001 /*
002 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
003 *
004 * This code is free software; you can redistribute it and/or modify it
005 * under the terms of the GNU General Public License version 2 only, as
006 * published by the Free Software Foundation. Sun designates this
007 * particular file as subject to the "Classpath" exception as provided
008 * by Sun in the LICENSE file that accompanied this code.
009 *
010 * This code is distributed in the hope that it will be useful, but WITHOUT
011 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
012 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
013 * version 2 for more details (a copy is included in the LICENSE file that
014 * accompanied this code).
015 *
016 * You should have received a copy of the GNU General Public License version
017 * 2 along with this work; if not, write to the Free Software Foundation,
018 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
019 *
020 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
021 * CA 95054 USA or visit www.sun.com if you need additional information or
022 * have any questions.
023 */
024
025 /*
026 * This file is available under and governed by the GNU General Public
027 * License version 2 only, as published by the Free Software Foundation.
028 * However, the following notice accompanied the original version of this
029 * file:
030 *
031 * Written by Doug Lea with assistance from members of JCP JSR-166
032 * Expert Group and released to the public domain, as explained at
033 * http://creativecommons.org/licenses/publicdomain
034 */
035
036 package java.util.concurrent;
037
038 import java.util.Collection;
039 import java.util.Queue;
040
041 /**
042 * A {@link java.util.Queue} that additionally supports operations
043 * that wait for the queue to become non-empty when retrieving an
044 * element, and wait for space to become available in the queue when
045 * storing an element.
046 *
047 * <p><tt>BlockingQueue</tt> methods come in four forms, with different ways
048 * of handling operations that cannot be satisfied immediately, but may be
049 * satisfied at some point in the future:
050 * one throws an exception, the second returns a special value (either
051 * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third
052 * blocks the current thread indefinitely until the operation can succeed,
053 * and the fourth blocks for only a given maximum time limit before giving
054 * up. These methods are summarized in the following table:
055 *
056 * <p>
057 * <table BORDER CELLPADDING=3 CELLSPACING=1>
058 * <tr>
059 * <td></td>
060 * <td ALIGN=CENTER><em>Throws exception</em></td>
061 * <td ALIGN=CENTER><em>Special value</em></td>
062 * <td ALIGN=CENTER><em>Blocks</em></td>
063 * <td ALIGN=CENTER><em>Times out</em></td>
064 * </tr>
065 * <tr>
066 * <td><b>Insert</b></td>
067 * <td>{@link #add add(e)}</td>
068 * <td>{@link #offer offer(e)}</td>
069 * <td>{@link #put put(e)}</td>
070 * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
071 * </tr>
072 * <tr>
073 * <td><b>Remove</b></td>
074 * <td>{@link #remove remove()}</td>
075 * <td>{@link #poll poll()}</td>
076 * <td>{@link #take take()}</td>
077 * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
078 * </tr>
079 * <tr>
080 * <td><b>Examine</b></td>
081 * <td>{@link #element element()}</td>
082 * <td>{@link #peek peek()}</td>
083 * <td><em>not applicable</em></td>
084 * <td><em>not applicable</em></td>
085 * </tr>
086 * </table>
087 *
088 * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
089 * Implementations throw <tt>NullPointerException</tt> on attempts
090 * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
091 * <tt>null</tt> is used as a sentinel value to indicate failure of
092 * <tt>poll</tt> operations.
093 *
094 * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
095 * time it may have a <tt>remainingCapacity</tt> beyond which no
096 * additional elements can be <tt>put</tt> without blocking.
097 * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
098 * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
099 *
100 * <p> <tt>BlockingQueue</tt> implementations are designed to be used
101 * primarily for producer-consumer queues, but additionally support
102 * the {@link java.util.Collection} interface. So, for example, it is
103 * possible to remove an arbitrary element from a queue using
104 * <tt>remove(x)</tt>. However, such operations are in general
105 * <em>not</em> performed very efficiently, and are intended for only
106 * occasional use, such as when a queued message is cancelled.
107 *
108 * <p> <tt>BlockingQueue</tt> implementations are thread-safe. All
109 * queuing methods achieve their effects atomically using internal
110 * locks or other forms of concurrency control. However, the
111 * <em>bulk</em> Collection operations <tt>addAll</tt>,
112 * <tt>containsAll</tt>, <tt>retainAll</tt> and <tt>removeAll</tt> are
113 * <em>not</em> necessarily performed atomically unless specified
114 * otherwise in an implementation. So it is possible, for example, for
115 * <tt>addAll(c)</tt> to fail (throwing an exception) after adding
116 * only some of the elements in <tt>c</tt>.
117 *
118 * <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
119 * any kind of "close" or "shutdown" operation to
120 * indicate that no more items will be added. The needs and usage of
121 * such features tend to be implementation-dependent. For example, a
122 * common tactic is for producers to insert special
123 * <em>end-of-stream</em> or <em>poison</em> objects, that are
124 * interpreted accordingly when taken by consumers.
