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 Queue} that additionally supports operations that wait for
  43  * the queue to become non-empty when retrieving an element, and wait
  44  * for space to become available in the queue when storing an element.
  45  *
  46  * <p>{@code BlockingQueue} methods come in four forms, with different ways
  47  * of handling operations that cannot be satisfied immediately, but may be
  48  * satisfied at some point in the future:
  49  * one throws an exception, the second returns a special value (either
  50  * {@code null} or {@code false}, depending on the operation), the third
  51  * blocks the current thread indefinitely until the operation can succeed,
  52  * and the fourth blocks for only a given maximum time limit before giving
  53  * up.  These methods are summarized in the following table:
  54  *
  55  * <table class="plain">
  56  * <caption>Summary of BlockingQueue methods</caption>
  57  *  <tr>
  58  *    <td></td>
  59  *    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
  60  *    <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
  61  *    <th scope="col" style="font-weight:normal; font-style:italic">Blocks</th>
  62  *    <th scope="col" style="font-weight:normal; font-style:italic">Times out</th>
  63  *  </tr>
  64  *  <tr>
  65  *    <th scope="row" style="text-align:left">Insert</th>
  66  *    <td>{@link #add(Object) add(e)}</td>
  67  *    <td>{@link #offer(Object) offer(e)}</td>
  68  *    <td>{@link #put(Object) put(e)}</td>
  69  *    <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
  70  *  </tr>
  71  *  <tr>
  72  *    <th scope="row" style="text-align:left">Remove</th>
  73  *    <td>{@link #remove() remove()}</td>
  74  *    <td>{@link #poll() poll()}</td>
  75  *    <td>{@link #take() take()}</td>
  76  *    <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
  77  *  </tr>
  78  *  <tr>
  79  *    <th scope="row" style="text-align:left">Examine</th>
  80  *    <td>{@link #element() element()}</td>
  81  *    <td>{@link #peek() peek()}</td>
  82  *    <td style="font-style: italic">not applicable</td>
  83  *    <td style="font-style: italic">not applicable</td>
  84  *  </tr>
  85  * </table>
  86  *
  87  * <p>A {@code BlockingQueue} does not accept {@code null} elements.
  88  * Implementations throw {@code NullPointerException} on attempts
  89  * to {@code add}, {@code put} or {@code offer} a {@code null}.  A
  90  * {@code null} is used as a sentinel value to indicate failure of
  91  * {@code poll} operations.
  92  *
  93  * <p>A {@code BlockingQueue} may be capacity bounded. At any given
  94  * time it may have a {@code remainingCapacity} beyond which no
  95  * additional elements can be {@code put} without blocking.
  96  * A {@code BlockingQueue} without any intrinsic capacity constraints always
  97  * reports a remaining capacity of {@code Integer.MAX_VALUE}.
  98  *
  99  * <p>{@code BlockingQueue} implementations are designed to be used
 100  * primarily for producer-consumer queues, but additionally support
 101  * the {@link Collection} interface.  So, for example, it is
 102  * possible to remove an arbitrary element from a queue using
 103  * {@code remove(x)}. However, such operations are in general
 104  * <em>not</em> performed very efficiently, and are intended for only
 105  * occasional use, such as when a queued message is cancelled.
 106  *
 107  * <p>{@code BlockingQueue} implementations are thread-safe.  All
 108  * queuing methods achieve their effects atomically using internal
 109  * locks or other forms of concurrency control. However, the
 110  * <em>bulk</em> Collection operations {@code addAll},
 111  * {@code containsAll}, {@code retainAll} and {@code removeAll} are
 112  * <em>not</em> necessarily performed atomically unless specified
 113  * otherwise in an implementation. So it is possible, for example, for
 114  * {@code addAll(c)} to fail (throwing an exception) after adding
 115  * only some of the elements in {@code c}.
 116  *
 117  * <p>A {@code BlockingQueue} does <em>not</em> intrinsically support
 118  * any kind of &quot;close&quot; or &quot;shutdown&quot; operation to
 119  * indicate that no more items will be added.  The needs and usage of
 120  * such features tend to be implementation-dependent. For example, a
 121  * common tactic is for producers to insert special
 122  * <em>end-of-stream</em> or <em>poison</em> objects, that are
 123  * interpreted accordingly when taken by consumers.
 124  *
 125  * <p>
 126  * Usage example, based on a typical producer-consumer scenario.
 127  * Note that a {@code BlockingQueue} can safely be used with multiple
 128  * producers and multiple consumers.
