How does Bean Validation work with Spring Boot 4?

// DTO with validation constraints:
public record CreateOrderRequest(
    @NotBlank(message = "Product ID is required")
    String productId,

    @Min(value = 1, message = "Quantity must be at least 1")
    @Max(value = 100, message = "Quantity cannot exceed 100")
    int quantity,

    @Email(message = "Invalid email address")
    @NotNull
    String customerEmail,

    @NotNull
    @Valid   // cascade validation into nested object
    AddressRequest shippingAddress
) {}

public record AddressRequest(
    @NotBlank String line1,
    @Nullable String line2,
    @NotBlank @Size(min=5, max=10) String postcode
) {}

// Controller: @Valid triggers validation before method body:
@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public OrderDto createOrder(@Valid @RequestBody CreateOrderRequest request) {
    return service.create(request);
}

// Handle validation errors globally:
@RestControllerAdvice
public class GlobalExceptionHandler {

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

// Service-level validation with @Validated:
@Service
@Validated  // enables method-level validation
public class OrderService {
    public Order getOrder(@NotBlank String id) {
        return repo.findById(id).orElseThrow();
    }
}