1 /*
   2  * Copyright (c) 2000, 2004, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.nio.channels;
  27 
  28 import java.io.Closeable;
  29 import java.io.IOException;
  30 import java.nio.channels.spi.SelectorProvider;
  31 import java.util.Set;
  32 
  33 
  34 /**
  35  * A multiplexor of {@link SelectableChannel} objects.
  36  *
  37  * <p> A selector may be created by invoking the {@link #open open} method of
  38  * this class, which will use the system's default {@link
  39  * java.nio.channels.spi.SelectorProvider </code>selector provider<code>} to
  40  * create a new selector.  A selector may also be created by invoking the
  41  * {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}
  42  * method of a custom selector provider.  A selector remains open until it is
  43  * closed via its {@link #close close} method.
  44  *
  45  * <a name="ks">
  46  *
  47  * <p> A selectable channel's registration with a selector is represented by a
  48  * {@link SelectionKey} object.  A selector maintains three sets of selection
  49  * keys:
  50  *
  51  * <ul>
  52  *
  53  *   <li><p> The <i>key set</i> contains the keys representing the current
  54  *   channel registrations of this selector.  This set is returned by the
  55  *   {@link #keys() keys} method. </p></li>
  56  *
  57  *   <li><p> The <i>selected-key set</i> is the set of keys such that each
  58  *   key's channel was detected to be ready for at least one of the operations
  59  *   identified in the key's interest set during a prior selection operation.
  60  *   This set is returned by the {@link #selectedKeys() selectedKeys} method.
  61  *   The selected-key set is always a subset of the key set. </p></li>
  62  *
  63  *   <li><p> The <i>cancelled-key</i> set is the set of keys that have been
  64  *   cancelled but whose channels have not yet been deregistered.  This set is
  65  *   not directly accessible.  The cancelled-key set is always a subset of the
  66  *   key set. </p></li>
  67  *
  68  * </ul>
  69  *
  70  * <p> All three sets are empty in a newly-created selector.
  71  *
  72  * <p> A key is added to a selector's key set as a side effect of registering a
  73  * channel via the channel's {@link SelectableChannel#register(Selector,int)
  74  * register} method.  Cancelled keys are removed from the key set during
  75  * selection operations.  The key set itself is not directly modifiable.
  76  *
  77  * <p> A key is added to its selector's cancelled-key set when it is cancelled,
  78  * whether by closing its channel or by invoking its {@link SelectionKey#cancel
  79  * cancel} method.  Cancelling a key will cause its channel to be deregistered
  80  * during the next selection operation, at which time the key will removed from
  81  * all of the selector's key sets.
  82  *
  83  * <a name="sks"><p> Keys are added to the selected-key set by selection
  84  * operations.  A key may be removed directly from the selected-key set by
  85  * invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}
  86  * method or by invoking the {@link java.util.Iterator#remove() remove} method
  87  * of an {@link java.util.Iterator </code>iterator<code>} obtained from the
  88  * set.  Keys are never removed from the selected-key set in any other way;
  89  * they are not, in particular, removed as a side effect of selection
  90  * operations.  Keys may not be added directly to the selected-key set. </p>
  91  *
  92  *
  93  * <a name="selop">
  94  * <h4>Selection</h4>
  95  *
  96  * <p> During each selection operation, keys may be added to and removed from a
  97  * selector's selected-key set and may be removed from its key and
  98  * cancelled-key sets.  Selection is performed by the {@link #select()}, {@link
  99  * #select(long)}, and {@link #selectNow()} methods, and involves three steps:
 100  * </p>
 101  *
 102  * <ol>
 103  *
 104  *   <li><p> Each key in the cancelled-key set is removed from each key set of
 105  *   which it is a member, and its channel is deregistered.  This step leaves
 106  *   the cancelled-key set empty. </p></li>
 107  *
 108  *   <li><p> The underlying operating system is queried for an update as to the
 109  *   readiness of each remaining channel to perform any of the operations
 110  *   identified by its key's interest set as of the moment that the selection
 111  *   operation began.  For a channel that is ready for at least one such
 112  *   operation, one of the following two actions is performed: </p>
 113  *
 114  *   <ol type=a>
 115  *
 116  *     <li><p> If the channel's key is not already in the selected-key set then
 117  *     it is added to that set and its ready-operation set is modified to
 118  *     identify exactly those operations for which the channel is now reported
 119  *     to be ready.  Any readiness information previously recorded in the ready
 120  *     set is discarded.  </p></li>
 121  *
 122  *     <li><p> Otherwise the channel's key is already in the selected-key set,
 123  *     so its ready-operation set is modified to identify any new operations
 124  *     for which the channel is reported to be ready.  Any readiness
 125  *     information previously recorded in the ready set is preserved; in other
 126  *     words, the ready set returned by the underlying system is
 127  *     bitwise-disjoined into the key's current ready set. </p></li>
 128  *
 129  *   </ol></li>
 130  *
 131  *   If all of the keys in the key set at the start of this step have empty
 132  *   interest sets then neither the selected-key set nor any of the keys'
 133  *   ready-operation sets will be updated.
