code: public static <T> Maybe<Set<T>> keys(CacheConfigBean cacheConfigBean, String key, Class<T> kClazz) {
          return keys(cacheConfigBean, key)
                  .map(keySet -> {
                      Set<T> _keys = new TreeSet<>();
                      keySet.forEach(_key -> _keys.add(Reflection.toType(_key, kClazz)));
                      return _keys;
                  });
      }
docs: retrial all the keys of the cached map
  
  @param cacheConfigBean datasource configuration
  @param key             cache key
  @param kClazz          the key's class
  @return all the keys of the cached map, or null if the key does not exist