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