BlockingQueue
public
interface
BlockingQueue
implements
Queue<E>
java.util.concurrent.BlockingQueue<E>
|
Known indirect subclasses
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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:
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.
Summary
Public methods |
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
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contains(Object o)
Returns true if this queue contains the specified element.
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abstract
int
|
drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them
to the given collection.
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abstract
int
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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.
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abstract
boolean
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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.
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abstract
boolean
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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.
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abstract
E
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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.
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abstract
void
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put(E e)
Inserts the specified element into this queue, waiting if necessary
for space to become available.
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abstract
int
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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.
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abstract
boolean
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remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present.
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abstract
E
|
take()
Retrieves and removes the head of this queue, waiting if necessary
until an element becomes available.
|
Inherited methods |
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.
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abstract
boolean
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offer(E e)
Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions.
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abstract
E
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peek()
Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
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abstract
E
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poll()
Retrieves and removes the head of this queue,
or returns null if this queue is empty.
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abstract
E
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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
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addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection
(optional operation).
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abstract
void
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clear()
Removes all of the elements from this collection (optional operation).
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abstract
boolean
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contains(Object o)
Returns true if this collection contains the specified element.
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abstract
boolean
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containsAll(Collection<?> c)
Returns true if this collection contains all of the elements
in the specified collection.
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abstract
boolean
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equals(Object o)
Compares the specified object with this collection for equality.
|
abstract
int
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hashCode()
Returns the hash code value for this collection.
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abstract
boolean
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isEmpty()
Returns true if this collection contains no elements.
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abstract
Iterator<E>
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iterator()
Returns an iterator over the elements in this collection.
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default
Stream<E>
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parallelStream()
Returns a possibly parallel Stream with this collection as its
source.
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abstract
boolean
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remove(Object o)
Removes a single instance of the specified element from this
collection, if it is present (optional operation).
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abstract
boolean
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removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).
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default
boolean
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removeIf(Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given
predicate.
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abstract
boolean
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retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the
specified collection (optional operation).
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abstract
int
|
size()
Returns the number of elements in this collection.
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default
Spliterator<E>
|
spliterator()
Creates a Spliterator over the elements in this collection.
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default
Stream<E>
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stream()
Returns a sequential Stream with this collection as its source.
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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.
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abstract
Object[]
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toArray()
Returns an array containing all of the elements in this collection.
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default
<T>
T[]
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toArray(IntFunction<T[]> generator)
Returns an array containing all of the elements in this collection,
using the provided generator function to allocate the returned array.
|
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From interface
java.lang.Iterable
default
void
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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.
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abstract
Iterator<E>
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iterator()
Returns an iterator over elements of type T .
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default
Spliterator<E>
|
spliterator()
Creates a Spliterator over the elements described by this
Iterable .
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Public methods
add
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 |
E : the element to add |
contains
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 : object to be checked for containment in this queue |
Returns |
boolean |
true if this queue contains the specified element |
drainTo
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 |
Collection : the collection to transfer elements into |
Returns |
int |
the number of elements transferred |
drainTo
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 |
Collection : the collection to transfer elements into |
maxElements |
int : the maximum number of elements to transfer |
Returns |
int |
the number of elements transferred |
offer
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 |
E : the element to add |
Returns |
boolean |
true if the element was added to this queue, else
false |
offer
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 |
E : the element to add |
timeout |
long : how long to wait before giving up, in units of
unit |
unit |
TimeUnit : a TimeUnit determining how to interpret the
timeout parameter |
Returns |
boolean |
true if successful, or false if
the specified waiting time elapses before space is available |
poll
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 |
long : how long to wait before giving up, in units of
unit |
unit |
TimeUnit : a TimeUnit determining how to interpret the
timeout parameter |
Returns |
E |
the head of this queue, or null if the
specified waiting time elapses before an element is available |
put
public abstract void put (E e)
Inserts the specified element into this queue, waiting if necessary
for space to become available.
Parameters |
e |
E : the element to add |
remainingCapacity
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 |
int |
the remaining capacity |
remove
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 |
Object : element to be removed from this queue, if present |
Returns |
boolean |
true if this queue changed as a result of the call |
take
public abstract E take ()
Retrieves and removes the head of this queue, waiting if necessary
until an element becomes available.
Returns |
E |
the head of this queue |