hexagonal-architecture-layers-java

Hexagonal architecture in Java isolates core business logic into a pure domain layer, coordinates use cases through application ports, and connects external technologies via infrastructure adapters. This enforces strict dependency direction, ensuring domain code remains independent of frameworks like Spring or JPA.

Refactoring a monolithic Spring application to clean architecture requires extracting pure domain models away from Spring annotations and JPA entities. You define application ports for use cases and wire infrastructure adapters for REST and persistence, ensuring the domain layer has no external framework dependencies.

Managing multiple persistence adapters like JPA and MongoDB in a single Java service is supported by defining clear application ports. You implement separate infrastructure adapters for each technology, allowing the domain and application layers to delegate I/O operations without depending on specific persistence frameworks.

Domain-driven design in this context requires clear definition of ports and adapters to maintain module isolation. By defining application ports for use cases and implementing infrastructure adapters, you enforce dependency direction and keep domain concerns strictly separated from external technologies.

A key limitation when applying hexagonal architecture to existing Java codebases is the need to strictly untangle tightly coupled domain and infrastructure code. Refactoring requires defining explicit ports and adapters to enforce dependency direction, which can be complex if current module boundaries are unclear.