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| 1 | /* | ||
| 2 | * Copyright (c) 2007, 2011, 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.file; | ||
| 27 | |||
| 28 | import java.util.List; | ||
| 29 | |||
| 30 | /** | ||
| 31 | * A token representing the registration of a {@link Watchable watchable} object | ||
| 32 | * with a {@link WatchService}. | ||
| 33 | * | ||
| 34 | * <p> A watch key is created when a watchable object is registered with a watch | ||
| 35 | * service. The key remains {@link #isValid valid} until: | ||
| 36 | * <ol> | ||
| 37 | * <li> It is cancelled, explicitly, by invoking its {@link #cancel cancel} | ||
| 38 | * method, or</li> | ||
| 39 | * <li> Cancelled implicitly, because the object is no longer accessible, | ||
| 40 | * or </li> | ||
| 41 | * <li> By {@link WatchService#close closing} the watch service. </li> | ||
| 42 | * </ol> | ||
| 43 | * | ||
| 44 | * <p> A watch key has a state. When initially created the key is said to be | ||
| 45 | * <em>ready</em>. When an event is detected then the key is <em>signalled</em> | ||
| 46 | * and queued so that it can be retrieved by invoking the watch service's {@link | ||
| 47 | * WatchService#poll() poll} or {@link WatchService#take() take} methods. Once | ||
| 48 | * signalled, a key remains in this state until its {@link #reset reset} method | ||
| 49 | * is invoked to return the key to the ready state. Events detected while the | ||
| 50 | * key is in the signalled state are queued but do not cause the key to be | ||
| 51 | * re-queued for retrieval from the watch service. Events are retrieved by | ||
| 52 | * invoking the key's {@link #pollEvents pollEvents} method. This method | ||
| 53 | * retrieves and removes all events accumulated for the object. When initially | ||
| 54 | * created, a watch key has no pending events. Typically events are retrieved | ||
| 55 | * when the key is in the signalled state leading to the following idiom: | ||
| 56 | * | ||
| 57 | * <pre> | ||
| 58 | * for (;;) { | ||
| 59 | * // retrieve key | ||
| 60 | * WatchKey key = watcher.take(); | ||
| 61 | * | ||
| 62 | * // process events | ||
| 63 | * for (WatchEvent<?> event: key.pollEvents()) { | ||
| 64 | * : | ||
| 65 | * } | ||
| 66 | * | ||
| 67 | * // reset the key | ||
| 68 | * boolean valid = key.reset(); | ||
| 69 | * if (!valid) { | ||
| 70 | * // object no longer registered | ||
| 71 | * } | ||
| 72 | * } | ||
| 73 | * </pre> | ||
| 74 | * | ||
| 75 | * <p> Watch keys are safe for use by multiple concurrent threads. Where there | ||
| 76 | * are several threads retrieving signalled keys from a watch service then care | ||
| 77 | * should be taken to ensure that the {@code reset} method is only invoked after | ||
| 78 | * the events for the object have been processed. This ensures that one thread | ||
| 79 | * is processing the events for an object at any time. | ||
| 80 | * | ||
| 81 | * @since 1.7 | ||
| 82 | */ | ||
| 83 | |||
| 84 | public interface WatchKey { | ||
| 85 | |||
| 86 | /** | ||
| 87 | * Tells whether or not this watch key is valid. | ||
| 88 | * | ||
| 89 | * <p> A watch key is valid upon creation and remains until it is cancelled, | ||
| 90 | * or its watch service is closed. | ||
| 91 | * | ||
| 92 | * @return {@code true} if, and only if, this watch key is valid | ||
| 93 | */ | ||
| 94 | boolean isValid(); | ||
| 95 | |||
| 96 | /** | ||
| 97 | * Retrieves and removes all pending events for this watch key, returning | ||
| 98 | * a {@code List} of the events that were retrieved. | ||
| 99 | * | ||
| 100 | * <p> Note that this method does not wait if there are no events pending. | ||
| 101 | * | ||
| 102 | * @return the list of the events retrieved; may be empty | ||
| 103 | */ | ||
| 104 | List<WatchEvent<?>> pollEvents(); | ||
| 105 | |||
| 106 | /** | ||
| 107 | * Resets this watch key. | ||
| 108 | * | ||
| 109 | * <p> If this watch key has been cancelled or this watch key is already in | ||
| 110 | * the ready state then invoking this method has no effect. Otherwise | ||
| 111 | * if there are pending events for the object then this watch key is | ||
| 112 | * immediately re-queued to the watch service. If there are no pending | ||
| 113 | * events then the watch key is put into the ready state and will remain in | ||
| 114 | * that state until an event is detected or the watch key is cancelled. | ||
| 115 | * | ||
| 116 | * @return {@code true} if the watch key is valid and has been reset, and | ||
| 117 | * {@code false} if the watch key could not be reset because it is | ||
| 118 | * no longer {@link #isValid valid} | ||
| 119 | */ | ||
| 120 | boolean reset(); | ||
| 121 | |||
| 122 | /** | ||
| 123 | * Cancels the registration with the watch service. Upon return the watch key | ||
| 124 | * will be invalid. If the watch key is enqueued, waiting to be retrieved | ||
| 125 | * from the watch service, then it will remain in the queue until it is | ||
| 126 | * removed. Pending events, if any, remain pending and may be retrieved by | ||
| 127 | * invoking the {@link #pollEvents pollEvents} method after the key is | ||
| 128 | * cancelled. | ||
| 129 | * | ||
| 130 | * <p> If this watch key has already been cancelled then invoking this | ||
| 131 | * method has no effect. Once cancelled, a watch key remains forever invalid. | ||
| 132 | */ | ||
| 133 | void cancel(); | ||
| 134 | |||
| 135 | /** | ||
| 136 | * Returns the object for which this watch key was created. This method will | ||
| 137 | * continue to return the object even after the key is cancelled. | ||
| 138 | * | ||
| 139 | * <p> As the {@code WatchService} is intended to map directly on to the | ||
| 140 | * native file event notification facility (where available) then many of | ||
| 141 | * details on how registered objects are watched is highly implementation | ||
| 142 | * specific. When watching a directory for changes for example, and the | ||
| 143 | * directory is moved or renamed in the file system, there is no guarantee | ||
| 144 | * that the watch key will be cancelled and so the object returned by this | ||
| 145 | * method may no longer be a valid path to the directory. | ||
| 146 | * | ||
| 147 | * @return the object for which this watch key was created | ||
| 148 | */ | ||
| 149 | Watchable watchable(); | ||
| 150 | } | ||