spring-data-jpa

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:

1. Extend the appropriate repository interface:

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

1. Use derived queries for simple conditions:

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

1. Implement custom queries with @Query:

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

Configure Entities

1. 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;
   }

1. 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.

1. Set up database auditing:

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

Apply Query Patterns

1. Use derived queries for simple conditions
2. Use @Query for complex queries
3. Return Optional<T> for single results
4. Use Pageable for pagination
5. Apply @Modifying for update/delete operations

Manage Transactions

1. Mark read-only operations with @Transactional(readOnly = true)
2. Use explicit transaction boundaries for modifying operations
3. 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](references/examples.md) - Complete code examples for common scenarios
• [Reference](references/reference.md) - 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.