 129  * <pre> {@code
 130  * class Producer implements Runnable {
 131  *   private final BlockingQueue queue;
 132  *   Producer(BlockingQueue q) { queue = q; }
 133  *   public void run() {
 134  *     try {
 135  *       while (true) { queue.put(produce()); }
 136  *     } catch (InterruptedException ex) { ... handle ...}
 137  *   }
 138  *   Object produce() { ... }
 139  * }
 140  *
 141  * class Consumer implements Runnable {
 142  *   private final BlockingQueue queue;
 143  *   Consumer(BlockingQueue q) { queue = q; }
 144  *   public void run() {
 145  *     try {
 146  *       while (true) { consume(queue.take()); }
 147  *     } catch (InterruptedException ex) { ... handle ...}
 148  *   }
 149  *   void consume(Object x) { ... }
 150  * }
 151  *
 152  * class Setup {
 153  *   void main() {
 154  *     BlockingQueue q = new SomeQueueImplementation();
 155  *     Producer p = new Producer(q);
 156  *     Consumer c1 = new Consumer(q);
 157  *     Consumer c2 = new Consumer(q);
 158  *     new Thread(p).start();
 159  *     new Thread(c1).start();
 160  *     new Thread(c2).start();
 161  *   }
 162  * }}</pre>
 163  *
 164  * <p>Memory consistency effects: As with other concurrent
 165  * collections, actions in a thread prior to placing an object into a
 166  * {@code BlockingQueue}
 167  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 168  * actions subsequent to the access or removal of that element from
 169  * the {@code BlockingQueue} in another thread.
 170  *
 171  * <p>This interface is a member of the
 172  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
 173  * Java Collections Framework</a>.
 174  *
 175  * @since 1.5
 176  * @author Doug Lea
 177  * @param <E> the type of elements held in this queue
 178  */
 179 public interface BlockingQueue<E> extends Queue<E> {
 180     /**
 181      * Inserts the specified element into this queue if it is possible to do
 182      * so immediately without violating capacity restrictions, returning
 183      * {@code true} upon success and throwing an
 184      * {@code IllegalStateException} if no space is currently available.
 185      * When using a capacity-restricted queue, it is generally preferable to
 186      * use {@link #offer(Object) offer}.
 187      *
 188      * @param e the element to add
 189      * @return {@code true} (as specified by {@link Collection#add})
 190      * @throws IllegalStateException if the element cannot be added at this
 191      *         time due to capacity restrictions
 192      * @throws ClassCastException if the class of the specified element
 193      *         prevents it from being added to this queue
 194      * @throws NullPointerException if the specified element is null
 195      * @throws IllegalArgumentException if some property of the specified
 196      *         element prevents it from being added to this queue
 197      */
 198     boolean add(E e);
 199 
 200     /**
 201      * Inserts the specified element into this queue if it is possible to do
 202      * so immediately without violating capacity restrictions, returning
 203      * {@code true} upon success and {@code false} if no space is currently
 204      * available.  When using a capacity-restricted queue, this method is
 205      * generally preferable to {@link #add}, which can fail to insert an
 206      * element only by throwing an exception.
 207      *
 208      * @param e the element to add
 209      * @return {@code true} if the element was added to this queue, else
 210      *         {@code false}
 211      * @throws ClassCastException if the class of the specified element
 212      *         prevents it from being added to this queue
 213      * @throws NullPointerException if the specified element is null
 214      * @throws IllegalArgumentException if some property of the specified
 215      *         element prevents it from being added to this queue
 216      */
 217     boolean offer(E e);
 218 
 219     /**
 220      * Inserts the specified element into this queue, waiting if necessary
 221      * for space to become available.
 222      *
 223      * @param e the element to add
 224      * @throws InterruptedException if interrupted while waiting
 225      * @throws ClassCastException if the class of the specified element
 226      *         prevents it from being added to this queue
 227      * @throws NullPointerException if the specified element is null
 228      * @throws IllegalArgumentException if some property of the specified
 229      *         element prevents it from being added to this queue
 230      */
 231     void put(E e) throws InterruptedException;
 232 
 233     /**
 234      * Inserts the specified element into this queue, waiting up to the
 235      * specified wait time if necessary for space to become available.
 236      *
 237      * @param e the element to add
 238      * @param timeout how long to wait before giving up, in units of
 239      *        {@code unit}
 240      * @param unit a {@code TimeUnit} determining how to interpret the
 241      *        {@code timeout} parameter
 242      * @return {@code true} if successful, or {@code false} if
 243      *         the specified waiting time elapses before space is available
 244      * @throws InterruptedException if interrupted while waiting
 245      * @throws ClassCastException if the class of the specified element
 246      *         prevents it from being added to this queue
 247      * @throws NullPointerException if the specified element is null
 248      * @throws IllegalArgumentException if some property of the specified
 249      *         element prevents it from being added to this queue
 250      */
 251     boolean offer(E e, long timeout, TimeUnit unit)
 252         throws InterruptedException;
 253 
 254     /**
 255      * Retrieves and removes the head of this queue, waiting if necessary
 256      * until an element becomes available.
