コンストラクターインジェクションと循環依存

@Service
public class OrderService {
    private final OrderRepository repo;
    private final EmailService email;
    // @Autowired is optional when there is only one constructor
    public OrderService(OrderRepository repo, EmailService email) {
        this.repo  = repo;
        this.email = email;
    }
}

// Unit test — no Spring context needed:
OrderRepository mockRepo  = mock(OrderRepository.class);
EmailService    mockEmail = mock(EmailService.class);
OrderService    service   = new OrderService(mockRepo, mockEmail);
// Test service logic directly

// Avoid in production code:
@Service
public class UserService {
    @Autowired  // hidden dependency
    private UserRepository repo; // cannot be final
}

@Service
public class NotificationService {
    private SmsSender smsSender; // optional
    @Autowired(required = false)
    public void setSmsSender(SmsSender s) { this.smsSender = s; }
}

// CIRCULAR:
@Service class A { A(B b) {} }
@Service class B { B(A a) {} }
// Spring throws: BeanCurrentlyInCreationException: Is there an unresolvable circular reference?

// Before: A <-> B
// After: A -> C, B -> C (no cycle)
@Service class SharedLogic { ... }
@Service class A { A(SharedLogic s) {} }
@Service class B { B(SharedLogic s) {} }

@Service
public class A {
    private final B b;
    public A(@Lazy B b) { this.b = b; } // breaks the cycle
}

@Service
public class A implements ApplicationContextAware {
    private ApplicationContext ctx;
    public void setApplicationContext(ApplicationContext c) { this.ctx = c; }
    public void doWork() { ctx.getBean(B.class).help(); }
}

@Service
public class A {
    private B b;
    @Autowired public void setB(B b) { this.b = b; } // Spring sets after construction
}

# application.properties (temporary workaround only):
spring.main.allow-circular-references=true