public class DatabaseConnection {
    private static volatile DatabaseConnection instance;

    private DatabaseConnection() {}

    public static DatabaseConnection getInstance() {
        if (instance == null) {
            synchronized (DatabaseConnection.class) {
                if (instance == null) {
                    instance = new DatabaseConnection();
                }
            }
        }
        return instance;
    }
}

public void readFile() throws FileNotFoundException { ... }

if (file == null) {
    throw new FileNotFoundException("Файл не найден");
}

@Service
@RequiredArgsConstructor
public class UserService {

    private final UserRepository userRepository;
    private final ApplicationEventPublisher eventPublisher;

    @Transactional
    public void register(UserDto dto) {
        User user = new User(dto.email());
        userRepository.save(user);

        eventPublisher.publishEvent(new UserRegisteredEvent(user));
    }
}

@Component
@RequiredArgsConstructor
public class EmailListener {

    private final EmailSender emailSender;
    private final SomeOtherService someOtherService;

    @EventListener 
    public void onUserRegistered(UserRegisteredEvent event) {
        emailSender.sendWelcome(event.user().getEmail());
        someOtherService.doSomething();
    }
}

@Component
public class UserContext {
    private static final ThreadLocal<String> currentUserId = new ThreadLocal<>();

    public static void set(String userId) { currentUserId.set(userId); }
    public static String get() { return currentUserId.get(); }
    public static void clear() { currentUserId.remove(); }
}

@Service
@RequiredArgsConstructor
public class OrderService {
    private final AuditLogger auditLogger;

    public CompletableFuture<Order> createOrder(String userId, OrderDto dto) {
        UserContext.set(userId);

        return CompletableFuture.supplyAsync(() -> {
            Order order = buildOrder(dto);
            auditLogger.log("Order created by: " + UserContext.get());
            return order;
        });
    }
}