web.xml的初始化参数:context-param,init-param

1、初始化参数
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("下面的参数是存放在servletcontext中的");
         System.out.println(getServletContext().getInitParameter("context/param"));
            System.out.println("下面的两个参数param1是在servlet中存放的");
         System.out.println(this.getInitParameter("param1"));
       }
}

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
2、初始化参数获取
<context-param>
<param-name>count</param-name>
<param-value>1200</param-value>
</context-param>

<servlet>
    <description>loginServlet</description>
    <display-name>loginServlet</display-name>
    <servlet-name>loginServlet</servlet-name>
    <servlet-class>loginServlet</servlet-class>
    <init-param>
      <description>系统初始化数</description>
      <param-name>num</param-name>
      <param-value>100</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>loginServlet</servlet-name>
    <url-pattern>/loginServlet</url-pattern>
</servlet-mapping>

获取参数的方法以及异同

String username=request.getParameter("username");
response.setCharacterEncoding("gb2312");
System.out.println("getServletContext()："+getServletContext());
ServletContext context=getServletConfig().getServletContext();
String num=context.getInitParameter("num");// <init-param> 获取不到值
String count=context.getInitParameter("count");//<context-param> 获取到值
System.out.println("num："+num);//null
System.out.println("count："+count);//1200
System.out.println("num："+getServletConfig().getInitParameter("num"));//100获取到值
System.out.println("count："+getServletConfig().getInitParameter("count"));//null获取不到值

System.out.println("request.getSession().getServletContext()："+request.getSession().getServletContext());
request.setAttribute("username", username);
PrintWriter out=response.getWriter();
out.println("获取的用户名是："+username);

补充：Web开发 , 其他 ,