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 36 package java.util.concurrent; 37 38 import java.util.Collection; 39 import 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>{@code BlockingQueue} 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 * {@code null} or {@code false}, 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 * <table class="plain"> 57 * <caption>Summary of BlockingQueue methods</caption> 58 * <tr> 59 * <td></td> 60 * <td style="text-align:center"><em>Throws exception</em></td> 61 * <td style="text-align:center"><em>Special value</em></td> 62 * <td style="text-align:center"><em>Blocks</em></td> 63 * <td style="text-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 {@code BlockingQueue} does not accept {@code null} elements. 89 * Implementations throw {@code NullPointerException} on attempts 90 * to {@code add}, {@code put} or {@code offer} a {@code null}. A 91 * {@code null} is used as a sentinel value to indicate failure of 92 * {@code poll} operations. 93 * 94 * <p>A {@code BlockingQueue} may be capacity bounded. At any given 95 * time it may have a {@code remainingCapacity} beyond which no 96 * additional elements can be {@code put} without blocking. 97 * A {@code BlockingQueue} without any intrinsic capacity constraints always 98 * reports a remaining capacity of {@code Integer.MAX_VALUE}. 99 * 100 * <p>{@code BlockingQueue} 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 * {@code remove(x)}. 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>{@code BlockingQueue} 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 {@code addAll}, 112 * {@code containsAll}, {@code retainAll} and {@code removeAll} are 113 * <em>not</em> necessarily performed atomically unless specified 114 * otherwise in an implementation. So it is possible, for example, for 115 * {@code addAll(c)} to fail (throwing an exception) after adding 116 * only some of the elements in {@code c}. 117 * 118 * <p>A {@code BlockingQueue} 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 {@code BlockingQueue} can safely be used with multiple 129 * producers and multiple consumers. 130 * <pre> {@code 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 * }}</pre> 164 * 165 * <p>Memory consistency effects: As with other concurrent 166 * collections, actions in a thread prior to placing an object into a 167 * {@code BlockingQueue} 168 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 169 * actions subsequent to the access or removal of that element from 170 * the {@code BlockingQueue} in another thread. 171 * 172 * <p>This interface is a member of the 173 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> 174 * Java Collections Framework</a>. 175 * 176 * @since 1.5 177 * @author Doug Lea 178 * @param <E> the type of elements held in this queue 179 */ 180 public interface BlockingQueue<E> extends Queue<E> { 181 /** 182 * Inserts the specified element into this queue if it is possible to do 183 * so immediately without violating capacity restrictions, returning 184 * {@code true} upon success and throwing an 185 * {@code IllegalStateException} if no space is currently available. 186 * When using a capacity-restricted queue, it is generally preferable to 187 * use {@link #offer(Object) offer}. 188 * 189 * @param e the element to add 190 * @return {@code true} (as specified by {@link Collection#add}) 191 * @throws IllegalStateException if the element cannot be added at this 192 * time due to capacity restrictions 193 * @throws ClassCastException if the class of the specified element 194 * prevents it from being added to this queue 195 * @throws NullPointerException if the specified element is null 196 * @throws IllegalArgumentException if some property of the specified 197 * element prevents it from being added to this queue 198 */ 199 boolean add(E e); 200 201 /** 202 * Inserts the specified element into this queue if it is possible to do 203 * so immediately without violating capacity restrictions, returning 204 * {@code true} upon success and {@code false} if no space is currently 205 * available. When using a capacity-restricted queue, this method is 206 * generally preferable to {@link #add}, which can fail to insert an 207 * element only by throwing an exception. 208 * 209 * @param e the element to add 210 * @return {@code true} if the element was added to this queue, else 211 * {@code false} 212 * @throws ClassCastException if the class of the specified element 213 * prevents it from being added to this queue 214 * @throws NullPointerException if the specified element is null 215 * @throws IllegalArgumentException if some property of the specified 216 * element prevents it from being added to this queue 217 */ 218 boolean offer(E e); 219 220 /** 221 * Inserts the specified element into this queue, waiting if necessary 222 * for space to become available. 223 * 224 * @param e the element to add 225 * @throws InterruptedException if interrupted while waiting 226 * @throws ClassCastException if the class of the specified element 227 * prevents it from being added to this queue 228 * @throws NullPointerException if the specified element is null 229 * @throws IllegalArgumentException if some property of the specified 230 * element prevents it from being added to this queue 231 */ 232 void put(E e) throws InterruptedException; 233 234 /** 235 * Inserts the specified element into this queue, waiting up to the 236 * specified wait time if necessary for space to become available. 