code: public synchronized static JsonMapper nonEmptyMapper() {
          if (nonEmptyJsonMapper == null) {
              nonEmptyJsonMapper = new JsonMapper(JsonInclude.Include.NON_EMPTY, false);
          }
          return nonEmptyJsonMapper;
      }
docs: 创建只输出非Null且非Empty(如List.isEmpty)的属性到Json字符串的Mapper,建议在外部接口中使用.