验证码的实现分为JSP页面和Java类Servlet两部分：
JSP页面：
<!DOCTYPE html>
<html>
<head>
<title>loginForm.html</title>
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="this is my page">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<!--<link rel="stylesheet" type="text/css" href="./styles.css">-->
</head>
<body>
<h3>带有验证码的登陆页面</h3>
<form method="post" action="login.yan" >
用户名：<input type="text" name="usreName" size="10"><br><br/>
密 码：<input type="password" name="password" size="10"><br><br>
验证码：<input type="text" name="userNum" size="4"><img src="loginForm"/><br/><br/>
<input type="submit" value="提交">
</form>
</body>
</html>
Java类：