 134  *
 135  *   <li><p> If any keys were added to the cancelled-key set while step (2) was
 136  *   in progress then they are processed as in step (1). </p></li>
 137  *
 138  * </ol>
 139  *
 140  * <p> Whether or not a selection operation blocks to wait for one or more
 141  * channels to become ready, and if so for how long, is the only essential
 142  * difference between the three selection methods. </p>
 143  *
 144  *
 145  * <h4>Concurrency</h4>
 146  *
 147  * <p> Selectors are themselves safe for use by multiple concurrent threads;
 148  * their key sets, however, are not.
 149  *
 150  * <p> The selection operations synchronize on the selector itself, on the key
 151  * set, and on the selected-key set, in that order.  They also synchronize on
 152  * the cancelled-key set during steps (1) and (3) above.
 153  *
 154  * <p> Changes made to the interest sets of a selector's keys while a
 155  * selection operation is in progress have no effect upon that operation; they
 156  * will be seen by the next selection operation.
 157  *
 158  * <p> Keys may be cancelled and channels may be closed at any time.  Hence the
 159  * presence of a key in one or more of a selector's key sets does not imply
 160  * that the key is valid or that its channel is open.  Application code should
 161  * be careful to synchronize and check these conditions as necessary if there
 162  * is any possibility that another thread will cancel a key or close a channel.
 163  *
 164  * <p> A thread blocked in one of the {@link #select()} or {@link
 165  * #select(long)} methods may be interrupted by some other thread in one of
 166  * three ways:
 167  *
 168  * <ul>
 169  *
 170  *   <li><p> By invoking the selector's {@link #wakeup wakeup} method,
 171  *   </p></li>
 172  *
 173  *   <li><p> By invoking the selector's {@link #close close} method, or
 174  *   </p></li>
 175  *
 176  *   <li><p> By invoking the blocked thread's {@link
 177  *   java.lang.Thread#interrupt() interrupt} method, in which case its
 178  *   interrupt status will be set and the selector's {@link #wakeup wakeup}
 179  *   method will be invoked. </p></li>
 180  *
 181  * </ul>
 182  *
 183  * <p> The {@link #close close} method synchronizes on the selector and all
 184  * three key sets in the same order as in a selection operation.
 185  *
 186  * <a name="ksc">
 187  *
 188  * <p> A selector's key and selected-key sets are not, in general, safe for use
 189  * by multiple concurrent threads.  If such a thread might modify one of these
 190  * sets directly then access should be controlled by synchronizing on the set
 191  * itself.  The iterators returned by these sets' {@link
 192  * java.util.Set#iterator() iterator} methods are <i>fail-fast:</i> If the set
 193  * is modified after the iterator is created, in any way except by invoking the
 194  * iterator's own {@link java.util.Iterator#remove() remove} method, then a
 195  * {@link java.util.ConcurrentModificationException} will be thrown. </p>
 196  *
 197  *
 198  * @author Mark Reinhold
 199  * @author JSR-51 Expert Group
 200  * @since 1.4
 201  *
 202  * @see SelectableChannel
 203  * @see SelectionKey
 204  */
 205 
 206 public abstract class Selector implements Closeable {
 207 
 208     /**
 209      * Initializes a new instance of this class.
 210      */
 211     protected Selector() { }
 212 
 213     /**
 214      * Opens a selector.
 215      *
 216      * <p> The new selector is created by invoking the {@link
 217      * java.nio.channels.spi.SelectorProvider#openSelector openSelector} method
 218      * of the system-wide default {@link
 219      * java.nio.channels.spi.SelectorProvider} object.  </p>
 220      *
 221      * @return  A new selector
 222      *
 223      * @throws  IOException
 224      *          If an I/O error occurs
 225      */
 226     public static Selector open() throws IOException {
 227         return SelectorProvider.provider().openSelector();
 228     }
 229 
 230     /**
 231      * Tells whether or not this selector is open.  </p>
 232      *
 233      * @return <tt>true</tt> if, and only if, this selector is open
 234      */
 235     public abstract boolean isOpen();
 236 
 237     /**
 238      * Returns the provider that created this channel.  </p>
 239      *
 240      * @return  The provider that created this channel
 241      */
 242     public abstract SelectorProvider provider();
 243 
 244     /**
 245      * Returns this selector's key set.
 246      *
 247      * <p> The key set is not directly modifiable.  A key is removed only after
 248      * it has been cancelled and its channel has been deregistered.  Any
 249      * attempt to modify the key set will cause an {@link
 250      * UnsupportedOperationException} to be thrown.
 251      *
 252      * <p> The key set is <a href="#ksc">not thread-safe</a>. </p>
 253      *
 254      * @return  This selector's key set
 255      *
 256      * @throws  ClosedSelectorException
 257      *          If this selector is closed
 258      */
 259     public abstract Set<SelectionKey> keys();
 260 
 261     /**
 262      * Returns this selector's selected-key set.
