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

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 — @ApplicationModuleListener ensures isolation

spring-boot-modulith