1 /*
2 * Copyright 2000-2004 Sun Microsystems, Inc. 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. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any questions.
24 */
25
26 package java.nio.channels;
27
28 import java.io.IOException;
29 import java.nio.channels.spi.SelectorProvider;
30 import java.util.Set;
31
32
33 /**
34 * A multiplexor of {@link SelectableChannel} objects.
35 *
36 * <p> A selector may be created by invoking the {@link #open open} method of
37 * this class, which will use the system's default {@link
38 * java.nio.channels.spi.SelectorProvider </code>selector provider<code>} to
39 * create a new selector. A selector may also be created by invoking the
40 * {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}
41 * method of a custom selector provider. A selector remains open until it is
42 * closed via its {@link #close close} method.
43 *
44 * <a name="ks">
45 *
46 * <p> A selectable channel's registration with a selector is represented by a
47 * {@link SelectionKey} object. A selector maintains three sets of selection
48 * keys:
49 *
50 * <ul>
51 *
52 * <li><p> The <i>key set</i> contains the keys representing the current
53 * channel registrations of this selector. This set is returned by the
54 * {@link #keys() keys} method. </p></li>
55 *
56 * <li><p> The <i>selected-key set</i> is the set of keys such that each
57 * key's channel was detected to be ready for at least one of the operations
58 * identified in the key's interest set during a prior selection operation.
59 * This set is returned by the {@link #selectedKeys() selectedKeys} method.
60 * The selected-key set is always a subset of the key set. </p></li>
61 *
62 * <li><p> The <i>cancelled-key</i> set is the set of keys that have been
63 * cancelled but whose channels have not yet been deregistered. This set is
64 * not directly accessible. The cancelled-key set is always a subset of the
65 * key set. </p></li>
66 *
67 * </ul>
68 *
69 * <p> All three sets are empty in a newly-created selector.
70 *
71 * <p> A key is added to a selector's key set as a side effect of registering a
72 * channel via the channel's {@link SelectableChannel#register(Selector,int)
73 * register} method. Cancelled keys are removed from the key set during
74 * selection operations. The key set itself is not directly modifiable.
75 *
76 * <p> A key is added to its selector's cancelled-key set when it is cancelled,
77 * whether by closing its channel or by invoking its {@link SelectionKey#cancel
78 * cancel} method. Cancelling a key will cause its channel to be deregistered
79 * during the next selection operation, at which time the key will removed from
80 * all of the selector's key sets.
81 *
82 * <a name="sks"><p> Keys are added to the selected-key set by selection
83 * operations. A key may be removed directly from the selected-key set by
84 * invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}
85 * method or by invoking the {@link java.util.Iterator#remove() remove} method
86 * of an {@link java.util.Iterator </code>iterator<code>} obtained from the
87 * set. Keys are never removed from the selected-key set in any other way;
88 * they are not, in particular, removed as a side effect of selection
89 * operations. Keys may not be added directly to the selected-key set. </p>
90 *
91 *
92 * <a name="selop">
93 * <h4>Selection</h4>
94 *
95 * <p> During each selection operation, keys may be added to and removed from a
96 * selector's selected-key set and may be removed from its key and
97 * cancelled-key sets. Selection is performed by the {@link #select()}, {@link
98 * #select(long)}, and {@link #selectNow()} methods, and involves three steps:
99 * </p>
100 *
101 * <ol>
102 *
103 * <li><p> Each key in the cancelled-key set is removed from each key set of
104 * which it is a member, and its channel is deregistered. This step leaves
105 * the cancelled-key set empty. </p></li>
106 *
107 * <li><p> The underlying operating system is queried for an update as to the
108 * readiness of each remaining channel to perform any of the operations
109 * identified by its key's interest set as of the moment that the selection
110 * operation began. For a channel that is ready for at least one such
111 * operation, one of the following two actions is performed: </p>
112 *
113 * <ol type=a>
114 *
115 * <li><p> If the channel's key is not already in the selected-key set then
116 * it is added to that set and its ready-operation set is modified to
117 * identify exactly those operations for which the channel is now reported
118 * to be ready. Any readiness information previously recorded in the ready
119 * set is discarded. </p></li>
120 *
121 * <li><p> Otherwise the channel's key is already in the selected-key set,
122 * so its ready-operation set is modified to identify any new operations
123 * for which the channel is reported to be ready. Any readiness
124 * information previously recorded in the ready set is preserved; in other
125 * words, the ready set returned by the underlying system is
126 * bitwise-disjoined into the key's current ready set. </p></li>
127 *
128 * </ol></li>
129 *
130 * If all of the keys in the key set at the start of this step have empty
131 * interest sets then neither the selected-key set nor any of the keys'
132 * ready-operation sets will be updated.
