Java example source code file (WatchService.java)

This example Java source code file (WatchService.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

closeable, interruptedexception, ioexception, override, threading, threads, timeunit, watchkey, watchservice

The WatchService.java Java example source code

/*
 * Copyright (c) 2007, 2013, 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 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, 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
 * 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
 *
 * <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;
}