E
- the type of elements held in this collectionpublic 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 |
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.
Modifier and Type | Method and Description |
---|---|
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.
|
boolean add(E e)
offer
.add
in interface Collection<E>
add
in interface Queue<E>
e
- the element to addCollection.add(E)
)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 queueboolean offer(E e)
add(E)
, which can fail to insert an
element only by throwing an exception.offer
in interface Queue<E>
e
- the element to addClassCastException
- 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 queuevoid put(E e) throws InterruptedException
e
- the element to addInterruptedException
- 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 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 parameterInterruptedException
- 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 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 parameterInterruptedException
- 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 presentClassCastException
- 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 queueClassCastException
- 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 intoUnsupportedOperationException
- 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 collectionint drainTo(Collection<? super E> c, int maxElements)
c
- the collection to transfer elements intomaxElements
- the maximum number of elements to transferUnsupportedOperationException
- 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 Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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