**Layers:** Controller = HTTP, validation, DTOs. Service = business logic, transactions. Repository = data access. Thin controller, fat service.
@RestController @RequestMapping("/api")
class DataController {
@Autowired DataService service;
@GetMapping("/items/{id}")
public ResponseEntity<Item> get(@PathVariable Long id) {
return ResponseEntity.ok(service.findById(id));
}
}
@Service
class DataService { public Item findById(Long id) { return repo.findById(id).orElseThrow()...
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like Walmart. The answer also includes follow-up discussion points that interviewers commonly explore.
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