 263      *
 264      * <p> Keys may be removed from, but not directly added to, the
 265      * selected-key set.  Any attempt to add an object to the key set will
 266      * cause an {@link UnsupportedOperationException} to be thrown.
 267      *
 268      * <p> The selected-key set is <a href="#ksc">not thread-safe</a>. </p>
 269      *
 270      * @return  This selector's selected-key set
 271      *
 272      * @throws  ClosedSelectorException
 273      *          If this selector is closed
 274      */
 275     public abstract Set<SelectionKey> selectedKeys();
 276 
 277     /**
 278      * Selects a set of keys whose corresponding channels are ready for I/O
 279      * operations.
 280      *
 281      * <p> This method performs a non-blocking <a href="#selop">selection
 282      * operation</a>.  If no channels have become selectable since the previous
 283      * selection operation then this method immediately returns zero.
 284      *
 285      * <p> Invoking this method clears the effect of any previous invocations
 286      * of the {@link #wakeup wakeup} method.  </p>
 287      *
 288      * @return  The number of keys, possibly zero, whose ready-operation sets
 289      *          were updated by the selection operation
 290      *
 291      * @throws  IOException
 292      *          If an I/O error occurs
 293      *
 294      * @throws  ClosedSelectorException
 295      *          If this selector is closed
 296      */
 297     public abstract int selectNow() throws IOException;
 298 
 299     /**
 300      * Selects a set of keys whose corresponding channels are ready for I/O
 301      * operations.
 302      *
 303      * <p> This method performs a blocking <a href="#selop">selection
 304      * operation</a>.  It returns only after at least one channel is selected,
 305      * this selector's {@link #wakeup wakeup} method is invoked, the current
 306      * thread is interrupted, or the given timeout period expires, whichever
 307      * comes first.
 308      *
 309      * <p> This method does not offer real-time guarantees: It schedules the
 310      * timeout as if by invoking the {@link Object#wait(long)} method. </p>
 311      *
 312      * @param  timeout  If positive, block for up to <tt>timeout</tt>
 313      *                  milliseconds, more or less, while waiting for a
 314      *                  channel to become ready; if zero, block indefinitely;
 315      *                  must not be negative
 316      *
 317      * @return  The number of keys, possibly zero,
 318      *          whose ready-operation sets were updated
 319      *
 320      * @throws  IOException
 321      *          If an I/O error occurs
 322      *
 323      * @throws  ClosedSelectorException
 324      *          If this selector is closed
 325      *
 326      * @throws  IllegalArgumentException
 327      *          If the value of the timeout argument is negative
 328      */
 329     public abstract int select(long timeout)
 330         throws IOException;
 331 
 332     /**
 333      * Selects a set of keys whose corresponding channels are ready for I/O
 334      * operations.
 335      *
 336      * <p> This method performs a blocking <a href="#selop">selection
 337      * operation</a>.  It returns only after at least one channel is selected,
 338      * this selector's {@link #wakeup wakeup} method is invoked, or the current
 339      * thread is interrupted, whichever comes first.  </p>
 340      *
 341      * @return  The number of keys, possibly zero,
 342      *          whose ready-operation sets were updated
 343      *
 344      * @throws  IOException
 345      *          If an I/O error occurs
 346      *
 347      * @throws  ClosedSelectorException
 348      *          If this selector is closed
 349      */
 350     public abstract int select() throws IOException;
 351 
 352     /**
 353      * Causes the first selection operation that has not yet returned to return
 354      * immediately.
 355      *
 356      * <p> If another thread is currently blocked in an invocation of the
 357      * {@link #select()} or {@link #select(long)} methods then that invocation
 358      * will return immediately.  If no selection operation is currently in
 359      * progress then the next invocation of one of these methods will return
 360      * immediately unless the {@link #selectNow()} method is invoked in the
 361      * meantime.  In any case the value returned by that invocation may be
 362      * non-zero.  Subsequent invocations of the {@link #select()} or {@link
 363      * #select(long)} methods will block as usual unless this method is invoked
 364      * again in the meantime.
 365      *
 366      * <p> Invoking this method more than once between two successive selection
 367      * operations has the same effect as invoking it just once.  </p>
 368      *
 369      * @return  This selector
 370      */
 371     public abstract Selector wakeup();
 372 
 373     /**
 374      * Closes this selector.
 375      *
 376      * <p> If a thread is currently blocked in one of this selector's selection
 377      * methods then it is interrupted as if by invoking the selector's {@link
 378      * #wakeup wakeup} method.
 379      *
 380      * <p> Any uncancelled keys still associated with this selector are
 381      * invalidated, their channels are deregistered, and any other resources
 382      * associated with this selector are released.
 383      *
 384      * <p> If this selector is already closed then invoking this method has no
 385      * effect.
 386      *
 387      * <p> After a selector is closed, any further attempt to use it, except by
 388      * invoking this method or the {@link #wakeup wakeup} method, will cause a
 389      * {@link ClosedSelectorException} to be thrown. </p>
 390      *
 391      * @throws  IOException
 392      *          If an I/O error occurs
 393      */
 394     public abstract void close() throws IOException;
 395 
 396 }