public static boolean isInteger(String str) {
        if (str == null || str.isEmpty()) {
            return false;
        }

        char ch = str.charAt(0);
        boolean sign = ch == '-' || ch == '+';
        if (sign) {
            if (str.length() == 1) {
                return false;
            }
        } else if (ch < '0' || ch > '9') {
            return false;
        }

        for (int i = 1; i < str.length(); ++i) {
            ch = str.charAt(i);
            if (ch < '0' || ch > '9') {
                return false;
            }
        }
        return true;
    }
decimal is integer, check has non-zero small

@param decimal
@return
core/src/main/java/com/alibaba/fastjson2/util/TypeUtils.java