访问页面时，判断用户是否登录，若没有登录，将跳转到登录页面（一）

当用户没有登录，直接访问servlet容器时，就会跳转到登陆页面去让用户登录。（还可以用filter过滤器进行统一过滤，那种方法很简单，建议使用过滤器拦截吗，见访问页面时，判断用户是否登录，若没有登录，将跳转到登录页面（二））

1.登录页面login.jsp

package com.servlet;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.javabean.User;

public class UserLoginServlet extends HttpServlet
{
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String username = (String)req.getParameter("username");
String password = (String)req.getParameter("password");
String authority = (String)req.getParameter("authority");

if("1".equals(authority))
{
//登录的是普通用户
if("zhangsan".equals(username) && "123".equals(password))
{
//将用户的信息放置到session中
User user = new User();
user.setUsername(username);
user.setPassword(password);
user.setAuthority(authority);

HttpSession session = req.getSession();
session.setAttribute("user", user);
req.getRequestDispatcher("index.jsp").forward(req, resp);
return;
}

}

else  if("2".equals(authority))
{
//登录的是系统管理员
if("lisi".equals(username) && "456".equals(password))
{
//将用户的信息放置到session中
User user = new User();
user.setUsername(username);
user.setPassword(password);
user.setAuthority(authority);

HttpSession session = req.getSession();
session.setAttribute("user", user);
req.getRequestDispatcher("index.jsp").forward(req, resp);
return;

}
}
//表示登录失败

System.out.println("shibai");

//请求转发模式
//            req.setAttribute("username", username);
//            req.setAttribute("authority",authority);
//            RequestDispatcher rd = req.getRequestDispatcher("login.jsp");
//            rd.forward(req, resp);

//重定向
resp.sendRedirect("login.jsp?error=true&username="+username+"&authority="+authority);

}
}

UpdateServlet