BlockingQueue.java revision 51b1b6997fd3f980076b8081f7f1165ccc2a4008
1/* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25/* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36package java.util.concurrent; 37 38import java.util.Collection; 39import java.util.Queue; 40 41/** 42 * A {@link java.util.Queue} that additionally supports operations 43 * that wait for the queue to become non-empty when retrieving an 44 * element, and wait for space to become available in the queue when 45 * storing an element. 46 * 47 * <p><tt>BlockingQueue</tt> methods come in four forms, with different ways 48 * of handling operations that cannot be satisfied immediately, but may be 49 * satisfied at some point in the future: 50 * one throws an exception, the second returns a special value (either 51 * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third 52 * blocks the current thread indefinitely until the operation can succeed, 53 * and the fourth blocks for only a given maximum time limit before giving 54 * up. These methods are summarized in the following table: 55 * 56 * <p> 57 * <table BORDER CELLPADDING=3 CELLSPACING=1> 58 * <tr> 59 * <td></td> 60 * <td ALIGN=CENTER><em>Throws exception</em></td> 61 * <td ALIGN=CENTER><em>Special value</em></td> 62 * <td ALIGN=CENTER><em>Blocks</em></td> 63 * <td ALIGN=CENTER><em>Times out</em></td> 64 * </tr> 65 * <tr> 66 * <td><b>Insert</b></td> 67 * <td>{@link #add add(e)}</td> 68 * <td>{@link #offer offer(e)}</td> 69 * <td>{@link #put put(e)}</td> 70 * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> 71 * </tr> 72 * <tr> 73 * <td><b>Remove</b></td> 74 * <td>{@link #remove remove()}</td> 75 * <td>{@link #poll poll()}</td> 76 * <td>{@link #take take()}</td> 77 * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> 78 * </tr> 79 * <tr> 80 * <td><b>Examine</b></td> 81 * <td>{@link #element element()}</td> 82 * <td>{@link #peek peek()}</td> 83 * <td><em>not applicable</em></td> 84 * <td><em>not applicable</em></td> 85 * </tr> 86 * </table> 87 * 88 * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements. 89 * Implementations throw <tt>NullPointerException</tt> on attempts 90 * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A 91 * <tt>null</tt> is used as a sentinel value to indicate failure of 92 * <tt>poll</tt> operations. 93 * 94 * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given 95 * time it may have a <tt>remainingCapacity</tt> beyond which no 96 * additional elements can be <tt>put</tt> without blocking. 97 * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always 98 * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>. 99 * 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 */ 181public 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) 278 throws InterruptedException; 279 280 /** 281 * Returns the number of additional elements that this queue can ideally 282 * (in the absence of memory or resource constraints) accept without 283 * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no intrinsic 284 * limit. 285 * 286 * <p>Note that you <em>cannot</em> always tell if an attempt to insert 287 * an element will succeed by inspecting <tt>remainingCapacity</tt> 288 * because it may be the case that another thread is about to 289 * insert or remove an element. 290 * 291 * @return the remaining capacity 292 */ 293 int remainingCapacity(); 294 295 /** 296 * Removes a single instance of the specified element from this queue, 297 * if it is present. More formally, removes an element <tt>e</tt> such 298 * that <tt>o.equals(e)</tt>, if this queue contains one or more such 299 * elements. 300 * Returns <tt>true</tt> if this queue contained the specified element 301 * (or equivalently, if this queue changed as a result of the call). 302 * 303 * @param o element to be removed from this queue, if present 304 * @return <tt>true</tt> if this queue changed as a result of the call 305 * @throws ClassCastException if the class of the specified element 306 * is incompatible with this queue 307 * (<a href="../Collection.html#optional-restrictions">optional</a>) 308 * @throws NullPointerException if the specified element is null 309 * (<a href="../Collection.html#optional-restrictions">optional</a>) 310 */ 311 boolean remove(Object o); 312 313 /** 314 * Returns <tt>true</tt> if this queue contains the specified element. 315 * More formally, returns <tt>true</tt> if and only if this queue contains 316 * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. 317 * 318 * @param o object to be checked for containment in this queue 319 * @return <tt>true</tt> if this queue contains the specified element 320 * @throws ClassCastException if the class of the specified element 321 * is incompatible with this queue 322 * (<a href="../Collection.html#optional-restrictions">optional</a>) 323 * @throws NullPointerException if the specified element is null 324 * (<a href="../Collection.html#optional-restrictions">optional</a>) 325 */ 326 public boolean contains(Object o); 327 328 /** 329 * Removes all available elements from this queue and adds them 330 * to the given collection. This operation may be more 331 * efficient than repeatedly polling this queue. A failure 332 * encountered while attempting to add elements to 333 * collection <tt>c</tt> may result in elements being in neither, 334 * either or both collections when the associated exception is 335 * thrown. Attempts to drain a queue to itself result in 336 * <tt>IllegalArgumentException</tt>. Further, the behavior of 337 * this operation is undefined if the specified collection is 338 * modified while the operation is in progress. 339 * 340 * @param c the collection to transfer elements into 341 * @return the number of elements transferred 342 * @throws UnsupportedOperationException if addition of elements 343 * is not supported by the specified collection 344 * @throws ClassCastException if the class of an element of this queue 345 * prevents it from being added to the specified collection 346 * @throws NullPointerException if the specified collection is null 347 * @throws IllegalArgumentException if the specified collection is this 348 * queue, or some property of an element of this queue prevents 349 * it from being added to the specified collection 350 */ 351 int drainTo(Collection<? super E> c); 352 353 /** 354 * Removes at most the given number of available elements from 355 * this queue and adds them to the given collection. A failure 356 * encountered while attempting to add elements to 357 * collection <tt>c</tt> may result in elements being in neither, 358 * either or both collections when the associated exception is 359 * thrown. Attempts to drain a queue to itself result in 360 * <tt>IllegalArgumentException</tt>. Further, the behavior of 361 * this operation is undefined if the specified collection is 362 * modified while the operation is in progress. 363 * 364 * @param c the collection to transfer elements into 365 * @param maxElements the maximum number of elements to transfer 366 * @return the number of elements transferred 367 * @throws UnsupportedOperationException if addition of elements 368 * is not supported by the specified collection 369 * @throws ClassCastException if the class of an element of this queue 370 * prevents it from being added to the specified collection 371 * @throws NullPointerException if the specified collection is null 372 * @throws IllegalArgumentException if the specified collection is this 373 * queue, or some property of an element of this queue prevents 374 * it from being added to the specified collection 375 */ 376 int drainTo(Collection<? super E> c, int maxElements); 377} 378