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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
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   * 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
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   */
  
  package java.nio.file;
  
  import java.io.Closeable;
  import java.io.IOException;
  import java.util.concurrent.TimeUnit;
  
  /**
   * A watch service that <em>watches</em> registered objects for changes and
   * events. For example a file manager may use a watch service to monitor a
   * directory for changes so that it can update its display of the list of files
   * when files are created or deleted.
   *
   * <p> A {@link Watchable} object is registered with a watch service by invoking
   * its {@link Watchable#register register} method, returning a {@link WatchKey}
   * to represent the registration. When an event for an object is detected the
   * key is <em>signalled</em>, and if not currently signalled, it is queued to
   * the watch service so that it can be retrieved by consumers that invoke the
   * {@link #poll() poll} or {@link #take() take} methods to retrieve keys
   * and process events. Once the events have been processed the consumer
   * invokes the key's {@link WatchKey#reset reset} method to reset the key which
   * allows the key to be signalled and re-queued with further events.
   *
   * <p> Registration with a watch service is cancelled by invoking the key's
   * {@link WatchKey#cancel cancel} method. A key that is queued at the time that
   * it is cancelled remains in the queue until it is retrieved. Depending on the
   * object, a key may be cancelled automatically. For example, suppose a
   * directory is watched and the watch service detects that it has been deleted
   * or its file system is no longer accessible. When a key is cancelled in this
   * manner it is signalled and queued, if not currently signalled. To ensure
   * that the consumer is notified the return value from the {@code reset}
   * method indicates if the key is valid.
   *
   * <p> A watch service is safe for use by multiple concurrent consumers. To
   * ensure that only one consumer processes the events for a particular object at
   * any time then care should be taken to ensure that the key's {@code reset}
   * method is only invoked after its events have been processed. The {@link
   * #close close} method may be invoked at any time to close the service causing
   * any threads waiting to retrieve keys, to throw {@code
   * ClosedWatchServiceException}.
   *
   * <p> File systems may report events faster than they can be retrieved or
   * processed and an implementation may impose an unspecified limit on the number
   * of events that it may accumulate. Where an implementation <em>knowingly</em>
   * discards events then it arranges for the key's {@link WatchKey#pollEvents
   * pollEvents} method to return an element with an event type of {@link
   * StandardWatchEventKinds#OVERFLOW OVERFLOW}. This event can be used by the
   * consumer as a trigger to re-examine the state of the object.
   *
   * <p> When an event is reported to indicate that a file in a watched directory
   * has been modified then there is no guarantee that the program (or programs)
   * that have modified the file have completed. Care should be taken to coordinate
   * access with other programs that may be updating the file.
   * The {@link java.nio.channels.FileChannel FileChannel} class defines methods
   * to lock regions of a file against access by other programs.
   *
   * <h2>Platform dependencies</h2>
   *
   * <p> The implementation that observes events from the file system is intended
   * to map directly on to the native file event notification facility where
   * available, or to use a primitive mechanism, such as polling, when a native
   * facility is not available. Consequently, many of the details on how events
   * are detected, their timeliness, and whether their ordering is preserved are
   * highly implementation specific. For example, when a file in a watched
   * directory is modified then it may result in a single {@link
   * StandardWatchEventKinds#ENTRY_MODIFY ENTRY_MODIFY} event in some
   * implementations but several events in other implementations. Short-lived
   * files (meaning files that are deleted very quickly after they are created)
   * may not be detected by primitive implementations that periodically poll the
   * file system to detect changes.
   *
   * <p> If a watched file is not located on a local storage device then it is
   * implementation specific if changes to the file can be detected. In particular,
   * it is not required that changes to files carried out on remote systems be
   * detected.
   *
   * @since 1.7
   *
   * @see FileSystem#newWatchService
   */
  
  public interface WatchService
      extends Closeable
  {
  
      /**
       * Closes this watch service.
       *
       * <p> If a thread is currently blocked in the {@link #take take} or {@link
       * #poll(long,TimeUnit) poll} methods waiting for a key to be queued then
       * it immediately receives a {@link ClosedWatchServiceException}. Any
       * valid keys associated with this watch service are {@link WatchKey#isValid
       * invalidated}.
       *
       * <p> After a watch service is closed, any further attempt to invoke
       * operations upon it will throw {@link ClosedWatchServiceException}.
       * If this watch service is already closed then invoking this method
       * has no effect.
       *
       * @throws  IOException
       *          if an I/O error occurs
       */
      @Override
      void close() throws IOException;
  
      /**
       * Retrieves and removes the next watch key, or {@code null} if none are
       * present.
       *
       * @return  the next watch key, or {@code null}
       *
       * @throws  ClosedWatchServiceException
       *          if this watch service is closed
       */
      WatchKey poll();
  
      /**
       * Retrieves and removes the next watch key, waiting if necessary up to the
       * specified wait time if none are yet present.
       *
       * @param   timeout
       *          how to wait before giving up, in units of unit
       * @param   unit
       *          a {@code TimeUnit} determining how to interpret the timeout
       *          parameter
       *
       * @return  the next watch key, or {@code null}
       *
       * @throws  ClosedWatchServiceException
       *          if this watch service is closed, or it is closed while waiting
       *          for the next key
       * @throws  InterruptedException
       *          if interrupted while waiting
       */
      WatchKey poll(long timeout, TimeUnit unit)
          throws InterruptedException;
  
      /**
       * Retrieves and removes next watch key, waiting if none are yet present.
       *
       * @return  the next watch key
       *
       * @throws  ClosedWatchServiceException
       *          if this watch service is closed, or it is closed while waiting
       *          for the next key
       * @throws  InterruptedException
       *          if interrupted while waiting
       */
      WatchKey take() throws InterruptedException;
  }
  