Data access without leaking JDBC upstairs

Repositories speak persistence; services speak use-cases

Spring Data JPA can derive queries from method names, and @Query handles the rest. The rule of thumb: repositories return domain objects or values, never ResultSets, never JPA-specific types leaking into controllers. Services orchestrate multi-step operations inside a @Transactional boundary.

interface InvoiceRepository extends JpaRepository<Invoice, Long> {
    List<Invoice> findByCustomerIdAndStatus(long customerId, InvoiceStatus status);

    @Query("select i from Invoice i where i.dueDate < :before and i.status = 'OPEN'")
    List<Invoice> findOverdue(@Param("before") LocalDate before);
}