Spring Data JPA Skill

Comprehensive guide to data access with Spring Data JPA including repository patterns, custom queries, and performance optimization.

Overview

This skill covers everything needed for production-ready database access with Spring Data JPA.

Parameters

Name	Type	Required	Default	Validation
database	enum	✗	postgresql	postgresql | mysql | h2
migration_tool	enum	✗	flyway	flyway | liquibase | none
fetch_strategy	enum	✗	lazy	lazy | eager

Topics Covered

Core (Must Know)

• Entities: @Entity, @Table, @Id, relationships
• Repositories: JpaRepository, derived queries
• Transactions: @Transactional, propagation

Intermediate

• Custom Queries: @Query with JPQL and native SQL
• Projections: Interface-based, class-based
• Auditing: @CreatedDate, @LastModifiedDate

Advanced

• Specifications: Dynamic query building
• Entity Graphs: Fetch optimization
• N+1 Prevention: JOIN FETCH strategies

Code Examples

Entity with Relationships

@Entity
@Table(name = "orders")
@EntityListeners(AuditingEntityListener.class)
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(unique = true, nullable = false)
    private String orderNumber;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id", nullable = false)
    private Customer customer;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<OrderItem> items = new ArrayList<>();

    @CreatedDate
    private LocalDateTime createdAt;

    @Version
    private Long version;

    public void addItem(OrderItem item) {
        items.add(item);
        item.setOrder(this);
    }
}

Repository with Custom Queries

public interface OrderRepository extends JpaRepository<Order, Long>,
                                         JpaSpecificationExecutor<Order> {

    Optional<Order> findByOrderNumber(String orderNumber);

    @Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.customer.id = :customerId")
    List<Order> findByCustomerWithItems(@Param("customerId") Long customerId);

    @EntityGraph(attributePaths = {"items", "customer"})
    Optional<Order> findWithDetailsById(Long id);

    @Modifying
    @Query("UPDATE Order o SET o.status = :status WHERE o.id = :id")
    int updateStatus(@Param("id") Long id, @Param("status") OrderStatus status);
}

Specification Pattern

public class OrderSpecifications {

    public static Specification<Order> hasStatus(OrderStatus status) {
        return (root, query, cb) ->
            status == null ? null : cb.equal(root.get("status"), status);
    }

    public static Specification<Order> createdAfter(LocalDateTime date) {
        return (root, query, cb) ->
            date == null ? null : cb.greaterThan(root.get("createdAt"), date);
    }
}

// Usage
Specification<Order> spec = Specification
    .where(OrderSpecifications.hasStatus(status))
    .and(OrderSpecifications.createdAfter(since));
Page<Order> orders = orderRepository.findAll(spec, pageable);

Troubleshooting

Failure Modes

Issue	Diagnosis	Fix
N+1 queries	Multiple SELECTs	Use @EntityGraph or JOIN FETCH
LazyInitializationException	Access outside session	Use @Transactional or DTO
Data not saved	Missing @Transactional	Add to service method

Debug Checklist

□ Enable SQL logging: spring.jpa.show-sql=true
□ Check for N+1 with hibernate.generate_statistics
□ Verify @Transactional on write operations
□ Review fetch types (LAZY vs EAGER)

Unit Test Template

@DataJpaTest
@Testcontainers
class OrderRepositoryTest {

    @Container
    @ServiceConnection
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15");

    @Autowired
    private OrderRepository orderRepository;

    @Test
    void shouldFindByOrderNumber() {
        Order order = new Order();
        order.setOrderNumber("ORD-001");
        orderRepository.save(order);

        Optional<Order> found = orderRepository.findByOrderNumber("ORD-001");
        assertThat(found).isPresent();
    }
}

Usage

Skill("spring-data-jpa")

Version History

Version	Date	Changes
2.0.0	2024-12-30	Specifications, auditing, performance patterns
1.0.0	2024-01-01	Initial release