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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 selector provider} 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"></a>
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"></a><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 iterator} 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"></a>
94   * <h2>Selection</h2>
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>
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>
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  * <h2>Concurrency</h2>
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"></a>
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.
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.
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 }