E - the type of elements held in this collection
A Queue that additionally supports operations
that wait for elements to exist when retrieving them, and wait for
space to exist when storing them.
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();
}
}
| Method Summary | |
|---|---|
boolean |
add(E o)
Adds the specified element to this queue if it is possible to do so immediately, returning true upon success, else throwing an IllegalStateException. |
int |
drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them into the given collection. |
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. |
boolean |
offer(E o)
Inserts the specified element into this queue, if possible. |
boolean |
offer(E o,
long timeout,
TimeUnit unit)
Inserts the specified element into this queue, waiting if necessary up to the specified wait time for space to become available. |
E |
poll(long timeout,
TimeUnit unit)
Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue. |
void |
put(E o)
Adds the specified element to this queue, waiting if necessary for space to become available. |
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. |
E |
take()
Retrieves and removes the head of this queue, waiting if no elements are present on this queue. |
| Methods inherited from interface java.util.Queue |
|---|
element, peek, poll, remove |
| Methods inherited from interface java.util.Collection |
|---|
addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray |
| Method Detail |
|---|
boolean offer(E o)
Collection.add(E), which can fail to
insert an element only by throwing an exception.
offer in interface Queueo - the element to add.
NullPointerException - if the specified element is null
boolean offer(E o,
long timeout,
TimeUnit unit)
throws InterruptedException
o - the element to addtimeout - how long to wait before giving up, in units of
unitunit - a TimeUnit determining how to interpret the
timeout parameter
InterruptedException - if interrupted while waiting.
NullPointerException - if the specified element is null.
E poll(long timeout,
TimeUnit unit)
throws InterruptedException
timeout - how long to wait before giving up, in units of
unitunit - a TimeUnit determining how to interpret the
timeout parameter
InterruptedException - if interrupted while waiting.
E take()
throws InterruptedException
InterruptedException - if interrupted while waiting.
void put(E o)
throws InterruptedException
o - the element to add
InterruptedException - if interrupted while waiting.
NullPointerException - if the specified element is null.int remainingCapacity()
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.
boolean add(E o)
add in interface Collectiono - the element
NullPointerException - if the specified element is null
IllegalStateException - if element cannot be addedint drainTo(Collection<? super E> c)
c - the collection to transfer elements into
NullPointerException - if c is null
IllegalArgumentException - if c is this queue
int drainTo(Collection<? super E> c,
int maxElements)
c - the collection to transfer elements intomaxElements - the maximum number of elements to transfer
NullPointerException - if c is null
IllegalArgumentException - if c is this queue