WatchService. This feature takes advantage of the OS file event notification facilities where available, or otherwise uses a "primitive" polling mechanism. In my blog article WatchService Using ScheduledExecutorService Example, I show an example of how to use the WatchService along with an ExecutorService to do file monitoring. This is really easy to implement and use.However, the question of how to do it on JDK 6 was posed to me for those who can't do an upgrade for business reasons. I did a quick Google search to see what was out there. Alas, some of the most popular search results were poor, or really badly implemented solutions. I did come across a nice piece of code done by Pascal Essiembre. I didn't find much more on the developer. I would give a link to his work if I were sure that what I found was him. Pascal wrote the code below which is EPL 1.0 so you are free to use it. I used it to check it out, and it works likely in the same manner as the "primitive" implementation in JDK7. Here is the code unmodified from the form I found. This is very nicely done. Good code lives on. I just wish this were more near the top of the search results.
Thanks Pascal for your code contribution to the Java community.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 | /******************************************************************************* * Copyright (c) 2007 Pascal Essiembre. * All rights reserved. This program and the accompanying materials * are made available under the terms of the Eclipse Public License v1.0 * which accompanies this distribution, and is available at * * Contributors: * Pascal Essiembre - initial API and implementation ******************************************************************************/import java.io.File;import java.io.FileNotFoundException;import java.net.URL;import java.util.Hashtable;import java.util.Timer;import java.util.TimerTask;/** * Class monitoring a {@link File} for changes. * * @author Pascal Essiembre */public class FileMonitor { private static final FileMonitor instance = new FileMonitor(); private Timer timer; private Hashtable<String, FileMonitorTask> timerEntries; /** * Gets the file monitor instance. * * @return file monitor instance */ public static FileMonitor getInstance() { return instance; } /** * Constructor. */ private FileMonitor() { // Create timer, run timer thread as daemon. timer = new Timer(true); timerEntries = new Hashtable<String, FileMonitorTask>(); } /** * Adds a monitored file with a {@link FileChangeListener}. * * @param listener * listener to notify when the file changed. * @param fileName * name of the file to monitor. * @param period * polling period in milliseconds. */ public void addFileChangeListener(FileChangeListener listener, String fileName, long period) throws FileNotFoundException { addFileChangeListener(listener, new File(fileName), period); } /** * Adds a monitored file with a FileChangeListener. * * @param listener * listener to notify when the file changed. * @param fileName * name of the file to monitor. * @param period * polling period in milliseconds. */ public void addFileChangeListener(FileChangeListener listener, File file, long period) throws FileNotFoundException { removeFileChangeListener(listener, file); FileMonitorTask task = new FileMonitorTask(listener, file); timerEntries.put(file.toString() + listener.hashCode(), task); timer.schedule(task, period, period); } /** * Remove the listener from the notification list. * * @param listener * the listener to be removed. */ public void removeFileChangeListener(FileChangeListener listener, String fileName) { removeFileChangeListener(listener, new File(fileName)); } /** * Remove the listener from the notification list. * * @param listener * the listener to be removed. */ public void removeFileChangeListener(FileChangeListener listener, File file) { FileMonitorTask task = timerEntries.remove(file.toString() + listener.hashCode()); if (task != null) { task.cancel(); } } /** * Fires notification that a file changed. * * @param listener * file change listener * @param file * the file that changed */ protected void fireFileChangeEvent(FileChangeListener listener, File file) { listener.fileChanged(file); } /** * File monitoring task. */ class FileMonitorTask extends TimerTask { FileChangeListener listener; File monitoredFile; long lastModified; public FileMonitorTask(FileChangeListener listener, File file) throws FileNotFoundException { this.listener = listener; this.lastModified = 0; monitoredFile = file; if (!monitoredFile.exists()) { // but is it on CLASSPATH? URL fileURL = listener.getClass().getClassLoader().getResource(file.toString()); if (fileURL != null) { monitoredFile = new File(fileURL.getFile()); } else { throw new FileNotFoundException("File Not Found: " + file); } } this.lastModified = monitoredFile.lastModified(); } public void run() { long lastModified = monitoredFile.lastModified(); if (lastModified != this.lastModified) { this.lastModified = lastModified; fireFileChangeEvent(this.listener, monitoredFile); } } }}/******************************************************************************* * Copyright (c) 2007 Pascal Essiembre. All rights reserved. This program and * the accompanying materials are made available under the terms of the Eclipse * Public License v1.0 which accompanies this distribution, and is available at * * Contributors: Pascal Essiembre - initial API and implementation ******************************************************************************//** * Listener interested in {@link File} changes. * * @author Pascal Essiembre */interface FileChangeListener { /** * Invoked when a file changes. * * @param fileName * name of changed file. */ public void fileChanged(File file);} |
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