java随机产生验证码，可以随机生成数字、大写字母、小写字母。还可以随机生成文字字体、及大小。在图片上面可能字体都不不同、大小不等(Java randomly generates verification code, which can generate numbers, uppercase letters and lowercase letters randomly. You can also randomly generate text fonts, and size. On the top of the picture, there may be different fonts and sizes.)