Service and DAO always implement interfaces

问题

In all the MVC projects I've seen, "service" and "DAO" classes always implemented their own interfaces. But almost all the times, I haven't seen a situation in which having this interface has been useful.

Is there any reason to use interfaces in these cases? What may be the consequence of not using interfaces in "service" and "DAO" classes? I can't imagine any consequences.

回答1:

Spring is an Inversion of Control container. This, in one sense, means that the implementation of classes you use doesn't fall on the application but on its configuration. If you have a class that needs a UserRepository to store User instances, it'd be something like

class UserService {
    @Autowired
    private UserRepository userRepository;
}

interface UserRepository {
    List<User> getUsers();
    User findUserBySIN(String SIN);
    List<User> getUsersInCountry(Long couyntryId);
}

And you would have a bean declared for it

<bean class="com.myapp.UserRepositoryHibernateImpl">
   ...
</bean>

Notice this bean is UserRepositoryHibernateImpl which would implement UserRepository.

At some point in the future of the world, the Hibernate project stops being supported and you really need a feature that is only available on Mybatis so you need to change implementations. Because your UserService class is using a UserRepository declared with the interface type, only the methods visible on the interface are visible to the class. So changing the actual polymorphic type of userRepository doesn't affect the rest of the client code. All you need to change (excluding creating the new class) is

<bean class="com.myapp.future.UserRepositoryMyBatisImpl">
   ...
</bean>

and your application still works.

回答2:

There are lots of arguments in favour of interfaces, see Google.

I can added to the points other people mentioned:

1. Imagine you change your DAO implementations from Hibernate to iBatis. Dependency to interface rather than implementation would be a great help for the service layer.
2. If you use AOP or proxies using JDK dynamic proxies then your classes must implement interfaces. This is not the case for CGLIB.
3. In the service layer if you want to release your methods to other clients to call, giving them "interface as a contract" would make more sense rather than implementations.
4. If you ever want to separate services.jar from daos.jar then having interfaces on your daos would save the services.jar from recompile in case daos.jar changes.

In short, it is just good to have interfaces!

回答3:

The interface-based implementation helps in mocking them in the test suite. In our project, while testing the service layer, we mock the DAOs and provide hard coded data instead of really connecting to the DB. The same argument applies to service layer as well.

回答4:

Using interfaces early on makes your application scalable-ready and consequences of not using it is sacrificing your application's scalability.

回答5:

I've been asking myself the exact same question recently, feeling that creating an interface even if I know there's ever going to be a single implementing class is silly and adds to the bloat (every Java programmer who tried a more pragmatic language will know the feeling). That's yet another compilation module, often only created to satisfy one's inner dogmatist.

Spring seems to have evolved into a module/component oriented framework where the programmer only creates the blocks and the framework assembles it all together. This is why having more than one bean matching the criteria is a problem and complicates things (you end up using qualifiers which kind of kill the purpose of DI). Programmers will naturally try to avoid type ambiguities to minimise amount of required configuration, ideally making any given block fit into only one "slot".

In my opinion, DI's biggest advantage is not that it makes it easy to change implementations (by simply changing type of declared class in the config XML), but that it allows easier separation of dependencies, thus making it easier to test each component in separation. You don't need one-child interfaces for that.

Since reverse-engineering a class to extract its interface would be a purely mechanical task, I wouldn't worry about "what if I need to add another implementation?" argument.

Disclaimer: opinion of a small-to-mid applications developer; I'm sure the situation changes with large projects.

来源：https://stackoverflow.com/questions/18477119/service-and-dao-always-implement-interfaces