content: /*
   * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.nio.file;
  
  import java.util.List;
  
  /**
   * A token representing the registration of a {@link Watchable watchable} object
   * with a {@link WatchService}.
   *
   * <p> A watch key is created when a watchable object is registered with a watch
   * service. The key remains {@link #isValid valid} until:
   * <ol>
   *   <li> It is cancelled, explicitly, by invoking its {@link #cancel cancel}
   *     method, or</li>
   *   <li> Cancelled implicitly, because the object is no longer accessible,
   *     or </li>
   *   <li> By {@link WatchService#close closing} the watch service. </li>
   * </ol>
   *
   * <p> A watch key has a state. When initially created the key is said to be
   * <em>ready</em>. When an event is detected then the key is <em>signalled</em>
   * and queued so that it can be retrieved by invoking the watch service's {@link
   * WatchService#poll() poll} or {@link WatchService#take() take} methods. Once
   * signalled, a key remains in this state until its {@link #reset reset} method
   * is invoked to return the key to the ready state. Events detected while the
   * key is in the signalled state are queued but do not cause the key to be
   * re-queued for retrieval from the watch service. Events are retrieved by
   * invoking the key's {@link #pollEvents pollEvents} method. This method
   * retrieves and removes all events accumulated for the object. When initially
   * created, a watch key has no pending events. Typically events are retrieved
   * when the key is in the signalled state leading to the following idiom:
   *
   * <pre>
   *     for (;;) {
   *         // retrieve key
   *         WatchKey key = watcher.take();
   *
   *         // process events
   *         for (WatchEvent&lt;?&gt; event: key.pollEvents()) {
   *             :
   *         }
   *
   *         // reset the key
   *         boolean valid = key.reset();
   *         if (!valid) {
   *             // object no longer registered
   *         }
   *     }
   * </pre>
   *
   * <p> Watch keys are safe for use by multiple concurrent threads. Where there
   * are several threads retrieving signalled keys from a watch service then care
   * should be taken to ensure that the {@code reset} method is only invoked after
   * the events for the object have been processed. This ensures that one thread
   * is processing the events for an object at any time.
   *
   * @since 1.7
   */
  
  public interface WatchKey {
  
      /**
       * Tells whether or not this watch key is valid.
       *
       * <p> A watch key is valid upon creation and remains until it is cancelled,
       * or its watch service is closed.
       *
       * @return  {@code true} if, and only if, this watch key is valid
       */
      boolean isValid();
  
      /**
       * Retrieves and removes all pending events for this watch key, returning
       * a {@code List} of the events that were retrieved.
       *
       * <p> Note that this method does not wait if there are no events pending.
       *
       * @return  the list of the events retrieved; may be empty
       */
      List<WatchEvent<?>> pollEvents();
  
      /**
       * Resets this watch key.
       *
       * <p> If this watch key has been cancelled or this watch key is already in
       * the ready state then invoking this method has no effect. Otherwise
       * if there are pending events for the object then this watch key is
       * immediately re-queued to the watch service. If there are no pending
       * events then the watch key is put into the ready state and will remain in
       * that state until an event is detected or the watch key is cancelled.
       *
       * @return  {@code true} if the watch key is valid and has been reset, and
       *          {@code false} if the watch key could not be reset because it is
       *          no longer {@link #isValid valid}
       */
      boolean reset();
  
      /**
       * Cancels the registration with the watch service. Upon return the watch key
       * will be invalid. If the watch key is enqueued, waiting to be retrieved
       * from the watch service, then it will remain in the queue until it is
       * removed. Pending events, if any, remain pending and may be retrieved by
       * invoking the {@link #pollEvents pollEvents} method after the key is
       * cancelled.
       *
       * <p> If this watch key has already been cancelled then invoking this
       * method has no effect.  Once cancelled, a watch key remains forever invalid.
       */
      void cancel();
  
      /**
       * Returns the object for which this watch key was created. This method will
       * continue to return the object even after the key is cancelled.
       *
       * <p> As the {@code WatchService} is intended to map directly on to the
       * native file event notification facility (where available) then many of
       * details on how registered objects are watched is highly implementation
       * specific. When watching a directory for changes for example, and the
       * directory is moved or renamed in the file system, there is no guarantee
       * that the watch key will be cancelled and so the object returned by this
       * method may no longer be a valid path to the directory.
       *
       * @return the object for which this watch key was created
       */
      Watchable watchable();
  }
  