@Component
public class AuditFilter implements Filter {

    private static final ThreadLocal<AuditContext> CONTEXT =
        new ThreadLocal<>();

    @Override
    public void doFilter(
        ServletRequest request,
        ServletResponse response,
        FilterChain chain
    ) throws IOException, ServletException {

        AuditContext ctx = new AuditContext(
            ((HttpServletRequest) request).getRequestURI(),
            System.currentTimeMillis()
        );
        CONTEXT.set(ctx);

        try {
            chain.doFilter(request, response);
        } finally {
            auditLog(CONTEXT.get());
        }
    }

    public static AuditContext current() {
        return CONTEXT.get();
    }

    private void auditLog(AuditContext ctx) {
        // пишем в БД...
    }
}

@Entity
public class Order {
    @Id
    private Long id;
    private Long userId;
    private LocalDateTime createdAt;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "order")
    private List<OrderItem> items;
}

@Entity
public class OrderItem {
    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private Order order;

    @ManyToOne(fetch = FetchType.LAZY)
    private Product product;

    private Integer quantity;
}

@RestController
@RequiredArgsConstructor
public class OrderController {

    private final OrderRepository orderRepository;

    @GetMapping("/orders")
    public List<OrderDto> getOrders(@RequestParam Long userId) {
        List<Order> orders = orderRepository.findByUserId(userId);

        return orders.stream()
            .map(order -> new OrderDto(
                order.getId(),
                order.getItems().stream()
                    .map(item -> new ItemDto(
                        item.getProduct().getName(),
                        item.getQuantity()
                    ))
                    .toList()
            ))
            .toList();
    }
}

// Событие:
public record UserRegistered(User user) {}

// Listener:
@Component
public class SendWelcomeEmail {
    @EventListener
    public void handle(UserRegistered event) {
        mailer.send(event.user().email(), "Welcome!");
    }
}

// Публикация события:
applicationContext.publishEvent(new UserRegistered(user));

@Value("${app.name}")
private String appName;

@Value("${app.timeout:30}")
private int timeout;  // 30 - default значение

@Value("#{systemProperties['user.name']}")
private String userName;  // SpEL выражение

@Service
@RequiredArgsConstructor
public class NotificationService {

    private final EmailClient emailClient;
    private final EventRepository eventRepository;
    private final Executor taskExecutor;

    public void notifyOrderPaid(Order order) {
        CompletableFuture.runAsync(
            () -> emailClient.sendOrderConfirmation(order),
            taskExecutor
        );

        CompletableFuture.runAsync(
            () -> {
                Event event = Event.orderPaid(order.getId());
                eventRepository.save(event);
            },
            taskExecutor
        );
    }
}