Building a REST API

@RestController
@RequestMapping("/api/products")
public class ProductController {
    private final ProductService service;
    public ProductController(ProductService service) { this.service = service; }

    @GetMapping                                   // GET /api/products?city=Pune
    public List<ProductDto> list(@RequestParam(required = false) String city) {
        return service.list(city);
    }

    @GetMapping("/{id}")                          // GET /api/products/42
    public ResponseEntity<ProductDto> one(@PathVariable long id) {
        return service.find(id)
                .map(ResponseEntity::ok)           // 200 with JSON body
                .orElse(ResponseEntity.notFound().build());   // 404
    }

    @PostMapping                                  // POST /api/products with a JSON body
    public ResponseEntity<ProductDto> create(@Valid @RequestBody NewProduct body) {
        ProductDto saved = service.create(body);
        return ResponseEntity.created(URI.create("/api/products/" + saved.id())).body(saved);   // 201
    }

    @DeleteMapping("/{id}")
    public ResponseEntity<Void> delete(@PathVariable long id) {
        service.delete(id);
        return ResponseEntity.noContent().build();  // 204
    }
}

import jakarta.validation.constraints.*;

public record NewProduct(
        @NotBlank String name,
        @NotBlank String city,
        @Min(1) long pricePaise,
        @PositiveOrZero int stock) { }
// With @Valid on the parameter, invalid input is rejected with 400 Bad Request before your code runs.
// Needs the spring-boot-starter-validation dependency.

@RestControllerAdvice
public class ApiErrors {
    @ExceptionHandler(ProductNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public Map<String, String> notFound(ProductNotFoundException e) {
        return Map.of("error", e.getMessage());
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String, String> invalid(MethodArgumentNotValidException e) {
        Map<String, String> errors = new LinkedHashMap<>();
        e.getBindingResult().getFieldErrors().forEach(f -> errors.put(f.getField(), f.getDefaultMessage()));
        return errors;
    }
}