Great question — this is a super common point of confusion when working with constructor-based dependency injection in Spring Boot. Let's break it down with clear rules and examples:

Core Rule (Spring 4.3+) #

Starting from Spring Framework 4.3, the @Autowired annotation on a constructor is optional in most cases. Spring will automatically detect and use a constructor for dependency injection based on how many constructors your class has.

Case 1: Class has only one constructor #

If your bean class (marked with @Component, @Service, @Repository, etc.) has exactly one constructor, Spring will automatically use it to inject dependencies—no @Autowired needed at all. This applies even if the constructor has multiple parameters that are Spring-managed beans.

Example:

@Service
public class UserService {
    private final UserRepository userRepo;

    // No @Autowired required here!
    public UserService(UserRepository userRepo) {
        this.userRepo = userRepo;
    }
}

Case 2: Class has multiple constructors #

If your class has more than one constructor, you must add @Autowired to the constructor you want Spring to use for dependency injection. Without this annotation, Spring won't know which constructor to pick and will throw an exception at startup.

Example:

@Service
public class OrderService {
    private final OrderRepository orderRepo;
    private final NotificationService notificationService;

    // This constructor will be used by Spring (thanks to @Autowired)
    @Autowired
    public OrderService(OrderRepository orderRepo, NotificationService notificationService) {
        this.orderRepo = orderRepo;
        this.notificationService = notificationService;
    }

    // This constructor won't be used by Spring unless you add @Autowired to it
    public OrderService(OrderRepository orderRepo) {
        this.orderRepo = orderRepo;
        this.notificationService = new NotificationService(); // Manual instantiation
    }
}

Additional Notes #

• This rule also applies to beans created via @Configuration classes (using @Bean methods). If the bean class has a single constructor, you don't need @Autowired when defining the bean.
• Omitting @Autowired for single constructors is generally recommended—it keeps your code cleaner and aligns with modern Spring best practices.

内容的提问来源于stack exchange，提问作者Vitalii