237 * 238 * @param e the element to add 239 * @param timeout how long to wait before giving up, in units of 240 * {@code unit} 241 * @param unit a {@code TimeUnit} determining how to interpret the 242 * {@code timeout} parameter 243 * @return {@code true} if successful, or {@code false} if 244 * the specified waiting time elapses before space is available 245 * @throws InterruptedException if interrupted while waiting 246 * @throws ClassCastException if the class of the specified element 247 * prevents it from being added to this queue 248 * @throws NullPointerException if the specified element is null 249 * @throws IllegalArgumentException if some property of the specified 250 * element prevents it from being added to this queue 251 */ 252 boolean offer(E e, long timeout, TimeUnit unit) 253 throws InterruptedException; 254 255 /** 256 * Retrieves and removes the head of this queue, waiting if necessary 257 * until an element becomes available. 258 * 259 * @return the head of this queue 260 * @throws InterruptedException if interrupted while waiting 261 */ 262 E take() throws InterruptedException; 263 264 /** 265 * Retrieves and removes the head of this queue, waiting up to the 266 * specified wait time if necessary for an element to become available. 267 * 268 * @param timeout how long to wait before giving up, in units of 269 * {@code unit} 270 * @param unit a {@code TimeUnit} determining how to interpret the 271 * {@code timeout} parameter 272 * @return the head of this queue, or {@code null} if the 273 * specified waiting time elapses before an element is available 274 * @throws InterruptedException if interrupted while waiting 275 */ 276 E poll(long timeout, TimeUnit unit) 277 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 {@code Integer.MAX_VALUE} 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 {@code remainingCapacity} 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 {@code e} such 297 * that {@code o.equals(e)}, if this queue contains one or more such 298 * elements. 299 * Returns {@code true} 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 {@code true} 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 306 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 307 * @throws NullPointerException if the specified element is null 308 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 309 */ 310 boolean remove(Object o); 311 312 /** 313 * Returns {@code true} if this queue contains the specified element. 314 * More formally, returns {@code true} if and only if this queue contains 315 * at least one element {@code e} such that {@code o.equals(e)}. 316 * 317 * @param o object to be checked for containment in this queue 318 * @return {@code true} if this queue contains the specified element 319 * @throws ClassCastException if the class of the specified element 320 * is incompatible with this queue 321 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 322 * @throws NullPointerException if the specified element is null 323 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 324 */ 325 boolean contains(Object o); 326 327 /** 328 * Removes all available elements from this queue and adds them 329 * to the given collection. This operation may be more 330 * efficient than repeatedly polling this queue. A failure 331 * encountered while attempting to add elements to 332 * collection {@code c} may result in elements being in neither, 333 * either or both collections when the associated exception is 334 * thrown. Attempts to drain a queue to itself result in 335 * {@code IllegalArgumentException}. Further, the behavior of 336 * this operation is undefined if the specified collection is 337 * modified while the operation is in progress. 338 * 339 * @param c the collection to transfer elements into 340 * @return the number of elements transferred 341 * @throws UnsupportedOperationException if addition of elements 342 * is not supported by the specified collection 343 * @throws ClassCastException if the class of an element of this queue 344 * prevents it from being added to the specified collection 345 * @throws NullPointerException if the specified collection is null 346 * @throws IllegalArgumentException if the specified collection is this 347 * queue, or some property of an element of this queue prevents 348 * it from being added to the specified collection 349 */ 350 int drainTo(Collection<? super E> c); 351 352 /** 353 * Removes at most the given number of available elements from 354 * this queue and adds them to the given collection. A failure 355 * encountered while attempting to add elements to 356 * collection {@code c} may result in elements being in neither, 357 * either or both collections when the associated exception is 358 * thrown. Attempts to drain a queue to itself result in 359 * {@code IllegalArgumentException}. Further, the behavior of 360 * this operation is undefined if the specified collection is 361 * modified while the operation is in progress. 362 * 363 * @param c the collection to transfer elements into 364 * @param maxElements the maximum number of elements to transfer 365 * @return the number of elements transferred 366 * @throws UnsupportedOperationException if addition of elements 367 * is not supported by the specified collection 368 * @throws ClassCastException if the class of an element of this queue 369 * prevents it from being added to the specified collection 370 * @throws NullPointerException if the specified collection is null 371 * @throws IllegalArgumentException if the specified collection is this 372 * queue, or some property of an element of this queue prevents 373 * it from being added to the specified collection 374 */ 375 int drainTo(Collection<? super E> c, int maxElements); 376 }