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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/licenses/publicdomain 5: */ 6: 7: package java.util.concurrent; 8: 9: import java.util.Collection; 10: import java.util.Queue; 11: 12: /** 13: * A {@link java.util.Queue} that additionally supports operations 14: * that wait for the queue to become non-empty when retrieving an 15: * element, and wait for space to become available in the queue when 16: * storing an element. 17: * 18: * <p><tt>BlockingQueue</tt> methods come in four forms, with different ways 19: * of handling operations that cannot be satisfied immediately, but may be 20: * satisfied at some point in the future: 21: * one throws an exception, the second returns a special value (either 22: * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third 23: * blocks the current thread indefinitely until the operation can succeed, 24: * and the fourth blocks for only a given maximum time limit before giving 25: * up. These methods are summarized in the following table: 26: * 27: * <p> 28: * <table BORDER CELLPADDING=3 CELLSPACING=1> 29: * <tr> 30: * <td></td> 31: * <td ALIGN=CENTER><em>Throws exception</em></td> 32: * <td ALIGN=CENTER><em>Special value</em></td> 33: * <td ALIGN=CENTER><em>Blocks</em></td> 34: * <td ALIGN=CENTER><em>Times out</em></td> 35: * </tr> 36: * <tr> 37: * <td><b>Insert</b></td> 38: * <td>{@link #add add(e)}</td> 39: * <td>{@link #offer offer(e)}</td> 40: * <td>{@link #put put(e)}</td> 41: * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> 42: * </tr> 43: * <tr> 44: * <td><b>Remove</b></td> 45: * <td>{@link #remove remove()}</td> 46: * <td>{@link #poll poll()}</td> 47: * <td>{@link #take take()}</td> 48: * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> 49: * </tr> 50: * <tr> 51: * <td><b>Examine</b></td> 52: * <td>{@link #element element()}</td> 53: * <td>{@link #peek peek()}</td> 54: * <td><em>not applicable</em></td> 55: * <td><em>not applicable</em></td> 56: * </tr> 57: * </table> 58: * 59: * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements. 60: * Implementations throw <tt>NullPointerException</tt> on attempts 61: * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A 62: * <tt>null</tt> is used as a sentinel value to indicate failure of 63: * <tt>poll</tt> operations. 64: * 65: * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given 66: * time it may have a <tt>remainingCapacity</tt> beyond which no 67: * additional elements can be <tt>put</tt> without blocking. 68: * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always 69: * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>. 70: * 71: * <p> <tt>BlockingQueue</tt> implementations are designed to be used 72: * primarily for producer-consumer queues, but additionally support 73: * the {@link java.util.Collection} interface. So, for example, it is 74: * possible to remove an arbitrary element from a queue using 75: * <tt>remove(x)</tt>. However, such operations are in general 76: * <em>not</em> performed very efficiently, and are intended for only 77: * occasional use, such as when a queued message is cancelled. 78: * 79: * <p> <tt>BlockingQueue</tt> implementations are thread-safe. All 80: * queuing methods achieve their effects atomically using internal 81: * locks or other forms of concurrency control. However, the 82: * <em>bulk</em> Collection operations <tt>addAll</tt>, 83: * <tt>containsAll</tt>, <tt>retainAll</tt> and <tt>removeAll</tt> are 84: * <em>not</em> necessarily performed atomically unless specified 85: * otherwise in an implementation. So it is possible, for example, for 86: * <tt>addAll(c)</tt> to fail (throwing an exception) after adding 87: * only some of the elements in <tt>c</tt>. 88: * 89: * <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support 90: * any kind of "close" or "shutdown" operation to 91: * indicate that no more items will be added. The needs and usage of 92: * such features tend to be implementation-dependent. For example, a 93: * common tactic is for producers to insert special 94: * <em>end-of-stream</em> or <em>poison</em> objects, that are 95: * interpreted accordingly when taken by consumers. 96: * 97: * <p> 98: * Usage example, based on a typical producer-consumer scenario. 99: * Note that a <tt>BlockingQueue</tt> can safely be used with multiple 100: * producers and multiple consumers. 101: * <pre> 102: * class Producer implements Runnable { 103: * private final BlockingQueue queue; 104: * Producer(BlockingQueue q) { queue = q; } 105: * public void run() { 106: * try { 107: * while (true) { queue.put(produce()); } 108: * } catch (InterruptedException ex) { ... handle ...} 109: * } 110: * Object produce() { ... } 111: * } 112: * 113: * class Consumer implements Runnable { 114: * private final BlockingQueue queue; 115: * Consumer(BlockingQueue q) { queue = q; } 116: * public void run() { 117: * try { 118: * while (true) { consume(queue.take()); } 119: * } catch (InterruptedException ex) { ... handle ...