1 /* 2 * Copyright (c) 2000, 2013, 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 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 */ Selector()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 */ open()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 */ isOpen()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 */ provider()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 */ keys()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 */ selectedKeys()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 */ selectNow()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 */ select(long timeout)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 */ select()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 */ wakeup()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 */ close()394 public abstract void close() throws IOException; 395 396 } 397