servlet登录验证并返回错误信息

index.jsp(登录界面)

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><base href="<%=basePath%>"><title>用户登录</title><meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0">    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page"><!--<link rel="stylesheet" type="text/css" href="styles.css">--><style type="text/css"></style></head>  <body><div><form action="loginServlet" method="post"><table align="center" ><tr><td>用户名:</td><td><input type="text" name="username"> <font color="red">${requestScope.namemsg}${requestScope.nameError}</font></td></tr><tr><td>密码:</td><td><input type="password" name="password"> <font color="red">${requestScope.pwdError}${requestScope.pwdmsg}</font></td></tr><tr><td></td><td><input type="submit" value="登录"><input type="reset" value="取消"></td></tr></table></form></div></body>
</html>

loginServlet(登录验证)

package com.dwx.servlet;
import java.io.IOException;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.dwx.bean.user;
import com.dwx.bean.userDao;/*** 用户登录验证*/
@WebServlet("/loginServlet")
public class loginServlet extends HttpServlet {private static final long serialVersionUID = 1L;/*** @see HttpServlet#HttpServlet()*/public loginServlet() {super();// TODO Auto-generated constructor stub}/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doPost(request, response);}/*** @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)*/protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String username=request.getParameter("username");String password=request.getParameter("password");//判断用户名、密码是否为空if(username==""||username.length()==0){request.setAttribute("namemsg","用户名为空!");request.getRequestDispatcher("index.jsp").forward(request, response);}if(password==""||password.length()==0){request.setAttribute("pwdmsg","密码为空!");request.getRequestDispatcher("index.jsp").forward(request, response);}//判断用户名、密码是否正确ResultSet rst=userDao.selectUser(username, password);try {if(rst.next()){if(password.equals(rst.getString("password"))){request.setAttribute("username",username);request.getRequestDispatcher("items.jsp").forward(request, response);}else{request.setAttribute("pwdError","密码不正确!");request.getRequestDispatcher("index.jsp").forward(request, response);}}else{request.setAttribute("nameError","用户名不存在!");request.getRequestDispatcher("index.jsp").forward(request, response);}} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}

userDao.java(查询用户)

package com.dwx.bean;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;
import com.dwx.util.dao;
public class userDao {public static ResultSet selectUser(String username,String password){Connection conn=dao.getConnection();String sql="select * from user where username=?";ResultSet rst=null;try{PreparedStatement prst=conn.prepareStatement(sql);prst.setString(1, username);rst=prst.executeQuery();}catch(Exception ex){ex.printStackTrace();}return rst;}
}

dao.java(建立数据库连接)

package com.dwx.util;
import java.sql.Connection;
import java.sql.DriverManager;
public class dao {public static Connection getConnection(){Connection conn=null;try{Class.forName("com.mysql.jdbc.Driver");      //加载驱动String url="jdbc:mysql://localhost:3306/db_item";String username="root";String password="root";//创建数据库连接conn=DriverManager.getConnection(url,username,password);}catch(Exception ex){ex.printStackTrace();}return conn;}
}

效果演示：

1、用户名为空

2、密码为空

3、用户名不存在

4、密码不正确

5、登录成功

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