BlockingQueue

public interface BlockingQueue implements Queue<E>

Known Indirect Subclasses

ArrayBlockingQueue<E>, BlockingDeque<E>, DelayQueue<E extends Delayed>, LinkedBlockingDeque<E>, LinkedBlockingQueue<E>, LinkedTransferQueue<E>, PriorityBlockingQueue<E>, SynchronousQueue<E>, TransferQueue<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:

Summary of BlockingQueue methods
	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 Method Summary

abstract 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.
abstract boolean	contains(Object o) Returns `true` if this queue contains the specified element.
abstract int	drainTo(Collection<? super E> c) Removes all available elements from this queue and adds them to the given collection.
abstract 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.
abstract 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.
abstract 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.
abstract 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.
abstract void	put(E e) Inserts the specified element into this queue, waiting if necessary for space to become available.
abstract 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.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this queue, if it is present.
abstract E	take() Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Inherited Method Summary

From interface java.util.Queue

abstract 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.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns `true` if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns `true` if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel `Stream` with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements in this collection.
Stream<E>	stream() Returns a sequential `Stream` with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type `T`.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements described by this `Iterable`.

Public Methods

public abstract 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.

Parameters

e	the element to add

Returns

true (as specified by Collection.add(E))

Throws

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

public abstract 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).

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)

public abstract 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.

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 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

public abstract 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. 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.

Parameters

c	the collection to transfer elements into
maxElements	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 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

public abstract 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. 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.

Parameters

e	the element to add

Returns

true if the element was added to this queue, else false

Throws

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

public abstract 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.

Parameters

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

Returns

true if successful, or false if the specified waiting time elapses before space is available

Throws

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

public abstract 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.

Parameters

timeout	how long to wait before giving up, in units of `unit`
unit	a `TimeUnit` determining how to interpret the `timeout` 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

public abstract void put (E e)

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 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

public abstract 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.

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

public abstract 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).

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)

public abstract E 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