Applying Hexagonal Architecture: Lessons from a Project Generator
This article shares a developer's journey in truly understanding and applying Hexagonal Architecture to a real-world Spring Boot project generator. It highlights the challenges of moving beyond theoretical knowledge to practical implementation, emphasizing the importance of isolating core domain logic from technical decisions and the utility of architectural guardrails for maintaining long-term integrity.
Read original on Dev.to #architectureThe Challenge of Applying Hexagonal Architecture
The author recounts a personal experience applying Hexagonal Architecture to "Codegen Blueprint", a Spring Boot project generator. Initial attempts, despite theoretical understanding, resulted in "spaghetti with hexagonal labels" due to a lack of clarity in defining ports, domains, and adapters. This highlights a common pitfall: knowing the vocabulary does not equate to understanding the practical application and underlying principles.
Core Principles Realized Through Practice
- Domain Isolation: The domain should be pure and stable, free from framework specifics (e.g., Spring) or infrastructure concerns (e.g., filesystem). Its sole purpose is to describe the core concepts and constraints that must survive technological changes.
- Application as Orchestration: The application layer coordinates use cases, depending only on abstract "ports" (interfaces) that define what is needed, rather than concrete implementations.
- Ports Define Needs, Adapters Provide Implementations: Ports articulate the core's requirements (e.g., "render templates"), while adapters encapsulate the replaceable external technologies and specific implementations (e.g., Maven POM generation, filesystem writing).
Key Insight
Hexagonal architecture isn't about folder names; it's about isolating decisions to protect the core business logic from external volatility and enable easier evolution.
Architectural Guardrails for Long-Term Integrity
A crucial lesson learned was the need for architecture guardrails. Business tests validate behavior, but not necessarily structural integrity. The author implemented generated ArchUnit tests that run at build time (e.g., `mvn verify`). These tests enforce architectural contracts, failing the build immediately if boundaries are violated (e.g., an adapter directly calling an application use case implementation instead of its port). This proactive feedback loop prevents silent architectural drift.
Architecture Violation
Adapters must not depend on application implementations
Expected: adapter → application.port
Found: adapter → application.usecase
BUILD FAILEDThis mechanism provides immediate, non-negotiable feedback, protecting the architecture from human fallibility under continuous change. It shifts the focus from just making the code work to ensuring its structure remains aligned with the intended design over time.