Resolving Circular Dependency in Real-World Applications

Circular dependency in Spring occurs when two or more beans depend on each other, creating a cycle that the framework cannot resolve during application startup. While this issue often arises due to improper design or tightly coupled components, there are multiple strategies to resolve it. In real-world applications, resolving circular dependencies is crucial to maintain code modularity and application stability.

Here, we explore practical approaches to handle circular dependencies effectively:

1. Using @Lazy Annotation

In real-time scenarios, lazy initialization is commonly used when one of the dependencies is optional or accessed infrequently. Marking one of the beans with @Lazy ensures that it is initialized only when required, thus breaking the dependency loop.

Example:

Imagine two services, OrderService and PaymentService, where each requires the other:

@Service
public class OrderService {
    private final PaymentService paymentService;

    @Autowired
    public OrderService(@Lazy PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    // Business logic
}

@Service
public class PaymentService {
    private final OrderService orderService;

    @Autowired
    public PaymentService(OrderService orderService) {
        this.orderService = orderService;
    }

    // Business logic
}
        

In this setup, OrderService defers the initialization of PaymentService until it is actually needed, resolving the circular dependency.

2. Using Setter Injection with @PostConstruct

Setter injection combined with the @PostConstruct annotation is another practical approach. This is particularly useful when you need to initialize beans after dependency injection.

Example:

Consider a scenario with UserService and NotificationService:

@Service
public class UserService {
    private NotificationService notificationService;

    @Autowired
    public void setNotificationService(NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    @PostConstruct
    public void init() {
        // Initialize resources or set up logic
    }
}

@Service
public class NotificationService {
    private UserService userService;

    @Autowired
    public void setUserService(UserService userService) {
        this.userService = userService;
    }

    @PostConstruct
    public void init() {
        // Initialize resources or set up logic
    }
}
        

Using setter injection eliminates the cycle during bean creation while maintaining the required references.

3. Defining Beans Explicitly in @Configuration

For applications that require precise control over bean creation, defining beans in a @Configuration class provides a clean and modular solution.

Example:

In a payment processing system:

@Configuration
public class AppConfig {

    @Bean
    public PaymentService paymentService(OrderService orderService) {
        return new PaymentService(orderService);
    }

    @Bean
    public OrderService orderService() {
        return new OrderService();
    }
}
        

This approach gives you full control over the instantiation order of beans, avoiding circular references during autowiring.

4. Refactoring the Design

Circular dependencies often indicate overly coupled components. Refactoring to introduce a mediator or a shared service can resolve the issue and improve code maintainability.

Example:

Suppose AuthService and AuditService have interdependencies. Introducing a MediatorService can help:

@Service
public class MediatorService {
    private final AuthService authService;
    private final AuditService auditService;

    @Autowired
    public MediatorService(AuthService authService, AuditService auditService) {
        this.authService = authService;
        this.auditService = auditService;
    }

    // Coordination logic between AuthService and AuditService
}
        

By moving shared logic to the mediator, you break the cycle and achieve a cleaner design.

5. Using ObjectFactory or Provider

For large-scale applications where dependencies are only occasionally required, using Spring’s ObjectFactory or JSR-330’s Provider can be a practical solution.

Example:

@Service
public class ReportService {
    private final ObjectFactory<ExportService> exportServiceFactory;

    @Autowired
    public ReportService(ObjectFactory<ExportService> exportServiceFactory) {
        this.exportServiceFactory = exportServiceFactory;
    }

    public void generateReport() {
        ExportService exportService = exportServiceFactory.getObject();
        exportService.export();
    }
}
        

This approach defers the creation of ExportService until it is explicitly needed, preventing initialization issues.

6. Using Interfaces and @Primary

When services implement a common interface, using @Primary helps Spring prioritize the correct implementation during autowiring.

Example:

public interface Notification {
    void notifyUser();
}

@Service
@Primary
public class EmailNotification implements Notification {
    @Override
    public void notifyUser() {
        // Email logic
    }
}

@Service
public class SMSNotification implements Notification {
    @Override
    public void notifyUser() {
        // SMS logic
    }
}
        

Conclusion

In real-time applications, resolving circular dependencies is critical for smooth application startup and maintenance. By combining approaches such as lazy initialization, setter injection, refactoring, and explicit bean definitions, developers can efficiently break dependency cycles while keeping their code modular and maintainable.