Spring Modulith for Bounded Contexts

Implements DDD bounded contexts as application modules with enforced boundaries and event-driven communication.

Core Concepts

| Concept | Description | |---------|-------------| | Application Module | Package-based boundary = bounded context | | Module API | Types in base package (public) | | Internal | Types in sub-packages (encapsulated) | | Events | Cross-module communication mechanism |

Module Structure

src/main/java/
├── com.example/
│   └── Application.java              ← @SpringBootApplication
├── com.example.order/                ← Module: order
│   ├── OrderService.java             ← Public API
│   ├── OrderCreated.java             ← Public event
│   ├── package-info.java             ← @ApplicationModule config
│   └── internal/                     ← Encapsulated
│       ├── OrderRepository.java
│       └── OrderEntity.java
├── com.example.inventory/            ← Module: inventory
│   ├── InventoryService.java
│   └── internal/
└── com.example.shipping/             ← Module: shipping

Types in com.example.order = public API Types in com.example.order.internal = hidden from other modules

Quick Patterns

See EXAMPLES.md for complete working examples including:

• Module Configuration with @ApplicationModule
• Event Publishing with domain event records
• Event Handling with @ApplicationModuleListener (Java + Kotlin)
• Module Verification Test with PlantUML generation
• Event Externalization for Kafka/AMQP

Spring Boot 4 / Modulith 2.0 Specifics

• @ApplicationModuleListener combines @Async + @Transactional(REQUIRES_NEW) + @TransactionalEventListener(AFTER_COMMIT)
• Event Externalization with @Externalized annotation for Kafka/AMQP
• JDBC event log ensures at-least-once delivery

Detailed References

• Examples: See EXAMPLES.md for complete working code examples
• Troubleshooting: See TROUBLESHOOTING.md for common issues and Boot 4 migration
• Module Structure: See references/MODULE-STRUCTURE.md for package conventions, named interfaces, dependency rules
• Event Patterns: See references/EVENTS.md for publishing, handling, externalization, testing with Scenario API

Anti-Pattern Checklist

| Anti-Pattern | Fix | |--------------|-----| | Direct bean injection across modules | Use events or expose API | | Synchronous cross-module calls | Use @ApplicationModuleListener | | Module dependencies not declared | Add allowedDependencies in @ApplicationModule | | Missing verification test | Add ApplicationModules.verify() test | | Internal types in public API | Move to .internal sub-package | | Events without data | Include all data handlers need |

Critical Reminders

1. One module = one bounded context — Mirror DDD boundaries
2. Events are the integration mechanism — Not direct method calls
3. Verify in CI — ApplicationModules.verify() catches boundary violations
4. Reference by ID — Never direct object references across modules
5. Transaction per module — @ApplicationModuleListener ensures isolation