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3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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  
26  package java.nio.channels.spi;
27  
28  import java.nio.channels.*;
29  
30  
31  /**
32   * Base implementation class for selection keys.
33   *
34   * <p> This class tracks the validity of the key and implements cancellation.
35   *
36   * @author Mark Reinhold
37   * @author JSR-51 Expert Group
38   * @since 1.4
39   */
40  
41  public abstract class AbstractSelectionKey
42      extends SelectionKey
43  {
44  
45      /**
46       * Initializes a new instance of this class.
47       */
48      protected AbstractSelectionKey() { }
49  
50      private volatile boolean valid = true;
51  
52      public final boolean isValid() {
53          return valid;
54      }
55  
56      void invalidate() {                                 // package-private
57          valid = false;
58      }
59  
60      /**
61       * Cancels this key.
62       *
63       * <p> If this key has not yet been cancelled then it is added to its
64       * selector's cancelled-key set while synchronized on that set.  </p>
65       */
66      public final void cancel() {
67          // Synchronizing "this" to prevent this key from getting canceled
68          // multiple times by different threads, which might cause race
69          // condition between selector's select() and channel's close().
70          synchronized (this) {
71              if (valid) {
72                  valid = false;
73                  ((AbstractSelector)selector()).cancel(this);
74              }
75          }
76      }
77  }