code: private void init() {
          writers.put(Boolean.class, new BooleanDataWriter());
          writers.put(Calendar.class, new CalendarDataWriter());
          writers.put(Date.class, new DateDataWriter());
          writers.put(Enum.class, new EnumDataWriter());
          writers.put(Number.class, new NumberDataWriter());
          writers.put(String.class, new StringDataWriter());
      }
docs: 初始化，注册默认的DataWriter