java.util.concurrent.BlockingQueue<E> |
Known Indirect Subclasses
ArrayBlockingQueue<E>,
BlockingDeque<E>,
DelayQueue<E extends Delayed>,
LinkedBlockingDeque<E>,
LinkedBlockingQueue<E>,
PriorityBlockingQueue<E>,
SynchronousQueue<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.
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.
Public Methods | |||||||||||
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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.
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Returns true if this queue contains the specified element.
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Removes all available elements from this queue and adds them
to the given collection.
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Removes at most the given number of available elements from
this queue and adds them to the given collection.
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Inserts the specified element into this queue, waiting up to the
specified wait time if necessary for space to become available.
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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.
| |||||||||||
Retrieves and removes the head of this queue, waiting up to the
specified wait time if necessary for an element to become available.
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Inserts the specified element into this queue, waiting if necessary
for space to become available.
| |||||||||||
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.
| |||||||||||
Removes a single instance of the specified element from this queue,
if it is present.
| |||||||||||
Retrieves and removes the head of this queue, waiting if necessary
until an element becomes available.
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[Expand]
Inherited Methods | |||||||||||
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From interface
java.lang.Iterable
| |||||||||||
From interface
java.util.Collection
| |||||||||||
From interface
java.util.Queue
|
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.
When using a capacity-restricted queue, it is generally preferable to
use offer
.
e | the element to add |
---|
add(E)
)IllegalStateException | if the element cannot be added at this time due to capacity restrictions |
---|---|
ClassCastException | if the class of the specified element prevents it from being added to this queue |
NullPointerException | if the specified element is null |
IllegalArgumentException | if some property of the specified element prevents it from being added to this queue |
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).
o | object to be checked for containment in this queue |
---|
ClassCastException | if the class of the specified element is incompatible with this queue (optional) |
---|---|
NullPointerException | if the specified element is null (optional) |
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 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 into |
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UnsupportedOperationException | if addition of elements is not supported by the specified collection |
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ClassCastException | if the class of an element of this queue prevents it from being added to the specified collection |
NullPointerException | if the specified collection is null |
IllegalArgumentException | 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 |
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 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 into |
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maxElements | the maximum number of elements to transfer |
UnsupportedOperationException | if addition of elements is not supported by the specified collection |
---|---|
ClassCastException | if the class of an element of this queue prevents it from being added to the specified collection |
NullPointerException | if the specified collection is null |
IllegalArgumentException | 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 |
Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
e | the element to add |
---|---|
timeout | how long to wait before giving up, in units of unit |
unit | a TimeUnit determining how to interpret the timeout parameter |
InterruptedException | if interrupted while waiting |
---|---|
ClassCastException | if the class of the specified element prevents it from being added to this queue |
NullPointerException | if the specified element is null |
IllegalArgumentException | if some property of the specified element prevents it from being added to this queue |
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. 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.
e | the element to add |
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ClassCastException | if the class of the specified element prevents it from being added to this queue |
---|---|
NullPointerException | if the specified element is null |
IllegalArgumentException | if some property of the specified element prevents it from being added to this queue |
Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
timeout | how long to wait before giving up, in units of unit |
---|---|
unit | a TimeUnit determining how to interpret the timeout parameter |
InterruptedException | if interrupted while waiting |
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Inserts the specified element into this queue, waiting if necessary for space to become available.
e | the element to add |
---|
InterruptedException | if interrupted while waiting |
---|---|
ClassCastException | if the class of the specified element prevents it from being added to this queue |
NullPointerException | if the specified element is null |
IllegalArgumentException | if some property of the specified element prevents it from being added to this queue |
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.
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).
o | element to be removed from this queue, if present |
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ClassCastException | if the class of the specified element is incompatible with this queue (optional) |
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NullPointerException | if the specified element is null (optional) |
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
InterruptedException | if interrupted while waiting |
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