125 *
126 * <p>
127 * Usage example, based on a typical producer-consumer scenario.
128 * Note that a <tt>BlockingQueue</tt> can safely be used with multiple
129 * producers and multiple consumers.
130 * <pre>
131 * class Producer implements Runnable {
132 * private final BlockingQueue queue;
133 * Producer(BlockingQueue q) { queue = q; }
134 * public void run() {
135 * try {
136 * while (true) { queue.put(produce()); }
137 * } catch (InterruptedException ex) { ... handle ...}
138 * }
139 * Object produce() { ... }
140 * }
141 *
142 * class Consumer implements Runnable {
143 * private final BlockingQueue queue;
144 * Consumer(BlockingQueue q) { queue = q; }
145 * public void run() {
146 * try {
147 * while (true) { consume(queue.take()); }
148 * } catch (InterruptedException ex) { ... handle ...}
149 * }
150 * void consume(Object x) { ... }
151 * }
152 *
153 * class Setup {
154 * void main() {
155 * BlockingQueue q = new SomeQueueImplementation();
156 * Producer p = new Producer(q);
157 * Consumer c1 = new Consumer(q);
158 * Consumer c2 = new Consumer(q);
159 * new Thread(p).start();
160 * new Thread(c1).start();
161 * new Thread(c2).start();
162 * }
163 * }
164 * </pre>
165 *
166 * <p>Memory consistency effects: As with other concurrent
167 * collections, actions in a thread prior to placing an object into a
168 * {@code BlockingQueue}
169 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
170 * actions subsequent to the access or removal of that element from
171 * the {@code BlockingQueue} in another thread.
172 *
173 * <p>This interface is a member of the
174 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
175 * Java Collections Framework</a>.
176 *
177 * @since 1.5
178 * @author Doug Lea
179 * @param <E> the type of elements held in this collection
180 */
181 public interface BlockingQueue<E> extends Queue<E> {
182 /**
183 * Inserts the specified element into this queue if it is possible to do
184 * so immediately without violating capacity restrictions, returning
185 * <tt>true</tt> upon success and throwing an
186 * <tt>IllegalStateException</tt> if no space is currently available.
187 * When using a capacity-restricted queue, it is generally preferable to
188 * use {@link #offer(Object) offer}.
189 *
190 * @param e the element to add
191 * @return <tt>true</tt> (as specified by {@link Collection#add})
192 * @throws IllegalStateException if the element cannot be added at this
193 * time due to capacity restrictions
194 * @throws ClassCastException if the class of the specified element
195 * prevents it from being added to this queue
196 * @throws NullPointerException if the specified element is null
197 * @throws IllegalArgumentException if some property of the specified
198 * element prevents it from being added to this queue
199 */
200 boolean add(E e);
201
202 /**
203 * Inserts the specified element into this queue if it is possible to do
204 * so immediately without violating capacity restrictions, returning
205 * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
206 * available. When using a capacity-restricted queue, this method is
207 * generally preferable to {@link #add}, which can fail to insert an
208 * element only by throwing an exception.
209 *
210 * @param e the element to add
211 * @return <tt>true</tt> if the element was added to this queue, else
212 * <tt>false</tt>
213 * @throws ClassCastException if the class of the specified element
214 * prevents it from being added to this queue
215 * @throws NullPointerException if the specified element is null
216 * @throws IllegalArgumentException if some property of the specified
217 * element prevents it from being added to this queue
218 */
219 boolean offer(E e);
220
221 /**
222 * Inserts the specified element into this queue, waiting if necessary
223 * for space to become available.
224 *
225 * @param e the element to add
226 * @throws InterruptedException if interrupted while waiting
227 * @throws ClassCastException if the class of the specified element
228 * prevents it from being added to this queue
229 * @throws NullPointerException if the specified element is null
230 * @throws IllegalArgumentException if some property of the specified
231 * element prevents it from being added to this queue
232 */
233 void put(E e) throws InterruptedException;
234
235 /**
236 * Inserts the specified element into this queue, waiting up to the
237 * specified wait time if necessary for space to become available.
238 *
239 * @param e the element to add
240 * @param timeout how long to wait before giving up, in units of
241 * <tt>unit</tt>
242 * @param unit a <tt>TimeUnit</tt> determining how to interpret the
243 * <tt>timeout</tt> parameter
244 * @return <tt>true</tt> if successful, or <tt>false</tt> if
245 * the specified waiting time elapses before space is available
246 * @throws InterruptedException if interrupted while waiting
247 * @throws ClassCastException if the class of the specified element
248 * prevents it from being added to this queue
249 * @throws NullPointerException if the specified element is null
250 * @throws IllegalArgumentException if some property of the specified
251 * element prevents it from being added to this queue
252 */
253 boolean offer(E e, long timeout, TimeUnit unit)
254 throws InterruptedException;
255
256 /**
257 * Retrieves and removes the head of this queue, waiting if necessary
258 * until an element becomes available.
