log4j日志输出到web项目指定文件夹

    尝试将log4j的文件日志输出到web工程制定目录，遇到了很多问题，最终搞定，下面是记录。
    其原理在于log4j的配置文件支持服务器的vm的环境变量，如${oss.log4j.path}，在log4j加载配置文件之前，先用System.setProperty("", "")设置好日志文件路径，这一操作通过一个初始的servlet来实现

1. log4j.properties文件

log4j.rootCategory=debug,stdout,ROLLFILE,R

log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] (%F:%L) -%m%n

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%p %t %c -%m%n

log4j.appender.R.File=${oss.log4j.path}logs/log.log
log4j.appender.R.MaxFileSize=100KB
log4j.appender.R.MaxBackupIndex=1

log4j.appender.ROLLFILE = org.apache.log4j.DailyRollingFileAppender
log4j.appender.ROLLFILE.File = ${oss.log4j.path}logs/log.html
log4j.appender.ROLLFILE.Append = true
log4j.appender.ROLLFILE.Threshold = DEBUG
log4j.appender.ROLLFILE.layout = org.apache.log4j.HTMLLayout 

2. servlet。配置web服务器的vm环境变量

package com.jesse.onlineshop.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;

import org.apache.log4j.Logger;

/**
 * 将配置信息读入系统变量
 * @author Administrator
 *
 */
public class SystemServlet extends HttpServlet {
	
	private static final Logger logger = Logger.getLogger(SystemServlet.class);
	
	private static final long serialVersionUID = -833322220864312415L;

	@Override
	public void init() throws ServletException {
        String rootPath = this.getServletContext().getRealPath("/");  
        
        logger.info(rootPath);
        
        String log4jPath = this.getServletConfig().getInitParameter("oss.log4j.path");  
        //若没有指定oss.log4j.path初始参数，则使用WEB的工程目录  
        log4jPath = (log4jPath==null||"".equals(log4jPath))?rootPath:log4jPath;  
        System.setProperty("oss.log4j.path", log4jPath);  
        super.init();  
	}

}

3. web.xml. 不需要配置servlet-mapping，load-on-startup设置为0，会在web容器启动时候就装入该servlet

	<servlet>
		<servlet-name>systemServlet</servlet-name>
		<servlet-class>com.jesse.onlineshop.servlet.SystemServlet</servlet-class>
		<init-param>
			<param-name>oss.log4j.path</param-name>
			<param-value></param-value>
		</init-param>
		<load-on-startup>0</load-on-startup>
	</servlet>

4. 结果


5. 疑惑
在log4j.properties配置文件中，日志文件路径需要配置成${oss.log4j.path}logs/log.html
在配制成${oss.log4j.path}/logs/log.html时不能输出，但是通过输出System.getProperty("oss.log4j.path")时输出的路径并不包含"/"。所以不理解为什么最终会在这里加上一个"/"的。