util/concurrent/BlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util.concurrent;

import java.util.Collection;
import java.util.Queue;

/**
 * A {@link java.util.Queue} that additionally supports operations
 * that wait for elements to exist when retrieving them, and wait for
 * space to exist when storing them.
 *
 * <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 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>
 *
 *
 * @since 1.5
 * @author Doug Lea
 */
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);

    /**
     * Adds all of the elements in the specified collection to this
     * queue if it is possible to do so.  The behavior of this
     * operation need not be atomic; a failure may occur after
     * adding only some elements.
     *
     * @param c collection whose elements are to be added to this queue
     * @return <tt>true</tt> if this queue changed as a result of the
     *         call.
     * @throws NullPointerException if <tt>c</tt> or any element in <tt>c</tt>
     * is <tt>null</tt>
     * @throws IllegalStateException if any element cannot be added.
     * 
     */
    boolean addAll(Collection<? extends E> c);

}
Revision:	1.21
Committed:	Fri Sep 26 11:37:10 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.20:	+1 -1 lines
Log Message:	Spellcheck
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain. Use, modify, and
4			* redistribute this code in any way without acknowledgement.
5			*/
6
7	tim	1.1	package java.util.concurrent;
8	tim	1.9
9			import java.util.Collection;
10	tim	1.1	import java.util.Queue;
11
12			/**
13	dl	1.4	* A {@link java.util.Queue} that additionally supports operations
14	brian	1.5	* that wait for elements to exist when retrieving them, and wait for
15	dholmes	1.10	* space to exist when storing them.
16	dl	1.2	*
17	dholmes	1.7	* <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
18	dholmes	1.10	* Implementations throw <tt>NullPointerException</tt> on attempts
19	dl	1.2	* to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
20			* <tt>null</tt> is used as a sentinel value to indicate failure of
21			* <tt>poll</tt> operations.
22			*
23	dholmes	1.7	* <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
24			* time it may have a <tt>remainingCapacity</tt> beyond which no
25	dl	1.2	* additional elements can be <tt>put</tt> without blocking.
26	dholmes	1.7	* A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
27			* reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
28	dl	1.2	*
29	dholmes	1.7	* <p> While <tt>BlockingQueue</tt> is designed to be used primarily
30	dl	1.18	* for producer-consumer queues, it additionally supports the {@link
31			* java.util.Collection} interface. So, for example, it is possible
32			* to remove an arbitrary element from a queue using
33	dl	1.6	* <tt>remove(x)</tt>. However, such operations are in general
34	dl	1.18	* <em>not</em> performed very efficiently, and are intended for only
35	dl	1.6	* occasional use, such as when a queued message is cancelled. Also,
36	dl	1.18	* the bulk operations, most notably <tt>addAll</tt> are <em>not</em>
37			* necessarily performed atomically, so it is possible for
38			* <tt>addAll(c)</tt> to fail (throwing an exception) after adding
39			* only some of the elements in <tt>c</tt>.
40	dl	1.2	*
41	dholmes	1.7	* <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
42	dl	1.2	* any kind of "close" or "shutdown" operation to
43			* indicate that no more items will be added. The needs and usage of
44	brian	1.5	* such features tend to be implementation-dependent. For example, a
45	dl	1.2	* common tactic is for producers to insert special
46			* <em>end-of-stream</em> or <em>poison</em> objects, that are
47			* interpreted accordingly when taken by consumers.
48	tim	1.1	*
49			* <p>
50	brian	1.5	* Usage example, based on a typical producer-consumer scenario.
51	tim	1.9	* Note that a <tt>BlockingQueue</tt> can safely be used with multiple
52	dholmes	1.7	* producers and multiple consumers.
53	tim	1.1	* <pre>
54			* class Producer implements Runnable {
55			* private final BlockingQueue queue;
56			* Producer(BlockingQueue q) { queue = q; }
57			* public void run() {
58			* try {
59			* while(true) { queue.put(produce()); }
60	tim	1.15	* } catch (InterruptedException ex) { ... handle ...}
61	tim	1.1	* }
62			* Object produce() { ... }
63			* }
64			*
65			* class Consumer implements Runnable {
66			* private final BlockingQueue queue;
67	dl	1.21	* Consumer(BlockingQueue q) { queue = q; }
68	tim	1.1	* public void run() {
69			* try {
70			* while(true) { consume(queue.take()); }
71	tim	1.15	* } catch (InterruptedException ex) { ... handle ...}
72	tim	1.1	* }
73			* void consume(Object x) { ... }
74			* }
75			*
76			* class Setup {
77			* void main() {
78			* BlockingQueue q = new SomeQueueImplementation();
79			* Producer p = new Producer(q);
80	brian	1.5	* Consumer c1 = new Consumer(q);
81			* Consumer c2 = new Consumer(q);
82	tim	1.1	* new Thread(p).start();
83	brian	1.5	* new Thread(c1).start();
84			* new Thread(c2).start();
85	tim	1.1	* }
86			* }
