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！
