layered-architecture

# Layered Architecture ## Layer Rules ``` @RestController ← HTTP only. No business logic. No JPA entities in responses. ↓ DTOs @Service ← All business logic lives here. Orchestrates repositories. ↓ Domain objects / Entities @Repository ← Data access only. No business logic. Returns entities or projections. ↓ JPA / JDBC Database ``` ## Controller Layer - Handles HTTP: parsing requests, validating input (`@Valid`), returning responses - Calls ONE service method per endpoint — no orchestration in controllers - Never returns `@Entity` classes directly — always map to response DTOs - Never injects `@Repository` — always goes through a `@Service` - Exception handling via `@ControllerAdvice`, never try/catch in controllers ```java // ✅ GOOD @PostMapping("/orders") public ResponseEntity<OrderResponse> createOrder(@Valid @RequestBody CreateOrderRequest request) { Order order = orderService.createOrder(request); return ResponseEntity.status(HttpStatus.CREATED).body(OrderResponse.from(order)); } // ❌ BAD — business logic in controller @PostMapping("/orders") public ResponseEntity<Order> createOrder(@RequestBody CreateOrderRequest request) { if (request.getItems().isEmpty()) throw new RuntimeException("No items"); Order order = orderRepository.save(new Order(request)); // direct repo access return ResponseEntity.ok(order); // returning entity } ``` ## Service Layer - Contains all business logic, validation rules,...