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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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  package java.nio.channels.spi;
  
  import java.nio.channels.*;
  
  
  /**
   * Base implementation class for selection keys.
   *
   * <p> This class tracks the validity of the key and implements cancellation.
   *
   * @author Mark Reinhold
   * @author JSR-51 Expert Group
   * @since 1.4
   */
  
  public abstract class AbstractSelectionKey
      extends SelectionKey
  {
  
      /**
       * Initializes a new instance of this class.
       */
      protected AbstractSelectionKey() { }
  
      private volatile boolean valid = true;
  
      public final boolean isValid() {
          return valid;
      }
  
      void invalidate() {                                 // package-private
          valid = false;
      }
  
      /**
       * Cancels this key.
       *
       * <p> If this key has not yet been cancelled then it is added to its
       * selector's cancelled-key set while synchronized on that set.  </p>
       */
      public final void cancel() {
          // Synchronizing "this" to prevent this key from getting canceled
          // multiple times by different threads, which might cause race
          // condition between selector's select() and channel's close().
          synchronized (this) {
              if (valid) {
                  valid = false;
                  ((AbstractSelector)selector()).cancel(this);
              }
          }
      }
  }
  