Implementing Repository Pattern

public interface CustomerRepository {
    Optional<Customer> findById(CustomerId id);
    Optional<Customer> findByEmail(Email email);
    void save(Customer customer);
    void delete(CustomerId id);
    Page<Customer> search(CustomerFilter filter, Pageable pageable);
}

public interface CrudRepository<T, ID> {
    Optional<T> findById(ID id);
    List<T> findAll();
    T save(T entity);
    void deleteById(ID id);
    long count();
    boolean existsById(ID id);
}

public class OrderSpecs {
    public static Specification<Order> byStatus(String s) {
        return (root, q, cb) -> cb.equal(root.get("status"), s);
    }
    public static Specification<Order> placedAfter(Instant t) {
        return (root, q, cb) -> cb.greaterThan(root.get("placedAt"), t);
    }
}
repo.findAll(byStatus("PAID").and(placedAfter(yesterday)));

Page<Order> page = orderRepo.findByStatus("OPEN",
    PageRequest.of(pageNum, size, Sort.by("placedAt").descending()));

public interface OrderRepoCustom { List<OrderSummary> reportFor(Instant from); }

public class OrderRepoCustomImpl implements OrderRepoCustom {
    @PersistenceContext private EntityManager em;
    public List<OrderSummary> reportFor(Instant from) {
        return em.createQuery("...", OrderSummary.class)
                 .setParameter("f", from).getResultList();
    }
}

public interface OrderRepository extends JpaRepository<Order, Long>, OrderRepoCustom {}