../../java/util/Collection.html#contains%28java.lang.Object%29) in interface [Collection](../../../java/util/Collection.html "interface in java.util")<[E](../../../java/util/concurrent/BlockingQueue.html "type parameter in BlockingQueue")>

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

Throws:

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element is incompatible with this queue (optional)

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null (optional)

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 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 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang") - if addition of elements is not supported by the specified collection

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of an element of this queue prevents it from being added to the specified collection

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection is null

[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang") - 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

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 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 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

maxElements - the maximum number of elements to transfer

Returns:

the number of elements transferred

Throws:

[UnsupportedOperationException](../../../java/lang/UnsupportedOperationException.html "class in java.lang") - if addition of elements is not supported by the specified collection

[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang") - if the class of an element of this queue prevents it from being added to the specified collection

[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection is null

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