E
- the type of elements held in this collection
public 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
Returns
true if this queue contains the specified element.
|
int |
drainTo(Collection
Removes all available elements from this queue and adds them to the given collection.
|
int |
drainTo(Collection
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
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
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
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, toArray
boolean 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 add
true
(as specified by
Collection.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
boolean 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 add
true
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 queue
NullPointerException
- if the specified element is null
IllegalArgumentException
- if some property of the specified element prevents it from being added to this queue
void put(E e) throws InterruptedException
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
boolean offer(E e, long timeout, TimeUnitunit) throws InterruptedException
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
true
if successful, or
false
if the specified waiting time elapses before space is available
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
E take() throws InterruptedException
InterruptedException
- if interrupted while waiting
E poll(long timeout, TimeUnitunit) throws InterruptedException
timeout
- how long to wait before giving up, in units of
unit
unit
- a
TimeUnit
determining how to interpret the
timeout
parameter
null
if the specified waiting time elapses before an element is available
InterruptedException
- if interrupted while waiting
int 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(Objecto)
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 present
true
if this queue changed as a result of the call
ClassCastException
- if the class of the specified element is incompatible with this queue (
optional)
NullPointerException
- if the specified element is null (
optional)
boolean contains(Objecto)
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 queue
true
if this queue contains the specified element
ClassCastException
- 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 into
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
int 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 into
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