login.jsp
<form action="login.jsp" method="post">
          账号：<input type="text" name="uname"><br>
          密码：<input type="password" name="upw"><br>
          <input type="submit" value="登    陆">            
        </form>
        
        <%
          if(request.getParameter("uname") != null && request.getParameter("upw") !=null){    
            
            String name = request.getParameter("uname");
            String pw = request.getParameter("upw");            
            
            if(name.equals("admin")&& pw.equals("1234")){
            
            //若用户成功登陆，则设置flag属性值为OK
            //则在欢迎页上，根据属性flag值的判定，可以知道用户是否登陆成功了
            
              session.setAttribute("flag","OK");
                
              //可以利用session把用户名传递给欢迎页
              session.setAttribute("uname",name);
            
            //登陆成功后，跳转至欢迎页
              response.sendRedirect("welcome.jsp");
            
            }else{            
        %>
          <h1>登陆失败！</h1>
        <%
            }
          }
        %>

welcome.jsp
<%-- 欢迎页：先判定用户是否已经成功登陆（根据属性flag是否被设置）
            
            若flag有值，说明登陆验证是成功的，则显示欢迎消息
            
            否则，3秒钟后转回 login.jsp页
            
     --%>

<%    
          if(session.getAttribute("flag")!= null){
%>
          <h1>欢迎<%= (String)session.getAttribute("uname") %>光临！</h1>
<%
          }else{            
%>        
            <h1>您还未登陆，请登陆后再访问！</h1>            
<%
          response.setHeader("refresh","3;url=login.jsp");
          }
%>