public 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.
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 .
While
BlockingQueue
implementations are
is
designed to be used primarily for producer-consumer queues,
but
it
additionally
support
supports
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.
Also, the bulk Collection operations, most notably
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
,
, are not necessarily performed atomically, so it is possible for
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(); } }
This interface is a member of the Java Collections Framework .
Method Summary | |
---|---|
boolean |
add
(
E
o) Adds the specified element to this queue if it is possible to do so immediately, returning true upon success, else throwing an IllegalStateException. |
int |
drainTo
(
Collection
<? super
E
> c) Removes all available elements from this queue and adds them into 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 into the given collection. |
boolean |
offer
(
E
o) Inserts the specified element into this queue, if possible. |
boolean |
offer
(
E
o, long timeout,
TimeUnit
unit) Inserts the specified element into this queue, waiting if necessary up to the specified wait time for space to become available. |
E |
poll
(long timeout,
TimeUnit
unit) Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue. |
void |
put
(
E
o) Adds the specified element to this queue, waiting if necessary for space to become available. |
int |
remainingCapacity
() Returns the number of 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. |
E |
take
() Retrieves and removes the head of this queue, waiting if no elements are present on this queue. |
Methods inherited from interface java.util. Queue |
---|
element , peek , poll , remove |
Methods inherited from interface java.util. Collection |
---|
addAll , clear , contains , containsAll , equals , hashCode , isEmpty , iterator , remove , removeAll , retainAll , size , toArray , toArray |
Method Detail |
---|
boolean offer(E o)
boolean offer(E o, long timeout, TimeUnit unit) throws InterruptedException
E poll(long timeout, TimeUnit unit) throws InterruptedException
E take() throws InterruptedException
void put(E o) throws InterruptedException
int remainingCapacity()
Note that you cannot always tell if an attempt to
add
an element will succeed by inspecting
remainingCapacity
because it may be the case that
another thread
a waiting consumer
is
about
ready
to
put
or
take
an element.
an element out of an otherwise full queue.
boolean add(E o)
int drainTo(Collection<? super E> c)
int drainTo(Collection<? super E> c, int maxElements)