🎯 ABOUT

This is a 'best practices' template project for Spring Boot with Hexagonal Architecture. However, it is an opinionated take on that.

DISCLAIMER: I'm by no means an expert on Spring Boot, one reason to do this is to learn it. Opinions are welcomed (with proper reasoning), check the contributing section to share your thoughts.

The project is mirrored on GitLab for CI demonstration purposes.

Have fun!

🗓️ Appointments

Example application to create appointments (REST API). Appointments are stored in a relational DB (Postgres), and their creation/deletion is published to a Kafka broker.

📘 Architecture

• Hexagonal/Onion/Clean Architecture
• OpenAPI code generation (server and client)

🧰 Stack

• Java 21
• Spring 3.3 (configurable server, 'undertow' by default)
  • Actuator (healthcheck, etc.)
• Flyway (chosen over Liquibase for its simplicity)
• Postgres
• Kafka

🏎️ Runtime

• Cloud Native Buildpacks (building)
• Docker Compose (local environment with the infrastructure)

🧪 Test

• ArchUnit (preferred over Java modules: it allows naming checks, etc.)
• Testcontainers (used to provide a test instance of Postgres and Kafka)
• Pitest (mutation testing, nightly)

⚒️ Development

• SDKMAN (allows to use simpler runners on CI)
• Maven Wrapper (Maven can be provided by SDKMAN, however, Maven Wrapper has better IDE support)
• Editorconfig (supported by a lot of editors, rules limited though)
• CI pipelines for GitHub and GitLab

📑 Requirements

• Docker Compose
• JDK 21+
• SDKMAN (optional, recommended)

🤔 Design Decisions

• Simplicity and pragmatism over Hexagonal Architectures 'by the book'.
• Minimal: don't use libraries to implement easy stuff (even if that's boring).
• Start small, split and refactor when features are added (I.e.: split services into use cases).
• Prefer flat structure (avoid empty parent packages if possible).
• Small coupling with Spring (easier to migrate, to other frameworks/toolkits).
• Not use Spring integrations if a library can be used directly.
• No Spring profiles (settings are loaded from the environment).
• Split API spec in different files for future modularity.
• Prefer service independence over code reuse (sharing libraries among microservices).
• Docker Compose profiles are used to separate infrastructure from a complete local environment.
• Atomicity in notifiers (with outbox pattern) should be done with a different notifier adapter.
• No input ports: they don't need to be decoupled, they just use the domain (and that's acceptable).

📖 Architecture

• Port: interface to set a boundary between application logic and implementation details.
• Adapter: port implementation to connect the application domain with the system's context.
• Domain: application logic and model entities.
• Service: implement operations with a common topic altogether. Usually calls driven ports.
• UseCase/Case: single operation service (isolate features). They can coexist with services.
• Output/Driven Adapter: implementation of ports called from the domain.
• Input/Driver Adapter: commands that call application logic (don't require a port).

📚 Design

• The REST API controller and client are generated from the OpenAPI spec at build time.
• domain holds business logic (services and/or use cases) and driven ports (interfaces).
• domain.model keeps the structures relevant to the application's domain. The more logic added to an entity, the better (it could be easily accessed by many different services, or use cases).
• output.{notifiers,repositories} driven adapters (implementations of driven ports).
• input.controllers driver adapter (adapters without interface).
• Subpackages can be created for different adapter implementations (to isolate their code).
• More information about each package rules can be found on their Javadoc documentation.

🎚️ Set up

• With SDKMAN: sdk env install
• If SDKMAN is not available, JDK 21+ must be installed.

▶️ Commands

All commands assume a Unix like OS.

The most important commands to operate the project are:

• Build: ./mvnw package
• Documentation: ./mvnw site
• Run: ./mvnw spring-boot:run
• Build image: ./mvnw verify or ./mvnw spring-boot:build-image

To run or deploy the application:

• Start infrastructure (for running locally): docker-compose up -d
• Run JAR locally: java -jar target/appointments-0.1.0.jar
• Run container: docker-compose --profile local up

🤖 Service Management

• You can check the API spec using Swagger UI.

Docker

At the docker-compose.yml you can find the information on how to run the application as a container, adjusting the configuration for running it on different environments.

Two Docker compose profiles are used:

• Default profile (no profile parameter): to allow starting only the infrastructure, this is useful to start the application from the IDE
• Local profile (--profile local): which also starts a container using the image of this application

Testing

The verification requests can be executed with: src/test/resources/requests.sh, or PORT=9090 src/test/resources/requests.sh if you want to run them to a different port.

The health check endpoint is: http://localhost:18080/actuator/health

Stress Testing (Gatling)

Gatling settings can be overridden creating a gatling.conf file at the test resources. The configuration options and their default values can be checked here.

Those parameters can also be overwritten by system properties from the command line. I.e.: -D gatling.core.encoding=utf-8

To run the Gatling test, execute ./mvnw gatling:test at the shell.