上面代码描述了Log4j的简单应用，其实使用Log4j也就是这样简单方便。当然除了上面的配置方法，还有其它，比如做一个J2EE应用，在J2EE应用使用Log4j，必须先在启动服务时加载Log4j的配置文件进行初始化，可以在web.xml中进行。 1、web应用的log4j使用基本上都采用：新建一个servlet，这个servlet在init函数中为log4j执行配置。一般就是读入配置文件。所以需要在web.xml中为这个servlet配置，同时设定load-on-startup为1。 2、这个servlet配置log4j就是读出配置文件，然后调用configure函数。这里有两个问题：一、需要知道文件在哪里；二、需要正确的文件类型 3、配置文件位置在web.xml中配置一个param即可，路径一般是相对于web的root目录 4、文件类型一般有两种，一个是Java的property文件，另一种是xml文件 配置文件的大致内容：log4j可以指定输出的log级别的最低等级，以及log的输出配置格式，每个log可以指定多个输出方式 (1) 创建Web工程，整个工程最后目录如下  (2）web.xml配置如下： ~~~ <?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0"> <display-name>LogLearning</display-name> <servlet> <servlet-name>Log4JTestServlet</servlet-name> <servlet-class>com.mucfc.Log4JTestServlet</servlet-class> </servlet> <!--用来启动 log4jConfigLocation的servlet --> <servlet> <servlet-name>Log4JInitServlet</servlet-name> <servlet-class>com.mucfc.Log4JInitServlet</servlet-class> <init-param> <param-name>log4j-properties-location</param-name> <param-value>/WEB-INF/classes/log4j.properties</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>Log4JTestServlet</servlet-name> <url-pattern>/test</url-pattern> </servlet-mapping> </web-app> ~~~ （3）配置文件log4j.properties ~~~ ### set log levels ### log4j.rootLogger = debug,stdout,D,E log4j.appender.stdout = org.apache.log4j.ConsoleAppender log4j.appender.stdout.Target = System.out log4j.appender.stdout.layout = org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n log4j.appender.D = org.apache.log4j.DailyRollingFileAppender log4j.appender.D.File = F://logs/log.log log4j.appender.D.Append = true log4j.appender.D.Threshold = DEBUG log4j.appender.D.layout = org.apache.log4j.PatternLayout log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n log4j.appender.E = org.apache.log4j.DailyRollingFileAppender log4j.appender.E.File =F://logs/error.log log4j.appender.E.Append = true log4j.appender.E.Threshold = ERROR log4j.appender.E.layout = org.apache.log4j.PatternLayout log4j.appender.E.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n ~~~ (4) web容器一来就初始化的servlet Log4JInitServlet.java ~~~ package com.mucfc; import java.io.File; import java.io.IOException; import javax.servlet.ServletConfig; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.log4j.BasicConfigurator; import org.apache.log4j.PropertyConfigurator; /** * Servlet implementation class Log4JInitServlet */ @WebServlet("/Log4JInitServlet") public class Log4JInitServlet extends HttpServlet { private static final long serialVersionUID = 1L; /** * @see HttpServlet#HttpServlet() */ public Log4JInitServlet() { super(); // TODO Auto-generated constructor stub } /** * @see Servlet#init(ServletConfig) */ public void init(ServletConfig config) throws ServletException { System.out.println("Log4JInitServlet 正在初始化 log4j日志设置信息"); String log4jLocation = config.getInitParameter("log4j-properties-location"); ServletContext sc = config.getServletContext(); if (log4jLocation == null) { System.err.println("*** 没有 log4j-properties-location 初始化的文件, 所以使用 BasicConfigurator初始化"); BasicConfigurator.configure(); } else { String webAppPath = sc.getRealPath("/"); String log4jProp = webAppPath + log4jLocation; File yoMamaYesThisSaysYoMama = new File(log4jProp); if (yoMamaYesThisSaysYoMama.exists()) { System.out.println("使用: " + log4jProp+"初始化日志设置信息"); PropertyConfigurator.configure(log4jProp); } else { System.err.println("*** " + log4jProp + " 文件没有找到， 所以使用 BasicConfigurator初始化"); BasicConfigurator.configure(); } } super.init(config); } /** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub } /** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */ protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub } } ~~~ 调用日志Log4JTestServlet,java ~~~ package com.mucfc; import java.io.IOException; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.log4j.Logger; /** * Servlet implementation class Log4JTestServlet */ @WebServlet("/Log4JTestServlet") public class Log4JTestServlet extends HttpServlet { private static final long serialVersionUID = 1L; private static Logger logger = Logger.getLogger(Log4JTestServlet.class); /** * @see HttpServlet#HttpServlet() */ public Log4JTestServlet() { super(); // TODO Auto-generated constructor stub } /** * @see Servlet#init(ServletConfig) */ public void init(ServletConfig config) throws ServletException { // TODO Auto-generated method stub } /** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // 记录debug级别的信息 logger.debug("This is debug message."); // 记录info级别的信息 logger.info("This is info message."); // 记录error级别的信息 logger.error("This is error message."); } /** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */ protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request,response); } } ~~~ 接下来就是运行了，来看看结果：  输出结果