Java优学网DI依赖注入入门解析：告别硬编码，拥抱Spring自动注入的便捷与高效

public class OrderService {

private PaymentService payment = new AlipayService();

}

@Component public class OrderService {

private final PaymentService paymentService;

// Spring会自动注入PaymentService实例
public OrderService(PaymentService paymentService) {
    this.paymentService = paymentService;
}

}

// 好的做法：使用构造函数注入 public class UserService {

private final UserRepository userRepository;
private final EmailService emailService;

public UserService(UserRepository userRepository, EmailService emailService) {
    this.userRepository = userRepository;
    this.emailService = emailService;
}

}

// 避免的做法：字段注入隐藏了依赖关系 public class UserService {

@Autowired
private UserRepository userRepository;
@Autowired
private EmailService emailService;

}