<form action="com.in.User" method="post">
<div class="loginbox">
<div class="errorbox"><i class="error"></i>用户名或密码错误，请重新输入！</div>
<div class="logongroup">
<input class="intext" type="text" name="UserName" placeholder="用户名" />
<i class="user"></i>
</div>
<div class="logongroup">
<input class="intext" type="password" name="PassWord" placeholder="密码" />
<i class="password"></i>
</div>
<div class="logongroup submitbox">
<button class="submitbtn" type="submit">登&nbsp;&nbsp;录</button>
</div>
</div>
</form>

package com.in;import java.io.IOException;
import java.io.PrintWriter;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;public class User extends HttpServlet {/*** Constructor of the object.*/public User() {super();}/**　　* Destruction of the servlet. <br>*/public void destroy() {super.destroy(); // Just puts "destroy" string in log// Put your code here
    }/*** The doGet method of the servlet. <br>** This method is called when a form has its tag value method equals to get.* * @param request the request send by the client to the server* @param response the response send by the server to the client* @throws ServletException if an error occurred* @throws IOException if an error occurred*/public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {HttpSession session=request.getSession();session.setAttribute("user","yes");request.getRequestDispatcher("default.jsp").forward(request, response);}/*** The doPost method of the servlet. <br>** This method is called when a form has its tag value method equals to post.* * @param request the request send by the client to the server* @param response the response send by the server to the client* @throws ServletException if an error occurred* @throws IOException if an error occurred*/public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {}/*** Initialization of the servlet. <br>** @throws ServletException if an error occurs*/public void init() throws ServletException {// Put your code here
    }}

<%String aa=(String)session.getAttribute("user");if(!"yes".equals(aa)||aa==null){response.sendRedirect("longin.jsp");}%>