133 *
134 * <li><p> If any keys were added to the cancelled-key set while step (2) was
135 * in progress then they are processed as in step (1). </p></li>
136 *
137 * </ol>
138 *
139 * <p> Whether or not a selection operation blocks to wait for one or more
140 * channels to become ready, and if so for how long, is the only essential
141 * difference between the three selection methods. </p>
142 *
143 *
144 * <h4>Concurrency</h4>
145 *
146 * <p> Selectors are themselves safe for use by multiple concurrent threads;
147 * their key sets, however, are not.
148 *
149 * <p> The selection operations synchronize on the selector itself, on the key
150 * set, and on the selected-key set, in that order. They also synchronize on
151 * the cancelled-key set during steps (1) and (3) above.
152 *
153 * <p> Changes made to the interest sets of a selector's keys while a
154 * selection operation is in progress have no effect upon that operation; they
155 * will be seen by the next selection operation.
156 *
157 * <p> Keys may be cancelled and channels may be closed at any time. Hence the
158 * presence of a key in one or more of a selector's key sets does not imply
159 * that the key is valid or that its channel is open. Application code should
160 * be careful to synchronize and check these conditions as necessary if there
161 * is any possibility that another thread will cancel a key or close a channel.
162 *
163 * <p> A thread blocked in one of the {@link #select()} or {@link
164 * #select(long)} methods may be interrupted by some other thread in one of
165 * three ways:
166 *
167 * <ul>
168 *
169 * <li><p> By invoking the selector's {@link #wakeup wakeup} method,
170 * </p></li>
171 *
172 * <li><p> By invoking the selector's {@link #close close} method, or
173 * </p></li>
174 *
175 * <li><p> By invoking the blocked thread's {@link
176 * java.lang.Thread#interrupt() interrupt} method, in which case its
177 * interrupt status will be set and the selector's {@link #wakeup wakeup}
178 * method will be invoked. </p></li>
179 *
180 * </ul>
181 *
182 * <p> The {@link #close close} method synchronizes on the selector and all
183 * three key sets in the same order as in a selection operation.
184 *
185 * <a name="ksc">
186 *
187 * <p> A selector's key and selected-key sets are not, in general, safe for use
188 * by multiple concurrent threads. If such a thread might modify one of these
189 * sets directly then access should be controlled by synchronizing on the set
190 * itself. The iterators returned by these sets' {@link
191 * java.util.Set#iterator() iterator} methods are <i>fail-fast:</i> If the set
192 * is modified after the iterator is created, in any way except by invoking the
193 * iterator's own {@link java.util.Iterator#remove() remove} method, then a
194 * {@link java.util.ConcurrentModificationException} will be thrown. </p>
195 *
196 *
197 * @author Mark Reinhold
198 * @author JSR-51 Expert Group
199 * @since 1.4
200 *
201 * @see SelectableChannel
202 * @see SelectionKey
203 */
204
205 public abstract class Selector {
206
207 /**
208 * Initializes a new instance of this class.
209 */
210 protected Selector() { }
211
212 /**
213 * Opens a selector.
214 *
215 * <p> The new selector is created by invoking the {@link
216 * java.nio.channels.spi.SelectorProvider#openSelector openSelector} method
217 * of the system-wide default {@link
218 * java.nio.channels.spi.SelectorProvider} object. </p>
219 *
220 * @return A new selector
221 *
222 * @throws IOException
223 * If an I/O error occurs
224 */
225 public static Selector open() throws IOException {
226 return SelectorProvider.provider().openSelector();
227 }
228
229 /**
230 * Tells whether or not this selector is open. </p>
231 *
232 * @return <tt>true</tt> if, and only if, this selector is open
233 */
234 public abstract boolean isOpen();
235
236 /**
237 * Returns the provider that created this channel. </p>
238 *
239 * @return The provider that created this channel
240 */
241 public abstract SelectorProvider provider();
242
243 /**
244 * Returns this selector's key set.
245 *
246 * <p> The key set is not directly modifiable. A key is removed only after
247 * it has been cancelled and its channel has been deregistered. Any
248 * attempt to modify the key set will cause an {@link
249 * UnsupportedOperationException} to be thrown.
250 *
251 * <p> The key set is <a href="#ksc">not thread-safe</a>. </p>
252 *
253 * @return This selector's key set
254 *
255 * @throws ClosedSelectorException
256 * If this selector is closed
257 */
258 public abstract Set<SelectionKey> keys();
259
260 /**
261 * Returns this selector's selected-key set.
