java.util.concurrent.BlockingQueue Maven / Gradle / Ivy
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* published by the Free Software Foundation. Oracle designates this
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*
* This code is distributed in the hope that it will be useful, but WITHOUT
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* accompanied this code).
*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.Collection;
import java.util.Queue;
/**
* A {@link java.util.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
* {@link #add add(e)}
* {@link #offer offer(e)}
* {@link #put put(e)}
* {@link #offer(Object, long, TimeUnit) offer(e, time, unit)}
*
*
* Remove
* {@link #remove remove()}
* {@link #poll poll()}
* {@link #take take()}
* {@link #poll(long, TimeUnit) poll(time, unit)}
*
*
* Examine
* {@link #element element()}
* {@link #peek 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 {@link java.util.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
* {@code BlockingQueue}
* happen-before
* actions subsequent to the access or removal of that element from
* the {@code BlockingQueue} in another thread.
*
*
This interface is a member of the
*
* Java Collections Framework.
*
* @since 1.5
* @author Doug Lea
* @param the type of elements held in this collection
*/
public interface BlockingQueue extends 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 {@link #offer(Object) offer}.
*
* @param e the element to add
* @return true (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
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 false if no space is currently
* available. When using a capacity-restricted queue, this method is
* generally preferable to {@link #add}, which can fail to insert an
* element only by throwing an exception.
*
* @param e the element to add
* @return true if the element was added to this queue, else
* false
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean offer(E e);
/**
* Inserts the specified element into this queue, waiting if necessary
* for space to become available.
*
* @param e the element to add
* @throws InterruptedException if interrupted while waiting
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
void put(E e) throws InterruptedException;
/**
* Inserts the specified element into this queue, waiting up to the
* specified wait time if necessary for space to become available.
*
* @param e the element to add
* @param timeout how long to wait before giving up, in units of
* unit
* @param unit a TimeUnit determining how to interpret the
* timeout parameter
* @return true if successful, or false if
* the specified waiting time elapses before space is available
* @throws InterruptedException if interrupted while waiting
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element becomes available.
*
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting
*/
E take() 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.
*
* @param timeout how long to wait before giving up, in units of
* unit
* @param unit a TimeUnit determining how to interpret the
* timeout parameter
* @return the head of this queue, or null if the
* specified waiting time elapses before an element is available
* @throws InterruptedException if interrupted while waiting
*/
E poll(long timeout, TimeUnit unit)
throws InterruptedException;
/**
* 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.
*
* @return the remaining capacity
*/
int remainingCapacity();
/**
* 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).
*
* @param o element to be removed from this queue, if present
* @return true if this queue changed as a result of the call
* @throws ClassCastException if the class of the specified element
* is incompatible with this queue
* (optional)
* @throws NullPointerException if the specified element is null
* (optional)
*/
boolean remove(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).
*
* @param o object to be checked for containment in this queue
* @return true if this queue contains the specified element
* @throws ClassCastException if the class of the specified element
* is incompatible with this queue
* (optional)
* @throws NullPointerException if the specified element is null
* (optional)
*/
public boolean contains(Object o);
/**
* 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.
*
* @param c the collection to transfer elements into
* @return the number of elements transferred
* @throws UnsupportedOperationException if addition of elements
* is not supported by the specified collection
* @throws ClassCastException if the class of an element of this queue
* prevents it from being added to the specified collection
* @throws NullPointerException if the specified collection is null
* @throws 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
*/
int drainTo(Collection c);
/**
* 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.
*
* @param c the collection to transfer elements into
* @param maxElements the maximum number of elements to transfer
* @return the number of elements transferred
* @throws UnsupportedOperationException if addition of elements
* is not supported by the specified collection
* @throws ClassCastException if the class of an element of this queue
* prevents it from being added to the specified collection
* @throws NullPointerException if the specified collection is null
* @throws 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
*/
int drainTo(Collection c, int maxElements);
}