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Spring Modulith 2.0 implementation for bounded contexts in Spring Boot 4. Use when structuring application modules, implementing @ApplicationModuleListener for event-driven communication, testing with Scenario API, enforcing module boundaries, or externalizing events to Kafka/AMQP. For modular monolith architecture decisions, see the domain-driven-design skill.
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EXAMPLES.mdTROUBLESHOOTING.mdreferences/EVENTS.mdreferences/MODULE-STRUCTURE.mdSpring 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
@Externalizedannotation 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
- One module = one bounded context — Mirror DDD boundaries
- Events are the integration mechanism — Not direct method calls
- Verify in CI —
ApplicationModules.verify()catches boundary violations - Reference by ID — Never direct object references across modules
- Transaction per module —
@ApplicationModuleListenerensures isolation