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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
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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
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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&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  
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 }