Servlet 实现登录页面，并在条件下跳转

Java代码 

package ServletDemo;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class DealServlet extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }

    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter out=resp.getWriter();
        resp.setContentType("text/html;charset =utf-8");//防止乱码
        String username = req.getParameter("username");//获取登录名
        String passWd = req.getParameter("passwd");//获取密码
        System.out.println("登录名" + username);
        System.out.println("密码"+ passWd);
        if(username.equals("") || passWd.equals("")){//判断密码和用户名是否为null
            String str = "You code is Error！！！";
//            req.setAttribute("Alert",str);
            //跳转到密码错误页面
//          req.getRequestDispatcher("passwdError.jsp").forward(req,resp);
            System.out.println("执行到没输入密码");
            out.println(str);
        } else{
            if(username.equals(passWd)){//判断用户名是否等于密码
                String str = "welcome!!!";
//              req.getRequestDispatcher("welcome.jsp").forward(req,resp);
                out.println(str);
                System.out.println("执行到密码等于用户名");
            }
            else{
                String str = "code Error,back to welcome";
//              req.setAttribute("Alert",str);
//              req.getRequestDispatcher("index.jsp").forward(req,resp);
                out.println(str);
                System.out.println("执行到密码错误");
            }
        }
    }
}

---

更新

package ServletDemo;
/**
 * 登录功能
 * 获取username和passwd
 * 将username和passwd封装
 * 调用UserDao的login方法，获取返回的User对象
 * 判断username和passwd的正确行，并根据条件跳转不同的页面
 */

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class DealServlet extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }
    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter out=resp.getWriter();
        //设置服务器端以UTF-8编码进行输出
        resp.setCharacterEncoding("GB2312");
        //设置浏览器以UTF-8编码进行接收,解决中文乱码问题
        resp.setContentType("text/html;charset = GB2312");//防止乱码
        String userName = req.getParameter("username");//获取登录名
        String passWd = req.getParameter("passwd");//获取密码
        System.out.println("登录名" + userName);
        System.out.println("密码"+ passWd);
        if(userName.equals("") || passWd.equals("")){//判断密码和用户名是否为null
//            String str = "密码错误！！！";
//            req.setAttribute("Alert",str);
            //跳转到密码错误页面
//          req.getRequestDispatcher("passwdError.jsp").forward(req,resp);
            resp.sendRedirect("http://localhost:8080/javaWeb_war_exploded/ServletDemo/PassWdError");//跳转页面
//            http://localhost:8080/javaWeb_war_exploded/ServletDemo/DateServlet
//            http://localhost:8080/javaWeb_war_exploded/ServletDemo/PassWdError
//            System.out.println("执行到没输入密码");//
//            out.println(str);
        } else{
            if(userName.equals(passWd)){//判断用户名是否等于密码
//                String str = "welcome!!!";
//              req.getRequestDispatcher("welcome.jsp").forward(req,resp);
//                out.println(str);
                //resp.sendRedirect("");
                
                System.out.println("执行到密码等于用户名");
            }
            else{
                String str = "code Error,back to welcome";
//              req.setAttribute("Alert",str);
//              req.getRequestDispatcher("index.jsp").forward(req,resp);
//                out.println(str);
                resp.sendRedirect("http://localhost:8080/javaWeb_war_exploded/ServletDemo/PassWdError");//跳转页面
                System.out.println("执行到密码错误");
            }
        }
    }
}

html文件 

<%@ page contentType="text/html;charset=utf-8" language="java" %>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>用户登录示例</title>
</head>
<body>
<div style="margin: auto; text-align: center;">
    <form action="ServletDemo/DealServlet" method="post">
        用户名：<input type="text" name="username" />
        密 码：<input type="password" name="passwd" />
                <input type="submit" value="登 录" />
    </form>
</div>
</body>
</html>