} 120: * } 121: * void consume(Object x) { ... } 122: * } 123: * 124: * class Setup { 125: * void main() { 126: * BlockingQueue q = new SomeQueueImplementation(); 127: * Producer p = new Producer(q); 128: * Consumer c1 = new Consumer(q); 129: * Consumer c2 = new Consumer(q); 130: * new Thread(p).start(); 131: * new Thread(c1).start(); 132: * new Thread(c2).start(); 133: * } 134: * } 135: * </pre> 136: * 137: * <p>Memory consistency effects: As with other concurrent 138: * collections, actions in a thread prior to placing an object into a 139: * {@code BlockingQueue} 140: * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 141: * actions subsequent to the access or removal of that element from 142: * the {@code BlockingQueue} in another thread. 143: * 144: * <p>This interface is a member of the 145: * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 146: * Java Collections Framework</a>. 147: * 148: * @since 1.5 149: * @author Doug Lea 150: * @param <E> the type of elements held in this collection 151: */ 152: public interface BlockingQueue<E> extends Queue<E> { 153: /** 154: * Inserts the specified element into this queue if it is possible to do 155: * so immediately without violating capacity restrictions, returning 156: * <tt>true</tt> upon success and throwing an 157: * <tt>IllegalStateException</tt> if no space is currently available. 158: * When using a capacity-restricted queue, it is generally preferable to 159: * use {@link #offer(Object) offer}. 160: * 161: * @param e the element to add 162: * @return <tt>true</tt> (as specified by {@link Collection#add}) 163: * @throws IllegalStateException if the element cannot be added at this 164: * time due to capacity restrictions 165: * @throws ClassCastException if the class of the specified element 166: * prevents it from being added to this queue 167: * @throws NullPointerException if the specified element is null 168: * @throws IllegalArgumentException if some property of the specified 169: * element prevents it from being added to this queue 170: */ 171: boolean add(E e); 172: 173: /** 174: * Inserts the specified element into this queue if it is possible to do 175: * so immediately without violating capacity restrictions, returning 176: * <tt>true</tt> upon success and <tt>false</tt> if no space is currently 177: * available. When using a capacity-restricted queue, this method is 178: * generally preferable to {@link #add}, which can fail to insert an 179: * element only by throwing an exception. 180: * 181: * @param e the element to add 182: * @return <tt>true</tt> if the element was added to this queue, else 183: * <tt>false</tt> 184: * @throws ClassCastException if the class of the specified element 185: * prevents it from being added to this queue 186: * @throws NullPointerException if the specified element is null 187: * @throws IllegalArgumentException if some property of the specified 188: * element prevents it from being added to this queue 189: */ 190: boolean offer(E e); 191: 192: /** 193: * Inserts the specified element into this queue, waiting if necessary 194: * for space to become available. 195: * 196: * @param e the element to add 197: * @throws InterruptedException if interrupted while waiting 198: * @throws ClassCastException if the class of the specified element 199: * prevents it from being added to this queue 200: * @throws NullPointerException if the specified element is null 201: * @throws IllegalArgumentException if some property of the specified 202: * element prevents it from being added to this queue 203: */ 204: void put(E e) throws InterruptedException; 205: 206: /** 207: * Inserts the specified element into this queue, waiting up to the 208: * specified wait time if necessary for space to become available. 209: * 210: * @param e the element to add 211: * @param timeout how long to wait before giving up, in units of 212: * <tt>unit</tt> 213: * @param unit a <tt>TimeUnit</tt> determining how to interpret the 214: * <tt>timeout</tt> parameter 215: * @return <tt>true</tt> if successful, or <tt>false</tt> if 216: * the specified waiting time elapses before space is available 217: * @throws InterruptedException if interrupted while waiting 218: * @throws ClassCastException if the class of the specified element 219: * prevents it from being added to this queue 220: * @throws NullPointerException if the specified element is null 221: * @throws IllegalArgumentException if some property of the specified 222: * element prevents it from being added to this queue 223: */ 224: boolean offer(E e, long timeout, TimeUnit unit) 225: throws InterruptedException; 226: 227: /** 228: * Retrieves and removes the head of this queue, waiting if necessary 229: * until an element becomes available. 230: * 231: * @return the head of this queue 232: * @throws InterruptedException if interrupted while waiting 233: */ 234: E take() throws InterruptedException; 235: 236: /** 237: * Retrieves and removes the head of this queue, waiting up to the 238: * specified wait time if necessary for an element to become available. 