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Spring Boot Modulith by joaquimscosta

Name: Spring Boot Modulith
Author: joaquimscosta

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, externalizing events to Kafka/AMQP, building a modular monolith, defining module dependencies, managing inter-module events, or organizing DDD modules as packages. Covers package-based module boundaries, named interfaces, event publication, and Scenario API testing. For architecture decisions, see domain-driven-design skill.

joaquimscosta Updated Sep 21, 2026

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
Workflow: See WORKFLOW.md for detailed step-by-step Modulith setup
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

Related Skills

Need	Skill
DDD concepts	`domain-driven-design`
Data layer per module	`spring-boot-data-ddd`
Module event testing	`spring-boot-testing`
REST APIs for modules	`spring-boot-web-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, externalizing events to Kafka/AMQP, building a modular monolith, defining module dependencies, managing inter-module events, or organizing DDD modules as packages. Covers package-based module boundaries, named interfaces, event publication, and Scenario API testing. For architecture decisions, see domain-driven-design skill.---# Spring Modulith for Bounded ContextsImplements 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 APITypes in `com.example.order.internal` = hidden from other modules## Quick PatternsSee [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- **Workflow**: See [WORKFLOW.md](WORKFLOW.md) for detailed step-by-step Modulith setup- **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## Related Skills| Need | Skill ||------|-------|| DDD concepts | `domain-driven-design` || Data layer per module | `spring-boot-data-ddd` || Module event testing | `spring-boot-testing` || REST APIs for modules | `spring-boot-web-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 Reminders1. **One module = one bounded context** — Mirror DDD boundaries2. **Events are the integration mechanism** — Not direct method calls3. **Verify in CI** — `ApplicationModules.verify()` catches boundary violations4. **Reference by ID** — Never direct object references across modules5. **Transaction per module** — `@ApplicationModuleListener` ensures isolation

Frequently asked questions