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.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 putting them.
 *
 * <p> <tt>BlockingQueues</tt> do not accept <tt>null</tt> elements.
 * Implementations throw <tt>IllegalArgumentException</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><tt>BlockingQueues</tt> may be capacity bounded. At any given
 * time they may have a <tt>remainingCapacity</tt> beyond which no
 * additional elements can be <tt>put</tt> without blocking.
 * BlockingQueues without any intrinsic capacity constraints always
 * report a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
 *
 * <p> While <tt>BlockingQueues</tt> are designed to be used primarily
 * as producer-consumer queues, they additionally support the
 * <tt>Collection</tt> interface.  So, for example, it is possible to
 * remove an arbitrary element from within 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>
 * 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><tt>BlockingQueue</tt>s do <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 Blocking queues 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;
 *   Concumer(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
 * @spec JSR-166
 * @revised $Date: 2003/06/23 02:26:16 $
 * @editor $Author: brian $
 * @author Doug Lea
 */
public interface BlockingQueue<E> extends Queue<E> {
    /**
     * Retrieve and remove the first element from the queue, waiting
     * if no objects are present on the queue.
     * @return the object
     * @throws InterruptedException if interrupted while waiting.
     */
    E take() throws InterruptedException;

    /**
     * Retrieve and remove the first element from the queue, waiting
     * if necessary up to a specified wait time if no objects are
     * present on the queue.
     * @param timeout how long to wait before giving up, in units of
     * <tt>unit</tt>
     * @param unit a TimeUnit determining how to interpret the timeout
     * parameter
     * @return the object, or <tt>null</tt> if the specified waiting
     * time elapses before an object is present.
     * @throws InterruptedException if interrupted while waiting.
     */
    E poll(long timeout, TimeUnit unit) 
        throws InterruptedException;

    /**
     * Add the given object to the queue, waiting if necessary for
     * space to become available.
     * @param x the object to add
     * @throws InterruptedException if interrupted while waiting.
     */
    void put(E x) throws InterruptedException;

    /**
     * Add the given object to the queue, waiting if necessary up to a
     * specified wait time for space to become available.
     * @param x the object to add
     * @param timeout how long to wait before giving up, in units of
     * <tt>unit</tt>
     * @param unit a TimeUnit determining how to interpret the timeout
     * 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.
     */
    boolean offer(E x, long timeout, TimeUnit unit) 
        throws InterruptedException;

    /**
     * Return 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.  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();

