jsp网页里的验证码实现(产生四位的随机字符,由0

代码说明：

jsp网页里的验证码实现(产生四位的随机字符,由0-9,a-z,A-Z构成.并把最终字符串放到session中保存以让后续页面验证真伪) -jsp pages in the Verification Code realize (4 random characters generated by the 0-9, az, AZ pose. and ultimately save the string into session in order to allow follow-up to verify the authenticity of the page)