From my notice board example application (Spring Data JPA version). You may notice that only 2 interface is defined only on repository layer

no implementation class on these interface is defined. So what actual instance is injected into info.saladlam.example.spring.noticeboard.service.MessageServiceImpl?

That instance is a proxy, with target class instance org.springframework.data.jpa.repository.support.SimpleJpaRepository and many advice/advisor/interceptor is attached. One important interceptor is org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor.

Definition of info.saladlam.example.spring.noticeboard.repository.MessageRepository

public interface MessageRepository extends JpaRepository<Message, Long> {

	@Query("SELECT m FROM Message m WHERE m.approvedBy IS NOT NULL AND m.publishDate <= :at AND (m.removeDate IS NULL OR m.removeDate > :at) ORDER BY m.publishDate DESC")
	List<Message> findPublished(@Param("at") LocalDateTime at);

	@Query("SELECT m FROM Message m WHERE m.approvedBy IS NULL ORDER BY m.publishDate DESC")
	List<Message> findWaitingApprove();

	@Query("SELECT m FROM Message m WHERE m.owner = :owner ORDER BY m.publishDate DESC")
	List<Message> findByOwner(@Param("owner") String owner);

}

and its parent interface org.springframework.data.jpa.repository.JpaRepository

@NoRepositoryBean
public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {

	List<T> findAll();
	List<T> findAll(Sort sort);
	List<T> findAllById(Iterable<ID> ids);
	<S extends T> List<S> saveAll(Iterable<S> entities);
	void flush();
	<S extends T> S saveAndFlush(S entity);
	void deleteInBatch(Iterable<T> entities);
	void deleteAllInBatch();
	T getOne(ID id);

	@Override
	<S extends T> List<S> findAll(Example<S> example);

	@Override
	<S extends T> List<S> findAll(Example<S> example, Sort sort);

}

For example, when a method findAll() defined in JpaRepository is called, as result method findAll() of SimpleJpaRepository is called. And when a method findWaitingApprove() defined in MessageRepository is called, interceptor org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor is involved. In method org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor.doInvoke()

		@Nullable
		private Object doInvoke(MethodInvocation invocation) throws Throwable {

			Method method = invocation.getMethod();
			Object[] arguments = invocation.getArguments();

			if (hasQueryFor(method)) {
				return queries.get(method).execute(arguments);
			}

			return invocation.proceed();
		}

queries is a map, which store org.springframework.data.jpa.repository.query.JpaQueryMethod instance with key method defined in MessageRepository.