Servlet中init-param与context-param的区别

web.xml文件中Servlet中配置如下:

<servlet>  
    <servlet-name>loginServlet</servlet-name>  
    <servlet-class>com.qcc.study.servlet02.LoginServlet</servlet-class>  
    <!-- 配置Servlet初始化参数 -->  
    <init-param>  
        <param-name>initParam</param-name>  
        <param-value>qcc</param-value>  
    </init-param>  
    <!-- Web容器启动时就加载并实例化该Servlet -->  
    <load-on-startup>0</load-on-startup>  
</servlet>  
<servlet-mapping>  
    <servlet-name>loginServlet</servlet-name>  
    <url-pattern>/login</url-pattern>     
</servlet-mapping>  
<!-- 配置ContextParam全局参数 -->  
<context-param>  
    <param-name>contextParam</param-name>  
    <param-value>admin</param-value>  
</context-param>

使用示例如下:

有login.jsp页面,代码如下:

<%@ page language="java" contentType="text/html; charset=UTF-8"  
    pageEncoding="UTF-8"%>  
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">  
<html>  
<head>  
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">  
<title>Insert title here</title>  
</head>  
<body>  
    <form action="login" method="post">  
        user:<input type="text" name="username" /><br/>  
        password:<input type="password" name="password" /><br/>  
        <input type="submit" value="Submit" />  
    </form>  
</body>  
</html>

当访问该jsp页面,并点击表单的提交按钮时,会将表单数据发送到login这个地址,这个login对应定义的serlvet的<url-mapping>的入口文件,请求被LoginServlet的doPost()方法处理,

LoginServlet的源码如下:

package com.qcc.study.servlet02;  
  
import java.io.IOException;  
import javax.servlet.ServletException;  
import javax.servlet.http.HttpServlet;  
import javax.servlet.http.HttpServletRequest;  
import javax.servlet.http.HttpServletResponse;  
  
public class LoginServlet extends HttpServlet {  
  
    @Override  
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)  
            throws ServletException, IOException {  
        //获取servlet初始化参数:  
        String initParam = getServletConfig().getInitParameter("initParam");  
        System.out.println("initParam: ---->" + initParam);  
        //获取全局初始化参数:  
        String contextParam = getServletContext().getInitParameter("contextParam");  
        System.out.println("contextParam: ---->" + contextParam);  
        //获取用户提交过来的数据:  
        String username = req.getParameter("username");  
        String password = req.getParameter("password");  
        System.out.println("用户提交的数据:" + username + " , " + password);  
    }  
}

二者的区别:
<init-parm>配置在<servlet>标签中,用来初始化当前的Servlet的,属于当前Servlet的配置,因此存放在servletConfig对象中;

通过getServletConfig().getInitParameter("initParam")的方式获取;

<context-param>直接配置在web.xml的<web-app>标签中,属于上下文参数,在整个web应用中都可以使用,它是全局的,因此存放在servletContext对象中(即application对象);

通过getServletContext().getInitParameter("contextParam")的方式获取;

打印结果:

initParam: ---->qcc  
contextParam: ---->admin  
用户提交的数据:admin , 123