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 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>
 *
 *
 * @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);

    /**
     * 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:	1.22
Committed:	Sun Oct 5 23:00:18 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.21:	+37 -15 lines
Log Message:	added drainTo; clarified various exception specs
#	Content
1	/*
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	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 elements to exist when retrieving them, and wait for
15	* space to exist when storing them.
16	*
17	* <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
18	* Implementations throw <tt>NullPointerException</tt> on attempts
19	* 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	* <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	* additional elements can be <tt>put</tt> without blocking.
26	* A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
27	* reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
28	*
29	* <p> While <tt>BlockingQueue</tt> is designed to be used primarily
30	* 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	* <tt>remove(x)</tt>. However, such operations are in general
34	* <em>not</em> performed very efficiently, and are intended for only
35	* occasional use, such as when a queued message is cancelled. Also,
36	* the bulk Collection operations, most notably <tt>addAll</tt> are
37	* <em>not</em> necessarily performed atomically, so it is possible
38	* for <tt>addAll(c)</tt> to fail (throwing an exception) after adding
39	* only some of the elements in <tt>c</tt>.
40	*
41	* <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
42	* any kind of "close" or "shutdown" operation to
43	* indicate that no more items will be added. The needs and usage of
44	* such features tend to be implementation-dependent. For example, a
45	* 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	*
49	* <p>
50	* Usage example, based on a typical producer-consumer scenario.
51	* Note that a <tt>BlockingQueue</tt> can safely be used with multiple
52	* producers and multiple consumers.
53	* <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	* } catch (InterruptedException ex) { ... handle ...}
61	* }
62	* Object produce() { ... }
63	* }
64	*
65	* class Consumer implements Runnable {
66	* private final BlockingQueue queue;
67	* Consumer(BlockingQueue q) { queue = q; }
68	* public void run() {
69	* try {
70	* while(true) { consume(queue.take()); }
71	* } catch (InterruptedException ex) { ... handle ...}
72	* }
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	* Consumer c1 = new Consumer(q);
81	* Consumer c2 = new Consumer(q);
82	* new Thread(p).start();
83	* new Thread(c1).start();
84	* new Thread(c2).start();
85	* }
86	* }
87	* </pre>
88	*
89	*
90	* @since 1.5
91	* @author Doug Lea
92	*/
93	public interface BlockingQueue<E> extends Queue<E> {
94
95	/**
96	* Inserts the specified element into this queue, if possible. When
97	* 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	* insert an element only by throwing an exception.
101	*
102	* @param o the element to add.
103	* @return <tt>true</tt> if it was possible to add the element to
104	* this queue, else <tt>false</tt>
105	* @throws NullPointerException if the specified element is <tt>null</tt>
106	*/
107	boolean offer(E o);
108
109	/**
110	* Inserts the specified element into this queue, waiting if necessary
111	* up to the specified wait time for space to become available.
112	* @param o the element to add
113	* @param timeout how long to wait before giving up, in units of
114	* <tt>unit</tt>
115	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
116	* <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	* @throws InterruptedException if interrupted while waiting.
120	* @throws NullPointerException if the specified element is <tt>null</tt>.
121	*/
122	boolean offer(E o, long timeout, TimeUnit unit)
123	throws InterruptedException;
124
125	/**
126	* Retrieves and removes the head of this queue, waiting
127	* if necessary up to the specified wait time if no elements are
128	* present on this queue.
129	* @param timeout how long to wait before giving up, in units of
130	* <tt>unit</tt>
131	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
132	* <tt>timeout</tt> parameter
133	* @return the head of this queue, or <tt>null</tt> if the
134	* specified waiting time elapses before an element is present.
135	* @throws InterruptedException if interrupted while waiting.
136	*/
137	E poll(long timeout, TimeUnit unit)
138	throws InterruptedException;
139
140	/**
141	* Retrieves and removes the head of this queue, waiting
142	* if no elements are present on this queue.
143	* @return the head of this queue
144	* @throws InterruptedException if interrupted while waiting.
145	*/
146	E take() throws InterruptedException;
147
148	/**
149	* Adds the specified element to this queue, waiting if necessary for
150	* space to become available.
151	* @param o the element to add
152	* @throws InterruptedException if interrupted while waiting.
153	* @throws NullPointerException if the specified element is <tt>null</tt>.
154	*/
155	void put(E o) throws InterruptedException;
156
157	/**
158	* Returns the number of elements that this queue can ideally (in
159	* the absence of memory or resource constraints) accept without
160	* blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
161	* intrinsic limit.
162	* <p>Note that you <em>cannot</em> always tell if
163	* 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	*/
169	int remainingCapacity();
170
171	/**
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
184	/**
185	* Removes all available elements from this queue and adds them
186	* into the given collection. This operation may be more
187	* efficient than repeatedly polling this queue. A failure
188	* encountered while attempting to <tt>add</tt> elements to
189	* collection <tt>c</tt> may result in elements being in neither,
190	* either or both collections when the associated exception is
191	* thrown. Attempts to drain a queue to itself result in
192	* <tt>IllegalArgumentException</tt>. Further, the behavior of
193	* this operation is undefined if the specified collection is
194	* modified while the operation is in progress.
195	*
196	* @param c the collection to transfer elements into
197	* @return the number of elements transferred.
198	* @throws NullPointerException if c is null
199	* @throws IllegalArgumentException if c is this queue
200	*
201	*/
202	int drainTo(Collection<? super E> c);
203
204	/**
205	* Removes at most the given number of available elements from
206	* this queue and adds them into the given collection. A failure
207	* encountered while attempting to <tt>add</tt> elements to
208	* collection <tt>c</tt> may result in elements being in neither,
209	* either or both collections when the associated exception is
210	* thrown. Attempts to drain a queue to itself result in
211	* <tt>IllegalArgumentException</tt>. Further, the behavior of
212	* this operation is undefined if the specified collection is
213	* modified while the operation is in progress.
214	*
215	* @param c the collection to transfer elements into
216	* @param maxElements the maximum number of elements to transfer
217	* @return the number of elements transferred.
218	* @throws NullPointerException if c is null
219	* @throws IllegalArgumentException if c is this queue
220	*/
221	int drainTo(Collection<? super E> c, int maxElements);
222	}