<%@ page language="java" import="java.util.*,java.sql.*" pageEncoding="UTF-8"%>
<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,javax.imageio.*"%>

<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<%!
Color getRandColor(int fc,int bc)//给定范围随机选颜色
{
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);
}
%>
<%
response.setHeader("Pragma","No-cache");//设置页面不缓冲
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();//获取图像上下文
Random random = new Random();//生成随机对象
g.setColor(getRandColor(200,250));
g.fillRect(0,0,width,height);
g.setFont(new Font("Times New Roman",Font.PLAIN,18));//设置字体
for(int i=0;i<155;i++)
{
int x=random.nextInt(width);
int y=random.nextInt(height);
int x1 = random.nextInt(12);
int y1 = random.nextInt(12);
g.drawLine(x,y,x+x1,y+y1);
}
//随机产生验证码
String sRand = "";
for(int i=0;i<4;i++)
{
String rand = String.valueOf(random.nextInt(10));
sRand+=rand;
//将验证码显示到图像中
g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));
//设置显示随机数的颜色
g.drawString(rand,13*i+6,16);
}
//将验证码存放到session中
session.setAttribute("rand",sRand);
//图像生效
g.dispose();
//输出图像到页面
ImageIO.write(image,"JPEG",response.getOutputStream());
out.clear();
out = pageContext.pushBody();

%>

可以在登录login.jsp下直接通过： 这种格式调用；
验证码:<input class="imgbutton" name="checknum" type="text" size="15" id ="checknum" maxlength="4"/>
<img src="checknum.jsp" alt="Change" border="1" onclick="changeCheckNum()"/><br/>
<input type="submit" name="submit" value="login"/>