239: * 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 the head of this queue, or <tt>null</tt> if the 245: * specified waiting time elapses before an element is available 246: * @throws InterruptedException if interrupted while waiting 247: */ 248: E poll(long timeout, TimeUnit unit) 249: throws InterruptedException; 250: 251: /** 252: * Returns the number of additional elements that this queue can ideally 253: * (in the absence of memory or resource constraints) accept without 254: * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no intrinsic 255: * limit. 256: * 257: * <p>Note that you <em>cannot</em> always tell if an attempt to insert 258: * an element will succeed by inspecting <tt>remainingCapacity</tt> 259: * because it may be the case that another thread is about to 260: * insert or remove an element. 261: * 262: * @return the remaining capacity 263: */ 264: int remainingCapacity(); 265: 266: /** 267: * Removes a single instance of the specified element from this queue, 268: * if it is present. More formally, removes an element <tt>e</tt> such 269: * that <tt>o.equals(e)</tt>, if this queue contains one or more such 270: * elements. 271: * Returns <tt>true</tt> if this queue contained the specified element 272: * (or equivalently, if this queue changed as a result of the call). 273: * 274: * @param o element to be removed from this queue, if present 275: * @return <tt>true</tt> if this queue changed as a result of the call 276: * @throws ClassCastException if the class of the specified element 277: * is incompatible with this queue (optional) 278: * @throws NullPointerException if the specified element is null (optional) 279: */ 280: boolean remove(Object o); 281: 282: /** 283: * Returns <tt>true</tt> if this queue contains the specified element. 284: * More formally, returns <tt>true</tt> if and only if this queue contains 285: * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. 286: * 287: * @param o object to be checked for containment in this queue 288: * @return <tt>true</tt> if this queue contains the specified element 289: * @throws ClassCastException if the class of the specified element 290: * is incompatible with this queue (optional) 291: * @throws NullPointerException if the specified element is null (optional) 292: */ 293: public boolean contains(Object o); 294: 295: /** 296: * Removes all available elements from this queue and adds them 297: * to the given collection. This operation may be more 298: * efficient than repeatedly polling this queue. A failure 299: * encountered while attempting to add elements to 300: * collection <tt>c</tt> may result in elements being in neither, 301: * either or both collections when the associated exception is 302: * thrown. Attempts to drain a queue to itself result in 303: * <tt>IllegalArgumentException</tt>. Further, the behavior of 304: * this operation is undefined if the specified collection is 305: * modified while the operation is in progress. 306: * 307: * @param c the collection to transfer elements into 308: * @return the number of elements transferred 309: * @throws UnsupportedOperationException if addition of elements 310: * is not supported by the specified collection 311: * @throws ClassCastException if the class of an element of this queue 312: * prevents it from being added to the specified collection 313: * @throws NullPointerException if the specified collection is null 314: * @throws IllegalArgumentException if the specified collection is this 315: * queue, or some property of an element of this queue prevents 316: * it from being added to the specified collection 317: */ 318: int drainTo(Collection<? super E> c); 319: 320: /** 321: * Removes at most the given number of available elements from 322: * this queue and adds them to the given collection. A failure 323: * encountered while attempting to add elements to 324: * collection <tt>c</tt> may result in elements being in neither, 325: * either or both collections when the associated exception is 326: * thrown. Attempts to drain a queue to itself result in 327: * <tt>IllegalArgumentException</tt>. Further, the behavior of 328: * this operation is undefined if the specified collection is 329: * modified while the operation is in progress. 330: * 331: * @param c the collection to transfer elements into 332: * @param maxElements the maximum number of elements to transfer 333: * @return the number of elements transferred 334: * @throws UnsupportedOperationException if addition of elements 335: * is not supported by the specified collection 336: * @throws ClassCastException if the class of an element of this queue 337: * prevents it from being added to the specified collection 338: * @throws NullPointerException if the specified collection is null 339: * @throws IllegalArgumentException if the specified collection is this 340: * queue, or some property of an element of this queue prevents 341: * it from being added to the specified collection 342: */ 343: int drainTo(Collection<? super E> c, int maxElements); 344: }
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