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.
|
addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArrayboolean add(E e)
true upon success and throwing an
IllegalStateException if no space is currently available.
When using a capacity-restricted queue, it is generally preferable to
use offer.add in interface Collection<E>add in interface Queue<E>e - the element to addtrue (as specified by Collection.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)
true 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.offer in interface Queue<E>e - the element to addtrue if the element was added to this queue, else
falseClassCastException - 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 parametertrue if successful, or false if
the specified waiting time elapses before space is availableInterruptedException - 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 parameternull if the
specified waiting time elapses before an element is availableInterruptedException - if interrupted while waitingint remainingCapacity()
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.
boolean remove(Object o)
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).remove in interface Collection<E>o - element to be removed from this queue, if presenttrue if this queue changed as a result of the callClassCastException - 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)
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).contains in interface Collection<E>o - object to be checked for containment in this queuetrue if this queue contains the specified elementClassCastException - 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 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 in
IllegalArgumentException. Further, the behavior of
this operation is undefined if the specified collection is
modified while the operation is in progress.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 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 in
IllegalArgumentException. Further, the behavior of
this operation is undefined if the specified collection is
modified while the operation is in progress.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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