code: public static boolean defined(String group, String unit) {
          return LocalUnitsManager.getLocalUnit(group, unit) != null ||
                  UnitDiscovery.singleton != null && UnitDiscovery.singleton.newestDefinition(Unit.fullName(group, unit)) != null;
      }
docs: Judge whether the specified unit is defined.
  tips: This method is for high performance high concurrent, frequently exception creation, throwing and catching is bad design.
  So we do not use {@link UnitRouter#newestDefinition(String)} to judge the unit.