生成有4個隨機數字和雜亂背景的圖片，數字和背景顏色會改變，服務器端刷新（用history.go(-1)也會變）
原型參考ALIBABA??
http://china.alibaba.com/member/showimage------------產生驗證碼圖片的文件-----image.jsp-------------------------------------------
<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" %>
<%!
Color getRandColor(int fc,int bc){//給定范圍獲得隨機顏色
????????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);
????????}
%>
<%
//設置頁面不緩存
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);
// 在內存中創建圖象
int width=60, height=20;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
// 獲取圖形上下文
Graphics g = image.getGraphics();
//生成隨機類
Random random = new Random();
// 設定背景色
g.setColor(getRandColor(200,250));
g.fillRect(0, 0, width, height);
//設定字體
g.setFont(new Font("Times New Roman",Font.PLAIN,18));
//畫邊框
//g.setColor(new Color());
//g.drawRect(0,0,width-1,height-1);
// 隨機產生155條干擾線，使圖象中的認證碼不易被其它程序探測到
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);
}
// 取隨機產生的認證碼(4位數字)
String sRand="";
for (int i=0;i<4;i++){
????String rand=String.valueOf(random.nextInt(10));
????sRand+=rand;
????// 將認證碼顯示到圖象中
????g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));//調用函數出來的顏色相同，可能是因為種子太接近，所以只能直接生成
????g.drawString(rand,13*i+6,16);
}
// 將認證碼存入SESSION
session.setAttribute("rand",sRand);
// 圖象生效
g.dispose();
// 輸出圖象到頁面
ImageIO.write(image, "JPEG", response.getOutputStream());
%>
---------------使用驗證碼圖片的文件---------a.jsp------------------------------------
<%@ page contentType="text/html;charset=gb2312" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>認證碼輸入頁面</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<META HTTP-EQUIV="Pragma" CONTENT="no-cache">
<META HTTP-EQUIV="Cache-Control" CONTENT="no-cache">
<META HTTP-EQUIV="Expires" CONTENT="0">
</head>
<body>
<form method=post action="check.jsp">
<table>
<tr>
<td align=left>系統產生的認證碼：</td>
<td><img border=0 src="image.jsp"></td>
</tr>
<tr>
<td align=left>輸入上面的認證碼：</td>
<td><input type=text name=rand maxlength=4 value=""></td>
</tr>
<tr>
<td colspan=2 align=center><input type=submit value="提交檢測"></td>
</tr>
</form>
</body>
</html>
-----------------驗證的頁面----------check.jsp
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<html>
<head>
<title>認證碼驗證頁面</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<META HTTP-EQUIV="Pragma" CONTENT="no-cache">
<META HTTP-EQUIV="Cache-Control" CONTENT="no-cache">
<META HTTP-EQUIV="Expires" CONTENT="0">
</head>
<body>
<%
String rand = (String)session.getAttribute("rand");
String input = request.getParameter("rand");
%>
系統產生的認證碼為： <%= rand %><br>
您輸入的認證碼為： <%= input %><br>
<br>
<%
??if (rand.equals(input)) {
%>
<font color=green>輸入相同，認證成功！</font>
<%
??} else {
%>
<font color=red>輸入不同，認證失敗！</font>
<%
??}
%>
</body>
</html>