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 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
|
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.
|
||||||||||
|
Returns
true
if this queue contains the specified element.
|
||||||||||
|
Removes all available elements from this queue and adds them
to the given collection.
|
||||||||||
|
Removes at most the given number of available elements from
this queue and adds them to the given collection.
|
||||||||||
|
Inserts the specified element into this queue, waiting up to the
specified wait time if necessary for space to become available.
|
||||||||||
|
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.
|
||||||||||
|
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.
|
[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 |
---|
true
(as specified by
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 |
---|
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 ) |
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 |
---|
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 |
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 |
---|---|
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
|
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 |
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 |
---|
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 |
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
|
null
if the
specified waiting time elapses before an element is available
InterruptedException | if interrupted while waiting |
---|
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 |
---|
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 ) |
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
InterruptedException | if interrupted while waiting |
---|