spring-data-jpa

SKILL.md

Spring Data JPA

Overview

Provides patterns for Spring Data JPA repositories, entity relationships, queries, pagination, auditing, and transactions.

When to Use

Creating repositories with CRUD operations, entity relationships, @Query annotations, pagination, auditing, or UUID primary keys.

Instructions

Create Repository Interfaces

To implement a repository interface:

Extend the appropriate repository interface:

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // Custom methods defined here
}

Use derived queries for simple conditions:

Optional<User> findByEmail(String email);
List<User> findByStatusOrderByCreatedDateDesc(String status);

Implement custom queries with @Query:

@Query("SELECT u FROM User u WHERE u.status = :status")
List<User> findActiveUsers(@Param("status") String status);

Configure Entities

Define entities with proper annotations:

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, length = 100)
    private String email;
}

Configure relationships using appropriate cascade types:

@OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
private List<Order> orders = new ArrayList<>();

Validation: Test cascade behavior with a small dataset before applying to production data. Verify delete operations don't cascade unexpectedly.

Set up database auditing:

@CreatedDate
@Column(nullable = false, updatable = false)
private LocalDateTime createdDate;

Apply Query Patterns

Use derived queries for simple conditions

Use @Query for complex queries

Return Optional for single results

Use Pageable for pagination

Apply @Modifying for update/delete operations

Manage Transactions

Mark read-only operations with @Transactional(readOnly = true)

Use explicit transaction boundaries for modifying operations

Specify rollback conditions when needed

Validate and Optimize

1. Verify entity configuration:

Test cascade behavior in a transaction before production deployment

Validate bidirectional relationships sync correctly

2. Optimize query performance:

Run EXPLAIN ANALYZE on queries against large tables

If performance issues detected: add indexes → verify with EXPLAIN → repeat

Use @EntityGraph to prevent N+1 queries

3. Validate pagination:

Ensure indexed columns support pagination queries

Test with large datasets to verify cursor stability

Examples

Basic CRUD Repository

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
    // Derived query
    List<Product> findByCategory(String category);

    // Custom query
    @Query("SELECT p FROM Product p WHERE p.price > :minPrice")
    List<Product> findExpensiveProducts(@Param("minPrice") BigDecimal minPrice);
}

Pagination Implementation

@Service
public class ProductService {
    private final ProductRepository repository;

    public Page<Product> getProducts(int page, int size) {
        Pageable pageable = PageRequest.of(page, size, Sort.by("name").ascending());
        return repository.findAll(pageable);
    }
}

Entity with Auditing

@Entity
@EntityListeners(AuditingEntityListener.class)
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @CreatedDate
    @Column(nullable = false, updatable = false)
    private LocalDateTime createdDate;

    @LastModifiedDate
    private LocalDateTime lastModifiedDate;

    @CreatedBy
    @Column(nullable = false, updatable = false)
    private String createdBy;
}

Best Practices

Entity Design

Use constructor injection exclusively (never field injection)

Prefer immutable fields with final modifiers

Use Java records (16+) or @Value for DTOs

Always provide proper @Id and @GeneratedValue annotations

Use explicit @Table and @Column annotations

Performance Optimization

Use appropriate fetch strategies (LAZY vs EAGER)

Implement pagination for large datasets

Use database indexes for frequently queried fields

Consider using @EntityGraph to avoid N+1 query problems

Reference Documentation

For comprehensive examples, detailed patterns, and advanced configurations, see:

Examples - Complete code examples for common scenarios

Reference - Detailed patterns and advanced configurations

Constraints and Warnings

Never expose JPA entities directly in REST APIs; always use DTOs to prevent lazy loading issues.

Avoid N+1 query problems by using @EntityGraph or JOIN FETCH in queries.

Be cautious with CascadeType.REMOVE on large collections as it can cause performance issues.

Do not use EAGER fetch type for collections; it can cause excessive database queries.

Avoid long-running transactions as they can cause database lock contention.

Use @Transactional(readOnly = true) for read operations to enable optimizations.

Be aware of the first-level cache; entities may not reflect database changes within the same transaction.

UUID primary keys can cause index fragmentation; consider using sequential UUIDs or Long IDs.

Pagination on large datasets requires proper indexing to avoid full table scans.

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