util/concurrent/BlockingQueue.java

/*
 * 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/licenses/publicdomain
 */

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.
 *
 * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
 * Implementations throw <tt>NullPointerException</tt> on attempts
 * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>.  A
 * <tt>null</tt> is used as a sentinel value to indicate failure of
 * <tt>poll</tt> operations.
 *
 * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
 * time it may have a <tt>remainingCapacity</tt> beyond which no
 * additional elements can be <tt>put</tt> without blocking.
 * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
 * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
 *
 * <p> While <tt>BlockingQueue</tt> is designed to be used primarily
 * for producer-consumer queues, it additionally supports the {@link
 * java.util.Collection} interface.  So, for example, it is possible
 * to remove an arbitrary element from a queue using
 * <tt>remove(x)</tt>. However, such operations are in general
 * <em>not</em> 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 <tt>addAll</tt>, are
 * <em>not</em> necessarily performed atomically, so it is possible
 * for <tt>addAll(c)</tt> to fail (throwing an exception) after adding
 * only some of the elements in <tt>c</tt>.
 *
 * <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
 * any kind of &quot;close&quot; or &quot;shutdown&quot; 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
 * <em>end-of-stream</em> or <em>poison</em> objects, that are
 * interpreted accordingly when taken by consumers.
 *
 * <p>
 * Usage example, based on a typical producer-consumer scenario.
 * Note that a <tt>BlockingQueue</tt> can safely be used with multiple
 * producers and multiple consumers.
 * <pre>
 * 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();
 *   }
 * }
 * </pre>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *  
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 */
public interface BlockingQueue<E> extends Queue<E> {

    /**
     * Inserts the specified element into this queue, if possible.  When
     * using queues that may impose insertion restrictions (for
     * example capacity bounds), method <tt>offer</tt> is generally
     * preferable to method {@link Collection#add}, which can fail to
     * insert an element only by throwing an exception.
     *
     * @param o the element to add.
     * @return <tt>true</tt> if it was possible to add the element to
     *         this queue, else <tt>false</tt>
     * @throws NullPointerException if the specified element is <tt>null</tt>
     */
    boolean offer(E o);
    
    /**
     * Inserts the specified element into this queue, waiting if necessary
     * up to the specified wait time for space to become available.
     * @param o the element to add
     * @param timeout how long to wait before giving up, in units of
     * <tt>unit</tt>
     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
     * <tt>timeout</tt> parameter
     * @return <tt>true</tt> if successful, or <tt>false</tt> if
     * the specified waiting time elapses before space is available.
     * @throws InterruptedException if interrupted while waiting.
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     */
    boolean offer(E o, long timeout, TimeUnit unit)
        throws InterruptedException;

    /**
     * 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.
     * @param timeout how long to wait before giving up, in units of
     * <tt>unit</tt>
     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
     * <tt>timeout</tt> parameter
     * @return the head of this queue, or <tt>null</tt> if the
     * specified waiting time elapses before an element is present.
     * @throws InterruptedException if interrupted while waiting.
     */
    E poll(long timeout, TimeUnit unit)
        throws InterruptedException;

    /**
     * Retrieves and removes the head of this queue, waiting
     * if no elements are present on this queue.
     * @return the head of this queue
     * @throws InterruptedException if interrupted while waiting.
     */
    E take() throws InterruptedException;

    /**
     * Adds the specified element to this queue, waiting if necessary for
     * space to become available.
     * @param o the element to add
     * @throws InterruptedException if interrupted while waiting.
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     */
    void put(E o) throws InterruptedException;

    /**
     * Returns the number of elements that this queue can ideally (in
     * the absence of memory or resource constraints) accept without
     * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
     * intrinsic limit.
     * <p>Note that you <em>cannot</em> always tell if
     * an attempt to <tt>add</tt> an element will succeed by
     * inspecting <tt>remainingCapacity</tt> because it may be the
     * case that a waiting consumer is ready to <tt>take</tt> an
     * element out of an otherwise full queue.
     * @return the remaining capacity
     */
    int remainingCapacity();

    /**
     * Adds the specified element to this queue if it is possible to
     * do so immediately, returning <tt>true</tt> upon success, else
     * throwing an IllegalStateException.  
     * @param o the element
     * @return <tt>true</tt> (as per the general contract of
     *         <tt>Collection.add</tt>).
     *
     * @throws NullPointerException if the specified element is <tt>null</tt>
     * @throws IllegalStateException if element cannot be added
     */
    boolean add(E o);

