1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent; 8 9 import java.util.Collection; 10 import java.util.Queue; 11 12 // BEGIN android-note 13 // removed link to collections framework docs 14 // END android-note 15 16 /** 17 * A {@link java.util.Queue} that additionally supports operations 18 * that wait for the queue to become non-empty when retrieving an 19 * element, and wait for space to become available in the queue when 20 * storing an element. 21 * 22 * <p>{@code BlockingQueue} methods come in four forms, with different ways 23 * of handling operations that cannot be satisfied immediately, but may be 24 * satisfied at some point in the future: 25 * one throws an exception, the second returns a special value (either 26 * {@code null} or {@code false}, depending on the operation), the third 27 * blocks the current thread indefinitely until the operation can succeed, 28 * and the fourth blocks for only a given maximum time limit before giving 29 * up. These methods are summarized in the following table: 30 * 31 * <p> 32 * <table BORDER CELLPADDING=3 CELLSPACING=1> 33 * <tr> 34 * <td></td> 35 * <td ALIGN=CENTER><em>Throws exception</em></td> 36 * <td ALIGN=CENTER><em>Special value</em></td> 37 * <td ALIGN=CENTER><em>Blocks</em></td> 38 * <td ALIGN=CENTER><em>Times out</em></td> 39 * </tr> 40 * <tr> 41 * <td><b>Insert</b></td> 42 * <td>{@link #add add(e)}</td> 43 * <td>{@link #offer offer(e)}</td> 44 * <td>{@link #put put(e)}</td> 45 * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> 46 * </tr> 47 * <tr> 48 * <td><b>Remove</b></td> 49 * <td>{@link #remove remove()}</td> 50 * <td>{@link #poll poll()}</td> 51 * <td>{@link #take take()}</td> 52 * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> 53 * </tr> 54 * <tr> 55 * <td><b>Examine</b></td> 56 * <td>{@link #element element()}</td> 57 * <td>{@link #peek peek()}</td> 58 * <td><em>not applicable</em></td> 59 * <td><em>not applicable</em></td> 60 * </tr> 61 * </table> 62 * 63 * <p>A {@code BlockingQueue} does not accept {@code null} elements. 64 * Implementations throw {@code NullPointerException} on attempts 65 * to {@code add}, {@code put} or {@code offer} a {@code null}. A 66 * {@code null} is used as a sentinel value to indicate failure of 67 * {@code poll} operations. 68 * 69 * <p>A {@code BlockingQueue} may be capacity bounded. At any given 70 * time it may have a {@code remainingCapacity} beyond which no 71 * additional elements can be {@code put} without blocking. 72 * A {@code BlockingQueue} without any intrinsic capacity constraints always 73 * reports a remaining capacity of {@code Integer.MAX_VALUE}. 74 * 75 * <p>{@code BlockingQueue} implementations are designed to be used 76 * primarily for producer-consumer queues, but additionally support 77 * the {@link java.util.Collection} interface. So, for example, it is 78 * possible to remove an arbitrary element from a queue using 79 * {@code remove(x)}. However, such operations are in general 80 * <em>not</em> performed very efficiently, and are intended for only 81 * occasional use, such as when a queued message is cancelled. 82 * 83 * <p>{@code BlockingQueue} implementations are thread-safe. All 84 * queuing methods achieve their effects atomically using internal 85 * locks or other forms of concurrency control. However, the 86 * <em>bulk</em> Collection operations {@code addAll}, 87 * {@code containsAll}, {@code retainAll} and {@code removeAll} are 88 * <em>not</em> necessarily performed atomically unless specified 89 * otherwise in an implementation. So it is possible, for example, for 90 * {@code addAll(c)} to fail (throwing an exception) after adding 91 * only some of the elements in {@code c}. 92 * 93 * <p>A {@code BlockingQueue} does <em>not</em> intrinsically support 94 * any kind of "close" or "shutdown" operation to 95 * indicate that no more items will be added. The needs and usage of 96 * such features tend to be implementation-dependent. For example, a 97 * common tactic is for producers to insert special 98 * <em>end-of-stream</em> or <em>poison</em> objects, that are 99 * interpreted accordingly when taken by consumers. 100 * 101 * <p> 102 * Usage example, based on a typical producer-consumer scenario. 103 * Note that a {@code BlockingQueue} can safely be used with multiple 104 * producers and multiple consumers. 105 * <pre> {@code 106 * class Producer implements Runnable { 107 * private final BlockingQueue queue; 108 * Producer(BlockingQueue q) { queue = q; } 109 * public void run() { 110 * try { 111 * while (true) { queue.put(produce()); } 112 * } catch (InterruptedException ex) { ... handle ...} 113 * } 114 * Object produce() { ... } 115 * } 116 * 117 * class Consumer implements Runnable { 118 * private final BlockingQueue queue; 119 * Consumer(BlockingQueue q) { queue = q; } 120 * public void run() { 121 * try { 122 * while (true) { consume(queue.take()); } 123 * } catch (InterruptedException ex) { ... handle ...