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。
log4j中已经实现了可以动态监控日志级别日志监视器
PropertyConfigurator.configureAndWatch(properties);
上面的properties是日志配置文件，例如常用的 log4j.properties ,log4j.xml。
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 Bogaert
package 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 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();
}
}
}
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。
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。