    /**
     * 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 <tt>add</tt> elements to
     * collection <tt>c</tt> 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
     * <tt>IllegalArgumentException</tt>. 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 NullPointerException if c is null
     * @throws IllegalArgumentException if c is this queue
     * 
     */
    int drainTo(Collection<? super E> c);
    
    /**
     * 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 <tt>add</tt> elements to
     * collection <tt>c</tt> 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
     * <tt>IllegalArgumentException</tt>. 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 NullPointerException if c is null
     * @throws IllegalArgumentException if c is this queue
     */
    int drainTo(Collection<? super E> c, int maxElements);
}
Revision:	1.28
Committed:	Tue Jan 27 11:36:31 2004 UTC (20 years, 4 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PFD
Changes since 1.27:	+4 -1 lines
Log Message:	Add Collection framework membership doc
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.Collection;
10	import java.util.Queue;
11
12	/**
13	* A {@link java.util.Queue} that additionally supports operations
14	* that wait for the queue to become non-empty when retrieving an element,
15	* and wait for space to become available in the queue when storing an
16	* element.
17	*
18	* <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
19	* Implementations throw <tt>NullPointerException</tt> on attempts
20	* to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
21	* <tt>null</tt> is used as a sentinel value to indicate failure of
22	* <tt>poll</tt> operations.
23	*
24	* <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
25	* time it may have a <tt>remainingCapacity</tt> beyond which no
26	* additional elements can be <tt>put</tt> without blocking.
27	* A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
28	* reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
29	*
30	* <p> While <tt>BlockingQueue</tt> is designed to be used primarily
31	* for producer-consumer queues, it additionally supports the {@link
32	* java.util.Collection} interface. So, for example, it is possible
33	* to remove an arbitrary element from a queue using
34	* <tt>remove(x)</tt>. However, such operations are in general
35	* <em>not</em> performed very efficiently, and are intended for only
36	* occasional use, such as when a queued message is cancelled. Also,
37	* the bulk Collection operations, most notably <tt>addAll</tt>, are
38	* <em>not</em> necessarily performed atomically, so it is possible
39	* for <tt>addAll(c)</tt> to fail (throwing an exception) after adding
40	* only some of the elements in <tt>c</tt>.
41	*
42	* <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
43	* any kind of "close" or "shutdown" operation to
44	* indicate that no more items will be added. The needs and usage of
45	* such features tend to be implementation-dependent. For example, a
46	* common tactic is for producers to insert special
47	* <em>end-of-stream</em> or <em>poison</em> objects, that are
48	* interpreted accordingly when taken by consumers.
49	*
50	* <p>
51	* Usage example, based on a typical producer-consumer scenario.
52	* Note that a <tt>BlockingQueue</tt> can safely be used with multiple
53	* producers and multiple consumers.
54	* <pre>
55	* class Producer implements Runnable {
56	* private final BlockingQueue queue;
57	* Producer(BlockingQueue q) { queue = q; }
58	* public void run() {
59	* try {
60	* while(true) { queue.put(produce()); }
61	* } catch (InterruptedException ex) { ... handle ...}
62	* }
63	* Object produce() { ... }
64	* }
65	*
66	* class Consumer implements Runnable {
67	* private final BlockingQueue queue;
68	* Consumer(BlockingQueue q) { queue = q; }
69	* public void run() {
70	* try {
71	* while(true) { consume(queue.take()); }
72	* } catch (InterruptedException ex) { ... handle ...}
73	* }
74	* void consume(Object x) { ... }
75	* }
76	*
77	* class Setup {
78	* void main() {
79	* BlockingQueue q = new SomeQueueImplementation();
80	* Producer p = new Producer(q);
81	* Consumer c1 = new Consumer(q);
82	* Consumer c2 = new Consumer(q);
83	* new Thread(p).start();
84	* new Thread(c1).start();
85	* new Thread(c2).start();
86	* }
87	* }
88	* </pre>
89	*
90	* <p>This interface is a member of the
91	* <a href="{@docRoot}/../guide/collections/index.html">
92	* Java Collections Framework</a>.
93	*
94	* @since 1.5
95	* @author Doug Lea
96	* @param <E> the type of elements held in this collection
97	*/
98	public interface BlockingQueue<E> extends Queue<E> {
99
100	/**
101	* Inserts the specified element into this queue, if possible. When
102	* using queues that may impose insertion restrictions (for
103	* example capacity bounds), method <tt>offer</tt> is generally
104	* preferable to method {@link Collection#add}, which can fail to
105	* insert an element only by throwing an exception.
106	*
107	* @param o the element to add.
108	* @return <tt>true</tt> if it was possible to add the element to