} 124 * } 125 * void consume(Object x) { ... } 126 * } 127 * 128 * class Setup { 129 * void main() { 130 * BlockingQueue q = new SomeQueueImplementation(); 131 * Producer p = new Producer(q); 132 * Consumer c1 = new Consumer(q); 133 * Consumer c2 = new Consumer(q); 134 * new Thread(p).start(); 135 * new Thread(c1).start(); 136 * new Thread(c2).start(); 137 * } 138 * }}</pre> 139 * 140 * <p>Memory consistency effects: As with other concurrent 141 * collections, actions in a thread prior to placing an object into a 142 * {@code BlockingQueue} 143 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 144 * actions subsequent to the access or removal of that element from 145 * the {@code BlockingQueue} in another thread. 146 * 147 * @since 1.5 148 * @author Doug Lea 149 * @param <E> the type of elements held in this collection 150 */ 151 public interface BlockingQueue<E> extends Queue<E> { 152 /** 153 * Inserts the specified element into this queue if it is possible to do 154 * so immediately without violating capacity restrictions, returning 155 * {@code true} upon success and throwing an 156 * {@code IllegalStateException} if no space is currently available. 157 * When using a capacity-restricted queue, it is generally preferable to 158 * use {@link #offer(Object) offer}. 159 * 160 * @param e the element to add 161 * @return {@code true} (as specified by {@link Collection#add}) 162 * @throws IllegalStateException if the element cannot be added at this 163 * time due to capacity restrictions 164 * @throws ClassCastException if the class of the specified element 165 * prevents it from being added to this queue 166 * @throws NullPointerException if the specified element is null 167 * @throws IllegalArgumentException if some property of the specified 168 * element prevents it from being added to this queue 169 */ add(E e)170 boolean add(E e); 171 172 /** 173 * Inserts the specified element into this queue if it is possible to do 174 * so immediately without violating capacity restrictions, returning 175 * {@code true} upon success and {@code false} if no space is currently 176 * available. When using a capacity-restricted queue, this method is 177 * generally preferable to {@link #add}, which can fail to insert an 178 * element only by throwing an exception. 179 * 180 * @param e the element to add 181 * @return {@code true} if the element was added to this queue, else 182 * {@code false} 183 * @throws ClassCastException if the class of the specified element 184 * prevents it from being added to this queue 185 * @throws NullPointerException if the specified element is null 186 * @throws IllegalArgumentException if some property of the specified 187 * element prevents it from being added to this queue 188 */ offer(E e)189 boolean offer(E e); 190 191 /** 192 * Inserts the specified element into this queue, waiting if necessary 193 * for space to become available. 194 * 195 * @param e the element to add 196 * @throws InterruptedException if interrupted while waiting 197 * @throws ClassCastException if the class of the specified element 198 * prevents it from being added to this queue 199 * @throws NullPointerException if the specified element is null 200 * @throws IllegalArgumentException if some property of the specified 201 * element prevents it from being added to this queue 202 */ put(E e)203 void put(E e) throws InterruptedException; 204 205 /** 206 * Inserts the specified element into this queue, waiting up to the 207 * specified wait time if necessary for space to become available. 208 * 209 * @param e the element to add 210 * @param timeout how long to wait before giving up, in units of 211 * {@code unit} 212 * @param unit a {@code TimeUnit} determining how to interpret the 213 * {@code timeout} parameter 214 * @return {@code true} if successful, or {@code false} if 215 * the specified waiting time elapses before space is available 216 * @throws InterruptedException if interrupted while waiting 217 * @throws ClassCastException if the class of the specified element 218 * prevents it from being added to this queue 219 * @throws NullPointerException if the specified element is null 220 * @throws IllegalArgumentException if some property of the specified 221 * element prevents it from being added to this queue 222 */ offer(E e, long timeout, TimeUnit unit)223 boolean offer(E e, long timeout, TimeUnit unit) 224 throws InterruptedException; 225 226 /** 227 * Retrieves and removes the head of this queue, waiting if necessary 228 * until an element becomes available. 229 * 230 * @return the head of this queue 231 * @throws InterruptedException if interrupted while waiting 232 */ take()233 E take() throws InterruptedException; 234 235 /** 236 * Retrieves and removes the head of this queue, waiting up to the 237 * specified wait time if necessary for an element to become available. 