 257      *
 258      * @return the head of this queue
 259      * @throws InterruptedException if interrupted while waiting
 260      */
 261     E take() throws InterruptedException;
 262 
 263     /**
 264      * Retrieves and removes the head of this queue, waiting up to the
 265      * specified wait time if necessary for an element to become available.
 266      *
 267      * @param timeout how long to wait before giving up, in units of
 268      *        {@code unit}
 269      * @param unit a {@code TimeUnit} determining how to interpret the
 270      *        {@code timeout} parameter
 271      * @return the head of this queue, or {@code null} if the
 272      *         specified waiting time elapses before an element is available
 273      * @throws InterruptedException if interrupted while waiting
 274      */
 275     E poll(long timeout, TimeUnit unit)
 276         throws InterruptedException;
 277 
 278     /**
 279      * Returns the number of additional elements that this queue can ideally
 280      * (in the absence of memory or resource constraints) accept without
 281      * blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic
 282      * limit.
 283      *
 284      * <p>Note that you <em>cannot</em> always tell if an attempt to insert
 285      * an element will succeed by inspecting {@code remainingCapacity}
 286      * because it may be the case that another thread is about to
 287      * insert or remove an element.
 288      *
 289      * @return the remaining capacity
 290      */
 291     int remainingCapacity();
 292 
 293     /**
 294      * Removes a single instance of the specified element from this queue,
 295      * if it is present.  More formally, removes an element {@code e} such
 296      * that {@code o.equals(e)}, if this queue contains one or more such
 297      * elements.
 298      * Returns {@code true} if this queue contained the specified element
 299      * (or equivalently, if this queue changed as a result of the call).
 300      *
 301      * @param o element to be removed from this queue, if present
 302      * @return {@code true} if this queue changed as a result of the call
 303      * @throws ClassCastException if the class of the specified element
 304      *         is incompatible with this queue
 305      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
 306      * @throws NullPointerException if the specified element is null
 307      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
 308      */
 309     boolean remove(Object o);
 310 
 311     /**
 312      * Returns {@code true} if this queue contains the specified element.
 313      * More formally, returns {@code true} if and only if this queue contains
 314      * at least one element {@code e} such that {@code o.equals(e)}.
 315      *
 316      * @param o object to be checked for containment in this queue
 317      * @return {@code true} if this queue contains the specified element
 318      * @throws ClassCastException if the class of the specified element
 319      *         is incompatible with this queue
 320      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
 321      * @throws NullPointerException if the specified element is null
 322      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
 323      */
 324     boolean contains(Object o);
 325 
 326     /**
 327      * Removes all available elements from this queue and adds them
 328      * to the given collection.  This operation may be more
 329      * efficient than repeatedly polling this queue.  A failure
 330      * encountered while attempting to add elements to
 331      * collection {@code c} may result in elements being in neither,
 332      * either or both collections when the associated exception is
 333      * thrown.  Attempts to drain a queue to itself result in
 334      * {@code IllegalArgumentException}. Further, the behavior of
 335      * this operation is undefined if the specified collection is
 336      * modified while the operation is in progress.
 337      *
 338      * @param c the collection to transfer elements into
 339      * @return the number of elements transferred
 340      * @throws UnsupportedOperationException if addition of elements
 341      *         is not supported by the specified collection
 342      * @throws ClassCastException if the class of an element of this queue
 343      *         prevents it from being added to the specified collection
 344      * @throws NullPointerException if the specified collection is null
 345      * @throws IllegalArgumentException if the specified collection is this
 346      *         queue, or some property of an element of this queue prevents
 347      *         it from being added to the specified collection
 348      */
 349     int drainTo(Collection<? super E> c);
 350 
 351     /**
 352      * Removes at most the given number of available elements from
 353      * this queue and adds them to the given collection.  A failure
 354      * encountered while attempting to add elements to
 355      * collection {@code c} may result in elements being in neither,
 356      * either or both collections when the associated exception is
 357      * thrown.  Attempts to drain a queue to itself result in
 358      * {@code IllegalArgumentException}. Further, the behavior of
 359      * this operation is undefined if the specified collection is
 360      * modified while the operation is in progress.
 361      *
 362      * @param c the collection to transfer elements into
 363      * @param maxElements the maximum number of elements to transfer
 364      * @return the number of elements transferred
 365      * @throws UnsupportedOperationException if addition of elements
 366      *         is not supported by the specified collection
 367      * @throws ClassCastException if the class of an element of this queue
 368      *         prevents it from being added to the specified collection
 369      * @throws NullPointerException if the specified collection is null
 370      * @throws IllegalArgumentException if the specified collection is this
 371      *         queue, or some property of an element of this queue prevents
 372      *         it from being added to the specified collection
 373      */
 374     int drainTo(Collection<? super E> c, int maxElements);
 375 }