Hibernate, how would we handle if we had two classes implementing the service interface?

You could accomplish this with Spring qualifiers. Consider these services:

@Service("parent")
public SomeServiceImpl implements SomeService { ... }

and this for the child class:

@Service("child")
public SomeChildServiceImpl extends SomeServiceImpl { ... }

When autowiring, use the @Qualifier annotation to specify which version you'd like:

@Autowired
@Qualifier("parent")
private SomeService parent;

@Autowired
@Qualifier("child")
private SomeService child;

Maybe you asked this question completely out of curiosity - there is serious value there. :) But, if you're looking to actually use this approach in the scenario you mentioned, I'd think carefully about object-oriented and service-DAO design principles. Typically you'd roll your service operations for a single resource and for collections of that resource into a single class. You could definitely create a base class for all your services, but that's about reducing the amount of code duplication by consolidating logic. Your controller shouldn't have to make a decision on which service "version" to use. Doing this forces your controller layer to know more about the service layer, which is a step away from "loose coupling".