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。

    

      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。