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Spring Boot Modulith by majiayu000 — Agent Skill

Name: Spring Boot Modulith
Rating: 4.2 (5 reviews)
Author: majiayu000

Spring Boot Modulith

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.

majiayu000 e0d0ac3 2 files · 4.5 KB Updated Sep 9, 2026 567 repo stars

File contents

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

---name: spring-boot-modulithdescription: 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](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](EXAMPLES.md) for complete working code examples - **Troubleshooting**: See [TROUBLESHOOTING.md](TROUBLESHOOTING.md) for common issues and Boot 4 migration - **Module Structure**: See [references/MODULE-STRUCTURE.md](references/MODULE-STRUCTURE.md) for package conventions, named interfaces, dependency rules - **Event Patterns**: See [references/EVENTS.md](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

Frequently asked questions