一个web项目web.xml的配置中

<context-param>的作用:web.xml的配置中<context-param>配置作用1. 启动一个WEB项目的时候,容器(如:Tomcat)会去读它的配置文件web.xml.读两个节点: <listener></listener> 和 <context-param></context-param>
2.紧接着,容器创建一个ServletContext(上下文),这个WEB项目所有部分都将共享这个上下文.
3.容器将<context-param></context-param>转化为键值对,并交给ServletContext.
4.容器创建<listener></listener>中的类实例,即创建监听.
5.在监听中会有contextInitialized(ServletContextEvent args)初始化方法,在这个方法中获得ServletContext = ServletContextEvent.getServletContext();context-param的值 = ServletContext.getInitParameter("context-param的键");
6.得到这个context-param的值之后,你就可以做一些操作了.注意,这个时候你的WEB项目还没有完全启动完成.这个动作会比所有的Servlet都要早.换句话说,这个时候,你对<context-param>中的键值做的操作,将在你的WEB项目完全启动之前被执行.
7.举例.你可能想在项目启动之前就打开数据库.那么这里就可以在<context-param>中设置数据库的连接方式,在监听类中初始化数据库的连接.
8.这个监听是自己写的一个类,除了初始化方法,它还有销毁方法.用于关闭应用前释放资源.比如说数据库连接的关闭.

如:<!-- 加载spring的配置文件 --><context-param>    <param-name>contextConfigLocation</param-name>    <param-value>/WEB-INF/applicationContext.xml,/WEB-INF/action-servlet.xml,/WEB-
INF/jason-servlet.xml</param-value></context-param><listener>    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener>

又如: --->自定义context-param,且自定义listener来获取这些信息
<context-param>    <param-name>urlrewrite</param-name>    <param-value>false</param-value></context-param><context-param>    <param-name>cluster</param-name>    <param-value>false</param-value></context-param><context-param>    <param-name>servletmapping</param-name>    <param-value>*.bbscs</param-value></context-param><context-param>    <param-name>poststoragemode</param-name>    <param-value>1</param-value></context-param><listener>    <listener-class>com.laoer.bbscs.web.servlet.SysListener</listener-class></listener>
public class SysListener extends HttpServlet implements ServletContextListener {
private static final Log logger = LogFactory.getLog(SysListener.class);
public void contextDestroyed(ServletContextEvent sce) {
   //用于在容器关闭时,操作}
//用于在容器开启时,操作
public void contextInitialized(ServletContextEvent sce) {   String rootpath = sce.getServletContext().getRealPath("/");   System.out.println("-------------rootPath:"+rootpath);
   if (rootpath != null) {    rootpath = rootpath.replaceAll("\\\\", "/");   } else {    rootpath = "/";   }   if (!rootpath.endsWith("/")) {    rootpath = rootpath + "/";   }   Constant.ROOTPATH = rootpath;   logger.info("Application Run Path:" + rootpath);   String urlrewrtie = sce.getServletContext().getInitParameter("urlrewrite");   boolean burlrewrtie = false;   if (urlrewrtie != null) {    burlrewrtie = Boolean.parseBoolean(urlrewrtie);   }   Constant.USE_URL_REWRITE = burlrewrtie;   logger.info("Use Urlrewrite:" + burlrewrtie);   其它略之....
   }
}   /*最终输出   -------------rootPath:D:\tomcat_bbs\webapps\BBSCS_8_0_3\   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]
Application Run Path:D:/tomcat_bbs/webapps/BBSCS_8_0_3/   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]
Use Urlrewrite:true   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]
Use Cluster:false   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]
SERVLET MAPPING:*.bbscs   2009-06-09 21:51:46,573 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]
Post Storage Mode:1   */
context-param和init-param区别web.xml里面可以定义两种参数：(1)application范围内的参数，存放在servletcontext中，在web.xml中配置如下：<context-param>           <param-name>context/param</param-name>           <param-value>avalible during application</param-value></context-param>
(2)servlet范围内的参数，只能在servlet的init()方法中取得，在web.xml中配置如下：<servlet>    <servlet-name>MainServlet</servlet-name>    <servlet-class>com.wes.controller.MainServlet</servlet-class>    <init-param>       <param-name>param1</param-name>       <param-value>avalible in servlet init()</param-value>    </init-param>    <load-on-startup>0</load-on-startup></servlet>
在servlet中可以通过代码分别取用：package com.wes.controller;
import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;
public class MainServlet extends HttpServlet ...{
    public MainServlet() ...{        super();     }    public void init() throws ServletException ...{         System.out.println("下面的两个参数param1是在servlet中存放的");         System.out.println(this.getInitParameter("param1"));         System.out.println("下面的参数是存放在servletcontext中的");        System.out.println(getServletContext().getInitParameter("context/param"));      }}

第一种参数在servlet里面可以通过getServletContext().getInitParameter("context/param")得到第二种参数只能在servlet的init()方法中通过this.getInitParameter("param1")取得.