java 监听文件内容变化
????? 有时候，我们需要确定某些文件是否有变化而做出一些对应的动作，例如，曾经开发的一款服务器中，由于模块比较多，在运行期间有时候需要单独的输出某个模块日志，但又不可能总是开着日志。
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????? log4j中已经实现了可以动态监控日志级别日志监视器
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PropertyConfigurator.configureAndWatch(properties);
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????? 上面的properties是日志配置文件，例如常用的 log4j.properties ,log4j.xml。
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? 但是这是系统级别的，然后看了一下源代码，
/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You under the Apache License, Version 2.0 * (the "License"); you may not use this file except in compliance with * the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */// Contributors: Mathias Bogaertpackage org.apache.log4j.helpers;import java.io.File;/** * Check every now and then that a certain file has not changed. If it has, then * call the {@link #doOnChange} method. * * @author Ceki Gülcü * @since version 0.9.1 */public abstract class FileWatchdog extends Thread {/** * The default delay between every file modification check, set to 60 * seconds. */static final public long DEFAULT_DELAY = 60000;/** * The name of the file to observe for changes. */protected String filename;/** * The delay to observe between every check. By default set * {@link #DEFAULT_DELAY}. */protected long delay = DEFAULT_DELAY;File file;long lastModif = 0;boolean warnedAlready = false;boolean interrupted = false;protected FileWatchdog(String filename) {super("FileWatchdog");this.filename = filename;file = new File(filename);setDaemon(true);checkAndConfigure();}/** * Set the delay to observe between each check of the file changes. */public void setDelay(long delay) {this.delay = delay;}abstract protected void doOnChange();protected void checkAndConfigure() {boolean fileExists;try {fileExists = file.exists();} catch (SecurityException e) {LogLog.warn("Was not allowed to read check file existance, file:["+ filename + "].");interrupted = true; // there is no point in continuingreturn;}if (fileExists) {long l = file.lastModified(); // this can also throw a// SecurityExceptionif (l > lastModif) { // however, if we reached this point thislastModif = l; // is very unlikely.doOnChange();warnedAlready = false;}} else {if (!warnedAlready) {LogLog.debug("[" + filename + "] does not exist.");warnedAlready = true;}}}public void run() {while (!interrupted) {try {Thread.sleep(delay);} catch (InterruptedException e) {// no interruption expected}checkAndConfigure();}}}?
? 这是一个抽象类，子类只要实现最关心的文件内容有变化的回调方法 doOnChange 就OK。
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? 因此，在一个工程中，如果想实现模块级别的日志控制，则我们可以附件一个配置文件，在这个配置文件中指定模块的日志级别， 在log4j中指定总的级别，然后在应用中实现文件内容监控，则可以做到动态切换，实现细粒度的控制。
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