E - the type of elements held in this collectionpublic interface BlockingQueue<E> extends Queue<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.
This interface is a member of the Java Collections Framework.
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. 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)
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.
offer in interface Queue<E>
e - the element to addtrue if the element was added to this queue, else false
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 queuevoid put(E e) throws InterruptedException
Inserts the specified element into this queue, waiting if necessary for space to become available.
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 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 addtimeout - how long to wait before giving up, in units of unit
unit - 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
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
InterruptedException - if interrupted while waitingE poll(long timeout,
TimeUnit unit)
throws InterruptedException 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 parameternull if the specified waiting time elapses before an element is availableInterruptedException - if interrupted while waitingint 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.
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)
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).
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)
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).
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)
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 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) 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 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
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