code: public static boolean isAlreadyRegisteredIn(ObjectMapper mapper) {
  
  		Assert.notNull(mapper, "ObjectMapper must not be null!");
  		return LinkMixin.class.equals(mapper.findMixInClassFor(Link.class));
  	}
docs: Returns whether the module was already registered in the given {@link ObjectMapper}.
  
  @param mapper must not be {@literal null}.
  @return