Kotlin + Spring Boot (with Gradle)

🍃 Learn Spring Boot and Gradle by building, not memorising. Five chapters: (1) Gradle — the build as a task graph, dependencies, configurations, the lifecycle, plugins; (2) Spring core — Inversion of Control, dependency injection, beans, `@Configuration`/`@Bean`, `@Value`, profiles, lifecycle; (3) Spring web — `@RestController`, `@GetMapping`/`@PostMapping`, path variables, request bodies, services, validation, exception handling; (4) data — repositories, the in-memory CrudRepository, persistence patterns; (5) testing & production — slice-style tests, configuration properties, actuator concepts. Spring challenges embed a faithful hand-rolled **mini-Spring runtime** (a reflection-based DI container + REST router + CrudRepository) so the `@Service`/`@RestController`/`@Bean` code you write genuinely wires and runs — graded by stdin/stdout on a self-hosted Kotlin 1.8.20 runner. Gradle challenges model the real build (task graphs, dependency resolution) in plain Kotlin.

66 challenges79 topics

How it works1Pick a challenge2Read the requirements3Code in a full IDE4Run the tests

66 challenges

Gradle is a task graph

Dependencies & the dependency tree

Configurations: which deps land on which classpath

Initialization, configuration, execution

Plugins: tasks, extensions, conventions

Source sets: main, test, and their classpaths

Version catalogs: one source of truth for versions

Multi-module builds & project dependencies

Inversion of Control & the container

Stereotypes: @Component, @Service, @Repository, @RestController

@Configuration classes & @Bean factory methods

@Value & Environment property injection

@Profile for environment-specific beans

Bean lifecycle: singleton scope & @PostConstruct

Why constructor injection

The ApplicationContext as the bean registry

Your first REST controller

Reading query parameters

Path variables

POST and the request body

Thin controllers, fat services

Rejecting bad input

Mapping exceptions to status codes

Capstone: a small REST resource

Repositories & CrudRepository

Custom repository methods

Services over repositories

Derived query methods

Transactions: all-or-nothing

Capstone: a small data domain

Unit-testing a service with a fake

Slice-testing a controller with MockMvc

Testing configuration and @Value

Testing profile-specific wiring

Health checks: aggregating component status

Capstone: an end-to-end tested app

AOP: cross-cutting concerns & proxies

Around advice: before and after

Web-layer advice: HandlerInterceptor

@Transactional is an aspect

Pointcuts: advising selectively

Capstone: stack two cross-cutting concerns

Authentication: who are you?

Authorization & roles

Method-level security

Never store plaintext passwords

The security filter chain

Capstone: a secured REST resource

Publish an event, a listener reacts

Events decouple side-effects

Routing by event type

Listening to a base type

The transactional outbox

Capstone: an event-driven order workflow

@Cacheable: compute once, reuse

Eviction: fighting staleness

@Scheduled: periodic background work

Retry: surviving transient failures

The circuit breaker

Capstone: a resilient client

Pagination: page & size

Filtering & sorting

Entity → DTO mapping

Content negotiation

A request logging interceptor

Capstone: a production-flavored resource