image.jsp

<%@ page contentType="image/jpeg"%>
<%@ page import="java.awt.*"%>
<%@ page import="java.awt.image.*"%>
<%@ page import="java.util.*"%>
<%@ page import="javax.imageio.*"%>
<%
 //在内存中创建图象
 int iWidth=130,iHeight=18;
 BufferedImage image=new BufferedImage(iWidth,iHeight,BufferedImage.TYPE_INT_RGB);
 //获取图形上下文
 Graphics g=image.getGraphics();
 //设定背景色
 g.setColor(Color.white);
 g.fillRect(0,0,iWidth,iHeight);
 //画边框
 g.setColor(Color.black);
 g.drawRect(0,0,iWidth-1,iHeight-1);
 //取随机产生的认证码(4位数字)
 String rand=request.getParameter("Rand");
 rand=rand.substring(0,rand.indexOf("."));
 switch(rand.length())
 {
  case 1:rand="000"+rand;break;
  case 2:rand="00"+rand;break;
  case 3:rand="0"+rand;break;
  default:rand=rand.substring(0,4);break;
 }
 //将认证码存入SESSION
 session.setAttribute("Rand",rand);
 //将认证码显示到图象中
 g.setColor(Color.black);
 g.setFont(new Font("Times New Roman",Font.PLAIN,18));
 g.drawString(rand,10,15);
 //随机产生88个干扰点,使图象中的认证码不易被其它程序探测到
 Random random=new Random();
 for(int iIndex=0;iIndex<88;iIndex++)
 {
  int x=random.nextInt(iWidth);
  int y=random.nextInt(iHeight);
  g.drawLine(x,y,x,y);
 }
 //图象生效
 g.dispose();
 //输出图象到页面
 ImageIO.write(image,"JPEG",response.getOutputStream());
%>

输出时应用验证码

<script>document.write("<img border=0 src='/image.jsp?Rand="+Math.random()*10000+"'>");</script><br>
   &nbsp;&nbsp;输认证码:&nbsp;<input name="Rand" maxlength=4 size="10"><br>

登录认证:

 String Rand=request.getParameter("Rand");
 String strRand=(String)session.getAttribute("Rand");
 if(!strRand.equals(Rand))
 {
  //未通过;
 }