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>

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>

<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 SysListenerextends HttpServlet implements ServletContextListener{

private static final Log logger = LogFactory.getLog(SysListener.class);

public voidcontextDestroyed(ServletContextEvent sce){

//用于在容器关闭时,操作
}

public voidcontextInitialized(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.