}
Revision:	1.6
Committed:	Tue Jun 24 14:34:47 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.5:	+36 -29 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	import java.util.Queue;
9
10	/**
11	* A {@link java.util.Queue} that additionally supports operations
12	* that wait for elements to exist when retrieving them, and wait for
13	* space to exist when putting them.
14	*
15	* <p> <tt>BlockingQueues</tt> do not accept <tt>null</tt> elements.
16	* Implementations throw <tt>IllegalArgumentException</tt> on attempts
17	* to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
18	* <tt>null</tt> is used as a sentinel value to indicate failure of
19	* <tt>poll</tt> operations.
20	*
21	* <p><tt>BlockingQueues</tt> may be capacity bounded. At any given
22	* time they may have a <tt>remainingCapacity</tt> beyond which no
23	* additional elements can be <tt>put</tt> without blocking.
24	* BlockingQueues without any intrinsic capacity constraints always
25	* report a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
26	*
27	* <p> While <tt>BlockingQueues</tt> are designed to be used primarily
28	* as producer-consumer queues, they additionally support the
29	* <tt>Collection</tt> interface. So, for example, it is possible to
30	* remove an arbitrary element from within a queue using
31	* <tt>remove(x)</tt>. However, such operations are in general
32	* <em>NOT</em> performed very efficiently, and are intended for only
33	* occasional use, such as when a queued message is cancelled. Also,
34	* the bulk operations, most notably <tt>addAll</tt> are <em>NOT</em>
35	* performed atomically, so it is possible for <tt>addAll(c)</tt> to
36	* fail (throwing an exception) after adding only some of the elements
37	* in <tt>c</tt>.
38	*
39	* <p><tt>BlockingQueue</tt>s do <em>not</em> intrinsically support
40	* any kind of "close" or "shutdown" operation to
41	* indicate that no more items will be added. The needs and usage of
42	* such features tend to be implementation-dependent. For example, a
43	* common tactic is for producers to insert special
44	* <em>end-of-stream</em> or <em>poison</em> objects, that are
45	* interpreted accordingly when taken by consumers.
46	*
47	* <p>
48	* Usage example, based on a typical producer-consumer scenario.
49	* Note that Blocking queues can safely be used with multiple producers
50	* and multiple consumers.
51	* <pre>
52	* class Producer implements Runnable {
53	* private final BlockingQueue queue;
54	* Producer(BlockingQueue q) { queue = q; }
55	* public void run() {
56	* try {
57	* while(true) { queue.put(produce()); }
58	* }
59	* catch (InterruptedException ex) { ... handle ...}
60	* }
61	* Object produce() { ... }
62	* }
63	*
64	* class Consumer implements Runnable {
65	* private final BlockingQueue queue;
66	* Concumer(BlockingQueue q) { queue = q; }
67	* public void run() {
68	* try {
69	* while(true) { consume(queue.take()); }
70	* }
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	* @spec JSR-166
92	* @revised $Date: 2003/06/23 02:26:16 $
93	* @editor $Author: brian $
94	* @author Doug Lea
95	*/
96	public interface BlockingQueue<E> extends Queue<E> {
97	/**
98	* Retrieve and remove the first element from the queue, waiting
99	* if no objects are present on the queue.
100	* @return the object
101	* @throws InterruptedException if interrupted while waiting.
102	*/
103	E take() throws InterruptedException;
104
105	/**
106	* Retrieve and remove the first element from the queue, waiting
107	* if necessary up to a specified wait time if no objects are
108	* present on the queue.
109	* @param timeout how long to wait before giving up, in units of
110	* <tt>unit</tt>
111	* @param unit a TimeUnit determining how to interpret the timeout
112	* parameter
113	* @return the object, or <tt>null</tt> if the specified waiting
114	* time elapses before an object is present.
115	* @throws InterruptedException if interrupted while waiting.
116	*/
117	E poll(long timeout, TimeUnit unit)
118	throws InterruptedException;
119
120	/**
121	* Add the given object to the queue, waiting if necessary for
122	* space to become available.
123	* @param x the object to add
124	* @throws InterruptedException if interrupted while waiting.
125	*/
126	void put(E x) throws InterruptedException;
127
128	/**
129	* Add the given object to the queue, waiting if necessary up to a
130	* specified wait time for space to become available.
131	* @param x the object to add
132	* @param timeout how long to wait before giving up, in units of
133	* <tt>unit</tt>
134	* @param unit a TimeUnit determining how to interpret the timeout
135	* parameter
136	* @return <tt>true</tt> if successful, or <tt>false</tt> if
137	* the specified waiting time elapses before space is available.
138	* @throws InterruptedException if interrupted while waiting.
139	*/
140	boolean offer(E x, long timeout, TimeUnit unit)
141	throws InterruptedException;
142
143	/**
144	* Return the number of elements that this queue can ideally (in
145	* the absence of memory or resource constraints) accept without
146	* blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
147	* intrinsic limit. Note that you <em>cannot</em> always tell if
148	* an attempt to <tt>add</tt> an element will succeed by
149	* inspecting <tt>remainingCapacity</tt> because it may be the
150	* case that a waiting consumer is ready to <tt>take</tt> an
151	* element out of an otherwise full queue.
152	* @return the remaining capacity
153	*/
154	int remainingCapacity();
155
156	}