code: public static BigDecimal calculate(String formula) throws Exception {
          formula = formula.replaceAll("\\s", "");
          if (isFormula(formula)) {
              return doCalculate(formula);
          } else {
              throw new Exception("calculation formula is not valid, please check up on it carefully.");
          }
      }
docs: 连续计算，使用默认精度（100）
  
  @param formula 表达式
  
  @return {@link BigDecimal}
  
  @throws Exception 异常