验证码是用来进行登录验证的一个有效方式，下面就为大家讲解如何在jsp中使用。下面仅介绍最简单的实现方法，稍后我们还会加入Ajax等来使验证更为正式。
本文直接使用jsp页面作为载体，也就是说直接把生成验证码的图片放在了jsp中来实现，感兴趣的朋友可以放在servlet中，这样更方便管理。
1.image.jsp---生成随机验证码图片的Jsp页面
<%@
page contentType="image/jpeg" import="java.awt.*,
java.awt.image.*,java.util.*,javax.imageio.*" %>
<%!
Color getRandColor(int fc,int bc)
{
Random random = new Random();
if(fc>255) fc=255;
if(bc>255) bc=255;
int r=fc+random.nextInt(bc-fc);
int g=fc+random.nextInt(bc-fc);
int b=fc+random.nextInt(bc-fc);
return new Color(r,g,b);
}
%>
<%
out.clear();//这句针对resin服务器，如果是tomacat可以不要这句
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);
int width=60, height=20;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();
Random random = new Random();
g.setColor(getRandColor(200,250));
g.fillRect(0, 0, width, height);
g.setFont(new Font("Times New Roman",Font.PLAIN,18));
g.setColor(getRandColor(160,200));
for (int i=0;i<155;i++)
{
int x = random.nextInt(width);
int y = random.nextInt(height);
int xl = random.nextInt(12);
int yl = random.nextInt(12);
g.drawLine(x,y,x+xl,y+yl);
}
String sRand="";
for (int i=0;i<4;i++){
String rand=String.valueOf(random.nextInt(10));
sRand+=rand;
g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));
g.drawString(rand,13*i+6,16);
}
// 将认证码存入SESSION
session.setAttribute("rand",sRand);
g.dispose();
ImageIO.write(image, "JPEG", response.getOutputStream());
%>
效果如下：
2.logic.jsp----登陆页面，在这里提供输入验证码然后提交并验证
，这个页面里加载image.jsp实现验证
<%@ page contentType="text/html;charset=gbk" %>
<%@ page language="java" import="java.sql.*" errorPage="" %>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>用户登录</title>
<script language="javascript">
function loadimage(){
document.getElementById("randImage").src = "image.jsp?"+Math.random();
}
</script>
</head>
<body>
<table width="256" border="0" cellpadding="0" cellspacing="0">
<!--DWLayoutTable-->
<form action="validate.jsp" method="post" name="loginForm">
<tr>
<td width="118" height="22" valign="middle" align="center"><input type="text" name="rand" size="15"></td>
<td width="138" valign="middle" align="center"><img alt="code..." name="randImage" id="randImage"
src="image.jsp" width="60" height="20" border="1" align="absmiddle"></td>
</tr>
<tr>
<td height="36" colspan="2" align="center" valign="middle"><a href="javascript:loadimage();"><font
class=pt95>看不清点我</font></a></td>
</tr>
<tr>
<td height="36" colspan="2" align="center" valign="middle"><input type="submit" name="login"
value="提交"></td>
</tr>
</form>
</table>
</body>
</html>
效果如下：
3.validate.jsp----用来验证输入的验证码是否正确
<%@
page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<%
String rand = (String)session.getAttribute("rand");
String input = request.getParameter("rand");
if(rand.equals(input)){
out.print("<script>alert('验证通过！');</script>");
} else{
out.print("<script>alert('请输入正确的验证码！');location.href='login.jsp';</script>");
}
%>
效果如下;
因为我之前输入的验证码不正确，所以如上显示。