Interface BlockingQueue<E>
- Type Parameters:
E
- the type of elements held in this queue
- All Superinterfaces:
Collection<E>
,Iterable<E>
,Queue<E>
- All Known Subinterfaces:
BlockingDeque<E>
,TransferQueue<E>
- All Known Implementing Classes:
ArrayBlockingQueue
,DelayQueue
,LinkedBlockingDeque
,LinkedBlockingQueue
,LinkedTransferQueue
,PriorityBlockingQueue
,SynchronousQueue
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.
- Since:
- 1.5
-
Method Summary
Modifier and TypeMethodDescriptionboolean
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
Returnstrue
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
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success andfalse
if no space is currently available.boolean
Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.void
Inserts the specified element into this queue, waiting if necessary for space to become available.int
Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, orInteger.MAX_VALUE
if there is no intrinsic limit.boolean
Removes a single instance of the specified element from this queue, if it is present.take()
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.Methods declared in interface java.util.Collection
addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray
-
Method Details
-
add
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 useoffer
.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byCollection.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
-
offer
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success andfalse
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.- Specified by:
offer
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
if the element was added to this queue, elsefalse
- 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
-
put
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
-
offer
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 ofunit
unit
- aTimeUnit
determining how to interpret thetimeout
parameter- Returns:
true
if successful, orfalse
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
-
take
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
-
poll
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
- aTimeUnit
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
- 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, orInteger.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
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an elemente
such thato.equals(e)
, if this queue contains one or more such elements. Returnstrue
if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
remove
in interfaceCollection<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)
-
contains
Returnstrue
if this queue contains the specified element. More formally, returnstrue
if and only if this queue contains at least one elemente
such thato.equals(e)
.- Specified by:
contains
in interfaceCollection<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)
-
drainTo
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 collectionc
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
- 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
-
drainTo
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 collectionc
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 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
-