1/*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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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).
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24 */
25
26package java.nio.file;
27
28import 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&lt;?&gt; 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
84public 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}
151