238 * 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 the head of this queue, or {@code null} if the 244 * specified waiting time elapses before an element is available 245 * @throws InterruptedException if interrupted while waiting 246 */ poll(long timeout, TimeUnit unit)247 E poll(long timeout, TimeUnit unit) 248 throws InterruptedException; 249 250 /** 251 * Returns the number of additional elements that this queue can ideally 252 * (in the absence of memory or resource constraints) accept without 253 * blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic 254 * limit. 255 * 256 * <p>Note that you <em>cannot</em> always tell if an attempt to insert 257 * an element will succeed by inspecting {@code remainingCapacity} 258 * because it may be the case that another thread is about to 259 * insert or remove an element. 260 * 261 * @return the remaining capacity 262 */ remainingCapacity()263 int remainingCapacity(); 264 265 /** 266 * Removes a single instance of the specified element from this queue, 267 * if it is present. More formally, removes an element {@code e} such 268 * that {@code o.equals(e)}, if this queue contains one or more such 269 * elements. 270 * Returns {@code true} if this queue contained the specified element 271 * (or equivalently, if this queue changed as a result of the call). 272 * 273 * @param o element to be removed from this queue, if present 274 * @return {@code true} if this queue changed as a result of the call 275 * @throws ClassCastException if the class of the specified element 276 * is incompatible with this queue 277 * (<a href="../Collection.html#optional-restrictions">optional</a>) 278 * @throws NullPointerException if the specified element is null 279 * (<a href="../Collection.html#optional-restrictions">optional</a>) 280 */ remove(Object o)281 boolean remove(Object o); 282 283 /** 284 * Returns {@code true} if this queue contains the specified element. 285 * More formally, returns {@code true} if and only if this queue contains 286 * at least one element {@code e} such that {@code o.equals(e)}. 287 * 288 * @param o object to be checked for containment in this queue 289 * @return {@code true} if this queue contains the specified element 290 * @throws ClassCastException if the class of the specified element 291 * is incompatible with this queue 292 * (<a href="../Collection.html#optional-restrictions">optional</a>) 293 * @throws NullPointerException if the specified element is null 294 * (<a href="../Collection.html#optional-restrictions">optional</a>) 295 */ contains(Object o)296 public boolean contains(Object o); 297 298 /** 299 * Removes all available elements from this queue and adds them 300 * to the given collection. This operation may be more 301 * efficient than repeatedly polling this queue. A failure 302 * encountered while attempting to add elements to 303 * collection {@code c} may result in elements being in neither, 304 * either or both collections when the associated exception is 305 * thrown. Attempts to drain a queue to itself result in 306 * {@code IllegalArgumentException}. Further, the behavior of 307 * this operation is undefined if the specified collection is 308 * modified while the operation is in progress. 309 * 310 * @param c the collection to transfer elements into 311 * @return the number of elements transferred 312 * @throws UnsupportedOperationException if addition of elements 313 * is not supported by the specified collection 314 * @throws ClassCastException if the class of an element of this queue 315 * prevents it from being added to the specified collection 316 * @throws NullPointerException if the specified collection is null 317 * @throws IllegalArgumentException if the specified collection is this 318 * queue, or some property of an element of this queue prevents 319 * it from being added to the specified collection 320 */ drainTo(Collection<? super E> c)321 int drainTo(Collection<? super E> c); 322 323 /** 324 * Removes at most the given number of available elements from 325 * this queue and adds them to the given collection. A failure 326 * encountered while attempting to add elements to 327 * collection {@code c} may result in elements being in neither, 328 * either or both collections when the associated exception is 329 * thrown. Attempts to drain a queue to itself result in 330 * {@code IllegalArgumentException}. Further, the behavior of 331 * this operation is undefined if the specified collection is 332 * modified while the operation is in progress. 333 * 334 * @param c the collection to transfer elements into 335 * @param maxElements the maximum number of elements to transfer 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 */ drainTo(Collection<? super E> c, int maxElements)346 int drainTo(Collection<? super E> c, int maxElements); 347 } 348