]log4j FileWatchdog工具类，用来监控文件，如有变动就执行特定的操作

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今天看到log4j里的一个工具类，用来监控文件，如有变动就执行特定的操作，简单且有用，只需要实现一个doOnChange()方法即可。

 从这个文件可以看出，其实log4j.properties或者log4j.xml是可以实现在线修改的，不用重启服务，只需在web.xml里加一个org.springframework.web.util.Log4jConfigListener，再用配置一个初始属性log4jRefreshInterval，即刷新log4j配置信息的间隔时间，单位是毫秒，如下：

 

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< context-param >          < param-name >log4jConfigLocation param-name >          < param-value >log4j.properties param-value >      context-param >      < context-param >          < param-name >log4jRefreshInterval param-name >          < param-value >60000 param-value >      context-param >      < listener >          < listener-class >org.springframework.web.util.Log4jConfigListener listener-class >      listener > 非web应用的话需要写一个类Foo继承org.apache.log4j.varia.ReloadingPropertyConfigurator，重写doConfigure方法，直接调用org.apache.log4j.PropertyConfigurator.configureAndWatch(String, long)； 然后在启动非web应用时加系统属性-Dlog4j.configuratorClass=Foo (这里要全路径)，用于指定log4j Configurator的实现类。 我自己写了一个MyLog4jConfigurator，已经测试通过，如下：   ? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 package com.mashiguang;   import java.net.URL;   import org.apache.log4j.PropertyConfigurator; import org.apache.log4j.spi.LoggerRepository; import org.apache.log4j.varia.ReloadingPropertyConfigurator;   public class MyLog4jConfigurator extends ReloadingPropertyConfigurator {            PropertyConfigurator delegate = new PropertyConfigurator();        @Override      public void doConfigure(URL url, LoggerRepository repository) {          delegate.configureAndWatch(url.getFile(), 1000 );      } }     FileWatchdog源码如下： /*   * 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 Bogaert   package org.apache.log4j.helpers;   import java.io.File; import org.apache.log4j.helpers.LogLog;   /**     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) {      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 continuing        return ;      }        if (fileExists) {        long l = file.lastModified(); // this can also throw a SecurityException        if (l > lastModif) {           // however, if we reached this point this      lastModif = 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();      }    } }