验证码的实现原理：
在一个Servlet中生成验证，并把验证码上的数据保存在Session，用户提交验证码之后，会提交给另外一个Servlet程序。在获取用户提交数据的Servlet中的从Session中把验证码取出，在取出的同时从Session中把验证码删除。
1.注册页面：register.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%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%>" rel="external nofollow" > <title>My JSP 'login.jsp' starting page</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" rel="external nofollow" > --> <script type="text/javascript"> function _changImg(){ document.getElementById("myimg").src = "/day11/checkImg?" + new Date().getMilliseconds(); } </script> </head> <body> <center> <form action="/day11/regist" method="post"> <table> <tr> <td>用户名<font color="red">*</font></td> <td><input type="text" name="name"/></td> </tr> <tr> <td>密码</td> <td><input type="password" name="password"/></td> </tr> <tr> <td>输入验证码</td> <td> <input type="text" name="form_checkcode" /> <img style="cursor: pointer;" alt="" src="/day11/checkImg" id="myimg" onclick="_changImg();"/> <a href="javascript:void(0);" rel="external nofollow" onclick="_changImg();">看不清，换一张</a> <font color="red">${imgError }</font> </td> </tr> <tr> <td><input type="submit" value="注册" /></td> </tr> </table> </form> </center> </body></html>2.生成验证码参考：cn.itcast.session.CheckImgServlet
public class CheckImgServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { int width = 120; int height = 40; // 先生成一张纸 BufferedImage bufi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); // 画笔 Graphics g = bufi.getGraphics(); // 设置背景颜色 // 修改画笔颜色 g.setColor(Color.white); // 填充 g.fillRect(0, 0, width, height); // 绘制边框 // 设置边框颜色 g.setColor(Color.red); // 画边框 g.drawRect(0, 0, width - 1, height - 1); // 准备一些数据 String data = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyz"; // 生成随机对象 Random r = new Random(); String checkcode = ""; // 生成4个随机的数字 for (int i = 0; i < 4; i++) { // 生成随机颜色 g.setColor(new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255))); // 设置字体 g.setFont(new Font("宋体", Font.ITALIC, 25)); String c = data.charAt(r.nextInt(data.length())) + ""; g.drawString(c, 10 + (20 * i), 30); checkcode += c; } // 将生成的验证码放到 session中 request.getSession().setAttribute("session_checkcode", checkcode); // 画干扰线 for (int i = 0; i < 8; i++) { // 设置随机颜色 g.setColor(new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255))); // 画线 两点确定一线 g.drawLine(r.nextInt(width), r.nextInt(height), r.nextInt(width), r.nextInt(height)); } // 将图片输出到浏览器 ImageIO.write(bufi, "jpg", response.getOutputStream()); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { }}3.验证码的验证参考：cn.itcast.session.RegistServlet
public class RegistServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // 校验验证码的有效性: 服务端生成的验证码 和 表单提交的验证码一致才算有效 // 服务端生成的验证码 保存在 session容器中 // 获得session中的验证码 HttpSession session = request.getSession(); String session_checkcode = (String) session.getAttribute("session_checkcode"); // 使用完后，迅速清除session中验证码的属性 session.removeAttribute("session_checkcode"); // 获得表单提交的验证码 String form_checkcode = request.getParameter("form_checkcode"); // 判断什么是非法 如果非法，终止 if (session_checkcode == null || !session_checkcode.equals(form_checkcode)) { // 说明验证码错误 request.setAttribute("imgError", "验证码错误！"); // 将request对象转发给 login.jsp request.getRequestDispatcher("/login.jsp").forward(request, response); // 结束当前方法 return; } // 如果合法， 继续校验用户名和密码的有效 String username = request.getParameter("username"); String password = request.getParameter("password"); }}以上所述是小编给大家介绍的通过Session案例分析一次性验证码登录问题，希望对大家有所帮助，如果大家有任何疑问欢迎给我留言，小编会及时回复大家的！
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