Spring Data - MongoDB - JUnit test

I would have a question concerning Spring Data - MongoDB and JUnit test.

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = { UserRepository.class, User.class })
public class MyJUnitTest {

The UserRepository looks like this:

@Repository
public interface UserRepository extends MongoRepository<User, String> {
    User findByUsername(final String username);
}

I get the following Exception:

Failed to instantiate [... .repository.UserRepository]: Specified class is an interface

My question now would be, how to do it, that UserRepository is instantiate although there is no implementation class because Spring Data does the implementation by its own? If I do not mark USerRepository with @Repository than Spring does not create a bean object Thanks a lot!