Hands-On workshop on Kotlin & Spring Boot

Purpose

This repository is a hands-on workshop. It shows how to build maintainable backend services with Kotlin and Spring Boot.

The training is split into progressive modules. You can start from Kotlin syntax only, then move to Spring, coroutines, and advanced error modeling.

What You Will Work On

• A Kotlin Spring Boot REST API for a pet shop
• HTTP service-to-service communication
• Database access
• Local development with Docker Compose
• API exploration with Bruno collections

Learning Path

Recommended order for beginners:

1. exo0-kotlin - Kotlin language fundamentals
2. exo1-spring - Kotlin + Spring + JPA
3. exo2-reactive - Coroutines and non-blocking I/O
4. exo3-advanced - Typed errors with Arrow Either

Project Structure

• master-one-pet-data/: supporting HTTP service used by the exercises, see Start dependencies section
• bruno/: Bruno collections for manual API testing, see API Testing with Bruno section
• exo0-kotlin/, exo1-spring/, exo2-reactive/ & exo3-advanced/ : workshop exercises (one module per exercise). Start here: Exercise #0: Kotlin syntax

Getting Started

Prerequisites

You need the following tools installed to run the exercises:

• Java 21 LTS

  We recommend SDKMAN to manage Java versions. Example installation: sdk install java 21.0.11-tem

• Docker & Docker Compose

  For instance, we have tested with Rancher Desktop

Optional but recommended:

• IntelliJ IDEA (Community is enough)
• Bruno (for manual API calls)

Verify your setup

Run these commands before starting the workshop:

java -version
./mvnw -version
docker --version
docker compose version

Expected outcome:

• Java version starts with 21
• Maven wrapper runs without downloading errors
• Docker engine is running and docker compose command is available

Start dependencies

1. Build image for master-one-pet-data service:

   docker compose build

2. Start master-one-pet-data (on 8081 port) and postgresql (on 5432 port) services:

   docker compose up

3. Validate they are reachable:

• master-one-pet-data: http://localhost:8081/actuator/health
• PostgreSQL: exposed on localhost:5432 (user/password/db: devsummit)

Build and run exercises

1. Build and run the exercise you want to work on (e.g. exo1-spring):

   ./mvnw -pl exo1-spring spring-boot:run

2. The service will be available at http://localhost:8080/ (for usage with Bruno).

3. You can also run integration tests (will use an ephemeral PostgreSQL database via TestContainer) with:

   ./mvnw -pl exo1-spring verify

Suggested session flow

For each exercise:

1. Read the module README
2. Run tests once before changing anything
3. Implement one step
4. Re-run tests
5. Validate behavior in Bruno

API Testing with Bruno

Bruno collections are available in bruno/.

1. Install Bruno
2. Import the bruno/ directory
3. Run the provided requests against your local services

Troubleshooting

• Port already used (8080, 8081, 5432): stop conflicting apps or change local port mapping.
• Docker dependency startup fails: re-run docker compose up and check container health.
• Tests fail with "Could not find a valid Docker environment" error. Check that you have followed Testcontainer configuration for Rancher Desktop because Testcontainers needs three environment variables to locate the Docker socket and connect to container ports inside the VM