code: public void setDefaultMapper(Type type, Mapper mapper) {
      if (mapper == null) {
        throw new IllegalArgumentException("mapper cannot be null");
      }
      lock.lock();
      try {
        cache.put(type, mapper);
      } finally {
        lock.unlock();
      }
    }
docs: Sets or registers the given mapper for the given type. This method must be called before
  performing any persistence operations, preferably, during application startup. Entities that
  were introspected before calling this method will NOT use the new mapper.
  
  @param type
  the type
  @param mapper
  the mapper to use for the given type