code: public static String generateVerifyCode(int verifySize, String sources){
          if(sources == null || sources.length() == 0){
              sources = VERIFY_CODES;
          }
          int codesLen = sources.length();
          Random rand = new Random(System.currentTimeMillis());
          StringBuilder verifyCode = new StringBuilder(verifySize);
          for(int i = 0; i < verifySize; i++){
              verifyCode.append(sources.charAt(rand.nextInt(codesLen-1)));
          }
          return verifyCode.toString();
      }
docs: 使用指定源生成验证码
  @param verifySize    验证码长度
  @param sources   验证码字符源
  @return