259 *
260 * @return the head of this queue
261 * @throws InterruptedException if interrupted while waiting
262 */
263 E take() throws InterruptedException;
264
265 /**
266 * Retrieves and removes the head of this queue, waiting up to the
267 * specified wait time if necessary for an element to become available.
268 *
269 * @param timeout how long to wait before giving up, in units of
270 * <tt>unit</tt>
271 * @param unit a <tt>TimeUnit</tt> determining how to interpret the
272 * <tt>timeout</tt> parameter
273 * @return the head of this queue, or <tt>null</tt> if the
274 * specified waiting time elapses before an element is available
275 * @throws InterruptedException if interrupted while waiting
276 */
277 E poll(long timeout, TimeUnit unit) throws InterruptedException;
278
279 /**
280 * Returns the number of additional elements that this queue can ideally
281 * (in the absence of memory or resource constraints) accept without
282 * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no intrinsic
283 * limit.
284 *
285 * <p>Note that you <em>cannot</em> always tell if an attempt to insert
286 * an element will succeed by inspecting <tt>remainingCapacity</tt>
287 * because it may be the case that another thread is about to
288 * insert or remove an element.
289 *
290 * @return the remaining capacity
291 */
292 int remainingCapacity();
293
294 /**
295 * Removes a single instance of the specified element from this queue,
296 * if it is present. More formally, removes an element <tt>e</tt> such
297 * that <tt>o.equals(e)</tt>, if this queue contains one or more such
298 * elements.
299 * Returns <tt>true</tt> if this queue contained the specified element
300 * (or equivalently, if this queue changed as a result of the call).
301 *
302 * @param o element to be removed from this queue, if present
303 * @return <tt>true</tt> if this queue changed as a result of the call
304 * @throws ClassCastException if the class of the specified element
305 * is incompatible with this queue (optional)
306 * @throws NullPointerException if the specified element is null (optional)
307 */
308 boolean remove(Object o);
309
310 /**
311 * Returns <tt>true</tt> if this queue contains the specified element.
312 * More formally, returns <tt>true</tt> if and only if this queue contains
313 * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
314 *
315 * @param o object to be checked for containment in this queue
316 * @return <tt>true</tt> if this queue contains the specified element
317 * @throws ClassCastException if the class of the specified element
318 * is incompatible with this queue (optional)
319 * @throws NullPointerException if the specified element is null (optional)
320 */
321 public boolean contains(Object o);
322
323 /**
324 * Removes all available elements from this queue and adds them
325 * to the given collection. This operation may be more
326 * efficient than repeatedly polling this queue. A failure
327 * encountered while attempting to add elements to
328 * collection <tt>c</tt> may result in elements being in neither,
329 * either or both collections when the associated exception is
330 * thrown. Attempts to drain a queue to itself result in
331 * <tt>IllegalArgumentException</tt>. Further, the behavior of
332 * this operation is undefined if the specified collection is
333 * modified while the operation is in progress.
334 *
335 * @param c the collection to transfer elements into
336 * @return the number of elements transferred
337 * @throws UnsupportedOperationException if addition of elements
338 * is not supported by the specified collection
339 * @throws ClassCastException if the class of an element of this queue
340 * prevents it from being added to the specified collection
341 * @throws NullPointerException if the specified collection is null
342 * @throws IllegalArgumentException if the specified collection is this
343 * queue, or some property of an element of this queue prevents
344 * it from being added to the specified collection
345 */
346 int drainTo(Collection<? super E> c);
347
348 /**
349 * Removes at most the given number of available elements from
350 * this queue and adds them to the given collection. A failure
351 * encountered while attempting to add elements to
352 * collection <tt>c</tt> may result in elements being in neither,
353 * either or both collections when the associated exception is
354 * thrown. Attempts to drain a queue to itself result in
355 * <tt>IllegalArgumentException</tt>. Further, the behavior of
356 * this operation is undefined if the specified collection is
357 * modified while the operation is in progress.
358 *
359 * @param c the collection to transfer elements into
360 * @param maxElements the maximum number of elements to transfer
361 * @return the number of elements transferred
362 * @throws UnsupportedOperationException if addition of elements
363 * is not supported by the specified collection
364 * @throws ClassCastException if the class of an element of this queue
365 * prevents it from being added to the specified collection
366 * @throws NullPointerException if the specified collection is null
367 * @throws IllegalArgumentException if the specified collection is this
368 * queue, or some property of an element of this queue prevents
369 * it from being added to the specified collection
370 */
371 int drainTo(Collection<? super E> c, int maxElements);
372 }
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