说明：  JSP验证码代码:生成有4个随机数字和杂乱背景的图片，数字和背景颜色会改变，服务器端刷新（用history.go(-1)也会变）
(Verification Code JSP code: has four randomly generated numbers and cluttered background images, figures and background color will change, server-side update (with history.go (-1) also will become))