<%@ 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"/>