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name: spring-boot-modulith description: 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.

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