/* * 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();}