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.
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.
While BlockingQueue is designed to be used primarily
for producer-consumer queues, it additionally supports 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. Also,
the bulk Collection operations, most notably addAll, are
not necessarily performed atomically, so it is possible
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();
}
}
Known Indirect Subclasses
public
int
drainTo(Collection<? super E> c, int maxElements)
Removes at most the given number of available elements from
this queue and adds them into 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.
public
int
drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them
into 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.
public
boolean
offer(E o)
Inserts the specified element into this queue, if possible. When
using queues that may impose insertion restrictions (for
example capacity bounds), method
offer is generally
preferable to method
add(E), which can fail to
insert an element only by throwing an exception.
Returns
- true if it was possible to add the element to
this queue, else false
public
int
remainingCapacity()
Returns the number of 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 add an element will succeed by
inspecting remainingCapacity because it may be the
case that a waiting consumer is ready to take an
element out of an otherwise full queue.