How do JPA and Hibernate work together?

JPA defines annotations and interfaces such as EntityManager and EntityTransaction. Hibernate implements them and adds features such as caching and criteria queries. A persistence context is a first-level cache that holds managed entities for the duration of a transaction.

@Entity
public class Order {
    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private Customer customer;

    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderLine> lines = new ArrayList<>();
}

@Query("select o from Order o join fetch o.customer where o.id = :id")
Optional<Order> findWithCustomer(long id);

Behaviour to understand

• Dirty checking flushes changed managed entities at transaction commit
• Lazy associations load on first access and throw LazyInitializationException outside a session
• The N+1 problem issues one query for a list and one per element for a lazy association
• JPQL operates on entities and fields, not table and column names
• Detached entities are no longer tracked and must be merged or reloaded

Keep entities inside the persistence layer and map them to DTOs at the boundary. That avoids leaking lazy state into serialization and keeps the API contract stable. Use projections for read-heavy endpoints instead of loading full graphs.