109	* this queue, else <tt>false</tt>
110	* @throws NullPointerException if the specified element is <tt>null</tt>
111	*/
112	boolean offer(E o);
113
114	/**
115	* Inserts the specified element into this queue, waiting if necessary
116	* up to the specified wait time for space to become available.
117	* @param o the element to add
118	* @param timeout how long to wait before giving up, in units of
119	* <tt>unit</tt>
120	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
121	* <tt>timeout</tt> parameter
122	* @return <tt>true</tt> if successful, or <tt>false</tt> if
123	* the specified waiting time elapses before space is available.
124	* @throws InterruptedException if interrupted while waiting.
125	* @throws NullPointerException if the specified element is <tt>null</tt>.
126	*/
127	boolean offer(E o, long timeout, TimeUnit unit)
128	throws InterruptedException;
129
130	/**
131	* Retrieves and removes the head of this queue, waiting
132	* if necessary up to the specified wait time if no elements are
133	* present on this queue.
134	* @param timeout how long to wait before giving up, in units of
135	* <tt>unit</tt>
136	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
137	* <tt>timeout</tt> parameter
138	* @return the head of this queue, or <tt>null</tt> if the
139	* specified waiting time elapses before an element is present.
140	* @throws InterruptedException if interrupted while waiting.
141	*/
142	E poll(long timeout, TimeUnit unit)
143	throws InterruptedException;
144
145	/**
146	* Retrieves and removes the head of this queue, waiting
147	* if no elements are present on this queue.
148	* @return the head of this queue
149	* @throws InterruptedException if interrupted while waiting.
150	*/
151	E take() throws InterruptedException;
152
153	/**
154	* Adds the specified element to this queue, waiting if necessary for
155	* space to become available.
156	* @param o the element to add
157	* @throws InterruptedException if interrupted while waiting.
158	* @throws NullPointerException if the specified element is <tt>null</tt>.
159	*/
160	void put(E o) throws InterruptedException;
161
162	/**
163	* Returns the number of elements that this queue can ideally (in
164	* the absence of memory or resource constraints) accept without
165	* blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
166	* intrinsic limit.
167	* <p>Note that you <em>cannot</em> always tell if
168	* an attempt to <tt>add</tt> an element will succeed by
169	* inspecting <tt>remainingCapacity</tt> because it may be the
170	* case that a waiting consumer is ready to <tt>take</tt> an
171	* element out of an otherwise full queue.
172	* @return the remaining capacity
173	*/
174	int remainingCapacity();
175
176	/**
177	* Adds the specified element to this queue if it is possible to
178	* do so immediately, returning <tt>true</tt> upon success, else
179	* throwing an IllegalStateException.
180	* @param o the element
181	* @return <tt>true</tt> (as per the general contract of
182	* <tt>Collection.add</tt>).
183	*
184	* @throws NullPointerException if the specified element is <tt>null</tt>
185	* @throws IllegalStateException if element cannot be added
186	*/
187	boolean add(E o);
188
189	/**
190	* Removes all available elements from this queue and adds them
191	* into the given collection. This operation may be more
192	* efficient than repeatedly polling this queue. A failure
193	* encountered while attempting to <tt>add</tt> elements to
194	* collection <tt>c</tt> may result in elements being in neither,
195	* either or both collections when the associated exception is
196	* thrown. Attempts to drain a queue to itself result in
197	* <tt>IllegalArgumentException</tt>. Further, the behavior of
198	* this operation is undefined if the specified collection is
199	* modified while the operation is in progress.
200	*
201	* @param c the collection to transfer elements into
202	* @return the number of elements transferred.
203	* @throws NullPointerException if c is null
204	* @throws IllegalArgumentException if c is this queue
205	*
206	*/
207	int drainTo(Collection<? super E> c);
208
209	/**
210	* Removes at most the given number of available elements from
211	* this queue and adds them into the given collection. A failure
212	* encountered while attempting to <tt>add</tt> elements to
213	* collection <tt>c</tt> may result in elements being in neither,
214	* either or both collections when the associated exception is
215	* thrown. Attempts to drain a queue to itself result in
216	* <tt>IllegalArgumentException</tt>. Further, the behavior of
217	* this operation is undefined if the specified collection is
218	* modified while the operation is in progress.
219	*
220	* @param c the collection to transfer elements into
221	* @param maxElements the maximum number of elements to transfer
222	* @return the number of elements transferred.
223	* @throws NullPointerException if c is null
224	* @throws IllegalArgumentException if c is this queue
225	*/
226	int drainTo(Collection<? super E> c, int maxElements);
227	}