262 *
263 * <p> Keys may be removed from, but not directly added to, the
264 * selected-key set. Any attempt to add an object to the key set will
265 * cause an {@link UnsupportedOperationException} to be thrown.
266 *
267 * <p> The selected-key set is <a href="#ksc">not thread-safe</a>. </p>
268 *
269 * @return This selector's selected-key set
270 *
271 * @throws ClosedSelectorException
272 * If this selector is closed
273 */
274 public abstract Set<SelectionKey> selectedKeys();
275
276 /**
277 * Selects a set of keys whose corresponding channels are ready for I/O
278 * operations.
279 *
280 * <p> This method performs a non-blocking <a href="#selop">selection
281 * operation</a>. If no channels have become selectable since the previous
282 * selection operation then this method immediately returns zero.
283 *
284 * <p> Invoking this method clears the effect of any previous invocations
285 * of the {@link #wakeup wakeup} method. </p>
286 *
287 * @return The number of keys, possibly zero, whose ready-operation sets
288 * were updated by the selection operation
289 *
290 * @throws IOException
291 * If an I/O error occurs
292 *
293 * @throws ClosedSelectorException
294 * If this selector is closed
295 */
296 public abstract int selectNow() throws IOException;
297
298 /**
299 * Selects a set of keys whose corresponding channels are ready for I/O
300 * operations.
301 *
302 * <p> This method performs a blocking <a href="#selop">selection
303 * operation</a>. It returns only after at least one channel is selected,
304 * this selector's {@link #wakeup wakeup} method is invoked, the current
305 * thread is interrupted, or the given timeout period expires, whichever
306 * comes first.
307 *
308 * <p> This method does not offer real-time guarantees: It schedules the
309 * timeout as if by invoking the {@link Object#wait(long)} method. </p>
310 *
311 * @param timeout If positive, block for up to <tt>timeout</tt>
312 * milliseconds, more or less, while waiting for a
313 * channel to become ready; if zero, block indefinitely;
314 * must not be negative
315 *
316 * @return The number of keys, possibly zero,
317 * whose ready-operation sets were updated
318 *
319 * @throws IOException
320 * If an I/O error occurs
321 *
322 * @throws ClosedSelectorException
323 * If this selector is closed
324 *
325 * @throws IllegalArgumentException
326 * If the value of the timeout argument is negative
327 */
328 public abstract int select(long timeout)
329 throws IOException;
330
331 /**
332 * Selects a set of keys whose corresponding channels are ready for I/O
333 * operations.
334 *
335 * <p> This method performs a blocking <a href="#selop">selection
336 * operation</a>. It returns only after at least one channel is selected,
337 * this selector's {@link #wakeup wakeup} method is invoked, or the current
338 * thread is interrupted, whichever comes first. </p>
339 *
340 * @return The number of keys, possibly zero,
341 * whose ready-operation sets were updated
342 *
343 * @throws IOException
344 * If an I/O error occurs
345 *
346 * @throws ClosedSelectorException
347 * If this selector is closed
348 */
349 public abstract int select() throws IOException;
350
351 /**
352 * Causes the first selection operation that has not yet returned to return
353 * immediately.
354 *
355 * <p> If another thread is currently blocked in an invocation of the
356 * {@link #select()} or {@link #select(long)} methods then that invocation
357 * will return immediately. If no selection operation is currently in
358 * progress then the next invocation of one of these methods will return
359 * immediately unless the {@link #selectNow()} method is invoked in the
360 * meantime. In any case the value returned by that invocation may be
361 * non-zero. Subsequent invocations of the {@link #select()} or {@link
362 * #select(long)} methods will block as usual unless this method is invoked
363 * again in the meantime.
364 *
365 * <p> Invoking this method more than once between two successive selection
366 * operations has the same effect as invoking it just once. </p>
367 *
368 * @return This selector
369 */
370 public abstract Selector wakeup();
371
372 /**
373 * Closes this selector.
374 *
375 * <p> If a thread is currently blocked in one of this selector's selection
376 * methods then it is interrupted as if by invoking the selector's {@link
377 * #wakeup wakeup} method.
378 *
379 * <p> Any uncancelled keys still associated with this selector are
380 * invalidated, their channels are deregistered, and any other resources
381 * associated with this selector are released.
382 *
383 * <p> If this selector is already closed then invoking this method has no
384 * effect.
385 *
386 * <p> After a selector is closed, any further attempt to use it, except by
387 * invoking this method or the {@link #wakeup wakeup} method, will cause a
388 * {@link ClosedSelectorException} to be thrown. </p>
389 *
390 * @throws IOException
391 * If an I/O error occurs
392 */
393 public abstract void close() throws IOException;
394
395 }
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