87			* </pre>
88			*
89			*
90			* @since 1.5
91	dl	1.6	* @author Doug Lea
92	tim	1.1	*/
93			public interface BlockingQueue<E> extends Queue<E> {
94	dholmes	1.7
95			/**
96	dl	1.20	* Inserts the specified element into this queue, if possible. When
97	dl	1.18	* using queues that may impose insertion restrictions (for
98			* example capacity bounds), method <tt>offer</tt> is generally
99			* preferable to method {@link Collection#add}, which can fail to
100	dl	1.19	* insert an element only by throwing an exception.
101	dl	1.18	*
102	dl	1.19	* @param o the element to add.
103	dl	1.18	* @return <tt>true</tt> if it was possible to add the element to
104	dl	1.19	* this queue, else <tt>false</tt>
105			* @throws NullPointerException if the specified element is <tt>null</tt>
106	dholmes	1.7	*/
107	tim	1.14	boolean offer(E o);
108	dl	1.20
109	tim	1.1	/**
110	dl	1.20	* Inserts the specified element into this queue, waiting if necessary
111	dl	1.18	* up to the specified wait time for space to become available.
112	dholmes	1.10	* @param o the element to add
113	dholmes	1.7	* @param timeout how long to wait before giving up, in units of
114			* <tt>unit</tt>
115	tim	1.9	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
116	dholmes	1.7	* <tt>timeout</tt> parameter
117			* @return <tt>true</tt> if successful, or <tt>false</tt> if
118			* the specified waiting time elapses before space is available.
119	tim	1.1	* @throws InterruptedException if interrupted while waiting.
120	dholmes	1.11	* @throws NullPointerException if the specified element is <tt>null</tt>.
121	tim	1.1	*/
122	dholmes	1.10	boolean offer(E o, long timeout, TimeUnit unit)
123	dholmes	1.7	throws InterruptedException;
124	tim	1.1
125			/**
126	dholmes	1.12	* Retrieves and removes the head of this queue, waiting
127	dholmes	1.7	* if necessary up to the specified wait time if no elements are
128			* present on this queue.
129	dl	1.6	* @param timeout how long to wait before giving up, in units of
130			* <tt>unit</tt>
131	tim	1.9	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
132	dholmes	1.7	* <tt>timeout</tt> parameter
133	tim	1.9	* @return the head of this queue, or <tt>null</tt> if the
134	dholmes	1.7	* specified waiting time elapses before an element is present.
135	tim	1.1	* @throws InterruptedException if interrupted while waiting.
136			*/
137	tim	1.9	E poll(long timeout, TimeUnit unit)
138	tim	1.1	throws InterruptedException;
139
140			/**
141	dholmes	1.12	* Retrieves and removes the head of this queue, waiting
142	dholmes	1.7	* if no elements are present on this queue.
143			* @return the head of this queue
144	tim	1.1	* @throws InterruptedException if interrupted while waiting.
145			*/
146	dholmes	1.7	E take() throws InterruptedException;
147
148	tim	1.1	/**
149	dholmes	1.12	* Adds the specified element to this queue, waiting if necessary for
150	dholmes	1.7	* space to become available.
151	dholmes	1.10	* @param o the element to add
152	tim	1.1	* @throws InterruptedException if interrupted while waiting.
153	dholmes	1.13	* @throws NullPointerException if the specified element is <tt>null</tt>.
154	tim	1.1	*/
155	dholmes	1.10	void put(E o) throws InterruptedException;
156	dholmes	1.7
157	dl	1.2	/**
158	dholmes	1.12	* Returns the number of elements that this queue can ideally (in
159	dl	1.2	* the absence of memory or resource constraints) accept without
160			* blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
161	tim	1.9	* intrinsic limit.
162	dholmes	1.7	* <p>Note that you <em>cannot</em> always tell if
163	dl	1.2	* an attempt to <tt>add</tt> an element will succeed by
164			* inspecting <tt>remainingCapacity</tt> because it may be the
165			* case that a waiting consumer is ready to <tt>take</tt> an
166			* element out of an otherwise full queue.
167			* @return the remaining capacity
168	dl	1.6	*/
169			int remainingCapacity();
170	tim	1.1
171	dl	1.18	/**
172			* Adds the specified element to this queue if it is possible to
173			* do so immediately, returning <tt>true</tt> upon success, else
174			* throwing an IllegalStateException.
175			* @param o the element
176			* @return <tt>true</tt> (as per the general contract of
177			* <tt>Collection.add</tt>).
178			*
179			* @throws NullPointerException if the specified element is <tt>null</tt>
180			* @throws IllegalStateException if element cannot be added
181			*/
182			boolean add(E o);
183	dholmes	1.7
184	dl	1.18	/**
185			* Adds all of the elements in the specified collection to this
186			* queue if it is possible to do so. The behavior of this
187			* operation need not be atomic; a failure may occur after
188			* adding only some elements.
189			*
190			* @param c collection whose elements are to be added to this queue
191			* @return <tt>true</tt> if this queue changed as a result of the
192			* call.
193			* @throws NullPointerException if <tt>c</tt> or any element in <tt>c</tt>
194			* is <tt>null</tt>
195			* @throws IllegalStateException if any element cannot be added.
196			*
197			*/
198			boolean addAll(Collection<? extends E> c);
199	dholmes	1.7
200	dl	1.18	}