简介：

同样需要用到EL表达式，那么就需要导入对应的jar包；

参考：https://www.cnblogs.com/0099-ymsml/p/16143473.html

使用session记录请求访问的信息，然后再通过过滤器中判断是否放行，不放行则打回登录页面；

首先需要创建的文件有：

AdminFilter：

package demoFilter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpFilter;
import javax.servlet.http.HttpServletRequest;

public class AdminFilter implements Filter {

    public void destroy() {
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        /**
         * 得到session 
         *         判断session域中是否存在admin，存在则放行 
         * 如果没有则返回登录页面。
         */
        // 向下转型
        HttpServletRequest req = (HttpServletRequest) request;
        String admin = (String) req.getSession().getAttribute("admin");
        System.out.println("AdminFilter:" + admin);
        if (admin != null) {
            chain.doFilter(request, response);
            System.out.println("AdminFilter:admin通过");
        }else {
            req.setAttribute("msg", "必须要管理员才能进入！！！");
            req.getRequestDispatcher("/login.jsp").forward(request, response);
        }
    }

    public void init(FilterConfig fConfig) throws ServletException {
    }

}

UserFilter：

package demoFilter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

public class UserFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        /**
         * 得到session 判断session域中是否存在admin，存在则放行 判断session域中是否存在username，存在则放行
         * 如果没有则返回登录页面。
         */
        // 向下转型
        HttpServletRequest req = (HttpServletRequest) request;
        String admin = (String) req.getSession().getAttribute("admin");
        System.out.println("AdminFilter:" + admin);
        if (admin != null) {
            chain.doFilter(request, response);
            System.out.println("UserFilter:admin通过");
            return;
        }    
        String user = (String) req.getSession().getAttribute("user");
        System.out.println("UserFilter:" + user);
        if (user != null) {
            chain.doFilter(request, response);
            System.out.println("UserFilter:user通过");
            return;
        } else {
            req.setAttribute("msg", "你只是游客，无法进入该页面！！");
            req.getRequestDispatcher("/login.jsp").forward(request, response);
        }
    }

    public void init(FilterConfig fConfig) throws ServletException {
    }

}

LoginServlet：

package demoServlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) 
            throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset:utf-8");
        
        /**
         * 获取用户名
         * 判断用户名是否是cdml
         *         是：则是管理员
         *             跳转到管理员页面
         *         不是：则是普通用户
         * 把登录的用户名称保存到session中
         * 然后转发到index.jsp
         */
        req.getSession().removeAttribute("admin");
        req.getSession().removeAttribute("user");
        String username = req.getParameter("username");
        System.out.println("username:" + username);
        if("cdml".equals(username)) {    // 管理员账号
            System.out.println("设置管理员session");
            req.getSession().setAttribute("admin", username);
            req.getRequestDispatcher("/admin/admin.jsp").forward(req, resp);
        }else if("cd".equals(username)) {    // 普通用户账号
            System.out.println("设置普通用户session");
            req.getSession().setAttribute("user", username);
            req.getRequestDispatcher("/user/user.jsp").forward(req, resp);
        }
        else {    // 游客
            req.getSession().setAttribute("username", username);
            req.getRequestDispatcher("/index.jsp").forward(req, resp);
        }
    }
}

admin.jsp：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>ADMIN</title>
</head>
<body>
<h1>管理员页面</h1>
<a href="<c:url value='/index.jsp'/>">toIndex</a><br/>
<a href="<c:url value='/user/user.jsp'/>">toUser</a><br/>
<a href="<c:url value='/admin/admin.jsp'/>">toAdmin</a><br/>
</body>
</html>

user.jsp：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>USER</title>
</head>
<body>
<h1>普通用户页面</h1>
<a href="<c:url value='/index.jsp'/>">toIndex</a><br/>
<a href="<c:url value='/user/user.jsp'/>">toUser</a><br/>
<a href="<c:url value='/admin/admin.jsp'/>">toAdmin</a><br/>
</body>
</html>

index.jsp：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>index</title>
</head>
<body>
<h1>游客页面</h1>
<a href="<c:url value='/index.jsp'/>">toIndex</a><br/>
<a href="<c:url value='/user/user.jsp'/>">toUser</a><br/>
<a href="<c:url value='/admin/admin.jsp'/>">toAdmin</a><br/>
</body>
</html>

login.jsp：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>login</title>
</head>
<body>
<h1 align="center">登录</h1>
${ msg }
<form action="<c:url value='/LoginServlet'/>" method="post" align="center">
用户名：<input type="text" name="username"/><br/>
<input type="submit" value="登录"/>
</form>
</body>
</html>

web.xml的配置：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>day041401</display-name>
  <filter>
  <filter-name>UserFilter</filter-name>
  <filter-class>demoFilter.UserFilter</filter-class>
  </filter>
  <filter-mapping>
  <filter-name>UserFilter</filter-name>
  <url-pattern>/user/*</url-pattern>
  </filter-mapping>
  <filter>
  <filter-name>AdminFilter</filter-name>
  <filter-class>demoFilter.AdminFilter</filter-class>
  </filter>
  <filter-mapping>
  <filter-name>AdminFilter</filter-name>
  <url-pattern>/admin/*</url-pattern>
  </filter-mapping>
</web-app>