/* * 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.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. * * <h4>Platform dependencies</h4> * * <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;}