JNR-013

Layered architecture

8 Lessons
4 h practice (estimate)

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On this page 8 topics

Overview
Learning outcomes
Lessons
Lab and exercises
Quiz
Common mistakes
Assessment
Career relevance

Overview

Topics

Controller
service
repository
DTO
domain/entity models

Learn first

4 h of practice (estimate)

Learning outcomes

Assign responsibilities to controller, service, repository, and domain layers.
Separate API DTOs from domain/entity models and map between them.
Organize code by feature with layers inside each feature.
Write a minimal @RestController that delegates to a service.

Lessons

Lab and exercises

Lab

JNR-013-LAB1 · Products feature

Outcomes: 1–4 · Time: ~4 h

In package com.shop.api.products, create Product, ProductRepository, InMemoryProductRepository, ProductService, ProductController.
Implement GET /products and GET /products/{id}.
Return ProductResponse DTOs through a mapper; never the domain class.
Throw a not-found exception from the service for an unknown id (the HTTP mapping comes in JNR-015).

Acceptance: the controller contains no business logic; domain objects are never serialized directly; the service has no HTTP types.

Exercises (assessment: layering review of a provided feature)

ID	Outcome	Task
JNR-013-EX01	1	Sort these into layers: parse JSON, check stock, find by id, set HTTP 404, compute total with tax.
JNR-013-EX02	1	Review a provided "fat controller" and list five violations.
JNR-013-EX03	2	Write `OrderResponse` and a mapper from `Order` that hides `internalNotes`.
JNR-013-EX04	2	Explain what breaks for clients if you rename a field of a serialized domain class.
JNR-013-EX05	3	Reorganize a package-by-layer tree into package-by-feature.
JNR-013-EX06	4	Write a `@RestController` with `GET /customers/{id}` that only delegates to a service.
JNR-013-EX07	3	Decide: may `orders` use `ProductRepository` directly? Justify and write the alternative.

Show solutions

EX01 Controller: parse JSON, set HTTP 404. Service: check stock, compute total with tax. Repository: find by id.
EX02 Typical violations: repository calls in the controller, business rules in it, a returned entity, try/catch and status logic, one DTO for everything.
EX03 record OrderResponse(long id, List<ItemResponse> items, BigDecimal total){} and static OrderResponse toResponse(Order o).
EX04 The JSON field name changes, so clients break; a DTO decouples this.
EX05 products/{ProductController, ProductService, ProductRepository, dto/} and the same for orders.
EX06 @RestController @RequestMapping("/customers") class CustomerController { ... @GetMapping("/{id}") CustomerResponse get(@PathVariable long id){ return service.get(id);} }
EX07 No: use ProductService (or a narrow interface) so the products feature controls its own data and rules.

Quiz

Outcomes: O1 layer responsibilities · O2 DTOs vs domain · O3 package by feature · O4 minimal controller.

Show answers

Q01: C
Q02: A
Q03: B
Q04: C
Q05: B
Q06: B
Q07: B
Q08: B
Q09: A
Q10: C

Practical part

Seven exercises JNR-013-EX01 to EX07 plus the lab. The assessment: review a provided feature's layering (EX02 is the template). Pass: quiz at least 8/10, review finds at least four violations, lab acceptance met.

Common mistakes

Fat controllers; services that return ResponseEntity; one DTO reused for everything.

Assessment

Layering review of a provided feature.

Career relevance

The "established architecture" the Junior gate asks the learner to work within; hexagonal architecture (MID-059).