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 |
|---|---|
| public boolean | add(E
the element to add e)Redeclares java. Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions, returning
|
| public boolean | Returns: true if this queue contains the specified elementobject to be checked for containment in this queue o)Redeclares java. Returns |
| public int | Returns: the number of elements transferredthe collection to transfer elements into c)Removes all available elements from this queue and adds them to the given collection. |
| public int | Returns: the number of elements transferredthe collection to transfer elements into c, int the maximum number of elements to transfer maxElements)Removes at most the given number of available elements from this queue and adds them to the given collection. |
| public boolean | Returns: true if the element was added to this queue, else
falsethe element to add e)Redeclares java. Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions, returning
|
| public boolean | Returns: true if successful, or false if
the specified waiting time elapses before space is availablethe element to add e, long how long to wait before giving up, in units of
timeout, TimeUnit unita unit)TimeUnit determining how to interpret the
timeout parameterInserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available. |
| public E | Returns: the head of this queue, ornull if the
specified waiting time elapses before an element is availablehow long to wait before giving up, in units of
timeout, TimeUnit unita unit)TimeUnit determining how to interpret the
timeout parameterRetrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available. |
| public void | put(E
the element to add e)Inserts the specified element into this queue, waiting if necessary for space to become available. |
| public int | Returns: the remaining capacityReturns the number of additional elements that this queue can ideally
(in the absence of memory or resource constraints) accept without
blocking, or |
| public boolean | Returns: true if this queue changed as a result of the callelement to be removed from this queue, if present o)Redeclares java. Removes a single instance of the specified element from this queue, if it is present. |
| public E | Returns: the head of this queueRetrieves and removes the head of this queue, waiting if necessary until an element becomes available. |
| add | back to summary |
|---|---|
| public boolean add(E e) Redeclares java. Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions, returning
| |
| contains | back to summary |
|---|---|
| public boolean contains(Object o) Redeclares java. Returns
| |
| drainTo | back to summary |
|---|---|
| public 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
| |
| drainTo | back to summary |
|---|---|
| public 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
| |
| offer | back to summary |
|---|---|
| public boolean offer(E e) Redeclares java. Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions, returning
| |
| offer | back to summary |
|---|---|
| public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
| |
| poll | back to summary |
|---|---|
| public E poll(long timeout, TimeUnit unit) throws InterruptedException Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
| |
| put | back to summary |
|---|---|
| public void put(E e) throws InterruptedException Inserts the specified element into this queue, waiting if necessary for space to become available.
| |
| remainingCapacity | back to summary |
|---|---|
| public 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 Note that you cannot always tell if an attempt to insert
an element will succeed by inspecting
| |
| remove | back to summary |
|---|---|
| public boolean remove(Object o) Redeclares java. Removes a single instance of the specified element from this queue,
if it is present. More formally, removes an element
| |
| take | back to summary |
|---|---|
| public E take() throws InterruptedException Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
| |