Kotlin — Spring Boot drills (extra practice)

▶ **Extra-practice drills that pair with the Kotlin Reactive course (Chapter 4: WebFlux & DI).** Learn the model there, then drill the annotations here. Spring Boot itself needs the JVM, so each challenge simulates the DispatcherServlet: the candidate writes handler logic with the real annotations shown as comments, and `main()` plays the role of the framework — feeding requests from stdin, printing responses to stdout. One annotation/pattern per challenge: `@RestController`, `@GetMapping` + `@PathVariable`, `@RequestParam`, `@RequestBody` + `@ResponseStatus(201)`, `@RequestHeader` auth gate, `@Valid` + custom `@Constraint`, `@ControllerAdvice` exception handlers, `@Transactional` rollback, `@Async` fire-and-forget, `@Cacheable`, `@Scheduled`, `@Profile` config, filter chain, constructor injection, `ResponseEntity` status branching, `Pageable` pagination + sort. 15–30 min each.

REST GET endpoint

Path variables and 404

Bean validation — @Valid

@ControllerAdvice — global error handler

Constructor injection + a fake repo

Pagination — Pageable & Page

Filter chain — OncePerRequestFilter

@Profile — env-specific beans

@Cacheable — memoize a slow service

@Scheduled — fixed-rate cron tick

@RequestBody + @ResponseStatus(201)

@RequestHeader auth gate

ResponseEntity status branching

Custom @Constraint validator

@Transactional rollback

@Async fire-and-forget

Pageable — pagination + sorting

Bug: @PathVariable name mismatch

Bug: 200 returned on resource create (should be 201)

Bug: NPE on missing optional query param

Bug: handler forgets to return the body

Bug: @ControllerAdvice catches the wrong exception