Spring Boot Data Layer for DDD

Implements DDD tactical patterns with Spring Data JPA and Spring Data JDBC in Spring Boot 4.

Technology Selection

Choose	When
Spring Data JPA	Complex queries, existing Hibernate expertise, need lazy loading
Spring Data JDBC	DDD-first design, simpler mapping, aggregate-per-table, no lazy loading

Spring Data JDBC enforces aggregate boundaries naturally—recommended for new DDD projects.

Core Workflow

1. Define aggregate root → Extend AbstractAggregateRoot<T> for domain events
2. Map value objects → Use @Embedded or @Converter for immutability
3. Create repository interface → One per aggregate root, extend appropriate base
4. Implement service layer → @Transactional on public methods, one aggregate per transaction
5. Add projections → Interface or record-based for read operations

Quick Patterns

See EXAMPLES.md for complete working examples including:

• Aggregate Root with AbstractAggregateRoot and domain events (Java + Kotlin)
• Repository with EntityGraph for N+1 prevention
• Transactional Service with proper boundaries
• Value Objects (Strongly-typed IDs, Money pattern)
• Projections for efficient read operations
• Auditing with automatic timestamps

Spring Boot 4 Specifics

• JSpecify null-safety: @NullMarked and @Nullable annotations
• AOT Repository Compilation: Enabled by default for faster startup
• Jakarta EE 11: All imports use jakarta.* namespace
• ListCrudRepository: New interface returning List<T> instead of Iterable<T>

ListCrudRepository (Spring Data 3.1+)

New repository interface returning List<T> for better API ergonomics:

// OLD: CrudRepository returns Iterable<T>
public interface UserRepository extends CrudRepository<User, Long> {
    Iterable<User> findAll();  // Requires conversion to List
}

// NEW: ListCrudRepository returns List<T>
public interface UserRepository extends ListCrudRepository<User, Long> {
    List<User> findAll();  // Direct List return
    List<User> findAllById(Iterable<Long> ids);  // Also List
}

// Can also extend both for full functionality
public interface UserRepository extends
    ListCrudRepository<User, Long>,
    ListPagingAndSortingRepository<User, Long> {
}

Benefits: No more StreamSupport.stream(iterable.spliterator(), false).toList() conversions.

Detailed References

• Examples: See EXAMPLES.md for complete working code examples
• Troubleshooting: See TROUBLESHOOTING.md for common issues and Boot 4 migration
• Aggregates & Entities: See references/AGGREGATES.md for complete patterns with value objects, typed IDs, auditing
• Repositories & Queries: See references/REPOSITORIES.md for custom queries, projections, specifications
• Transactions: See references/TRANSACTIONS.md for propagation, isolation, cross-aggregate consistency

Anti-Pattern Checklist

Anti-Pattern	Fix
FetchType.EAGER on associations	Use LAZY + @EntityGraph when needed
Returning entities from controllers	Convert to DTOs in service layer
@Transactional on private methods	Use public methods (proxy limitation)
Missing readOnly = true on queries	Add for read operations (performance)
Direct aggregate-to-aggregate references	Reference by ID only
Multiple aggregates in one transaction	Use domain events for eventual consistency

Critical Reminders

1. One aggregate per transaction — Cross-aggregate changes via domain events
2. Repository per aggregate root — Never for child entities
3. Value objects are immutable — No setters, return new instances
4. Flush before events — Call repository.save() before events dispatch
5. Test with @DataJpaTest — Use TestEntityManager for setup