@Autowired Annotation in Spring Explained

Direct Answer

@Autowired marks an injection point that Spring should satisfy from matching beans. Candidate selection starts by type, then considers qualifier metadata and priorities such as @Primary; an injection-point name can be a fallback. A required dependency that cannot be resolved causes context creation to fail, often through an UnsatisfiedDependencyException whose cause explains whether the bean was missing or ambiguous.

Code Example

@Service
public class OrderService {
    private final CustomerRepository customerRepo; // required, immutable
    private PaymentGateway paymentGateway;         // optional

    // Constructor injection — preferred for required dependencies.
    // Since Spring 4.3, @Autowired is optional on a single constructor.
    @Autowired
    public OrderService(CustomerRepository customerRepo) {
        this.customerRepo = customerRepo;
    }

    // Setter injection — used for optional dependencies.
    @Autowired
    public void setPaymentGateway(PaymentGateway gateway) {
        this.paymentGateway = gateway;
    }

    public void processOrder(Long customerId, Order order) {
        Customer customer = customerRepo.findById(customerId);
        paymentGateway.charge(customer, order.getTotal());
    }
}

@Repository
public class CustomerRepository {
    public Customer findById(Long id) { /* JDBC logic */ return null; }
}

Common Mistakes & Pitfalls

• Mixing the new keyword with @Autowired: Creating a service with new OrderService() bypasses Spring entirely — Spring never injects dependencies into objects you instantiate yourself. Let Spring create the bean by marking it @Service or @Component.
• Autowiring into static fields: Spring's injection works on instance members only. Static fields belong to the class, not to a Spring-managed instance, so @Autowired on a static field silently stays null.
• Forgetting the stereotype on the injected class: If CustomerRepository isn't annotated with @Repository or @Component, Spring never creates a bean for it, and @Autowired throws NoSuchBeanDefinitionException at startup.