How does Spring handle the situation when multiple Beans implement the same interface?

When multiple beans in the Spring context implement the same interface, and an autowiring attempt is made (for example, using @Autowired), ambiguity arises because Spring does not know which bean to choose.

How does Spring handle this situation:

• If no specific bean is indicated, a NoUniqueBeanDefinitionException will be thrown.
• To resolve this, you can:
  • Use the @Qualifier annotation with the name of the desired bean.
  • Mark one bean as @Primary, so it will be chosen by default.
  • Inject a collection of all beans of this type (List<Interface>, Map<String, Interface>) to get all implementations.

Example with @Qualifier:

@Autowired
@Qualifier("myServiceImpl")
private MyService myService;

Example with @Primary:

@Service
@Primary
public class PrimaryServiceImpl implements MyService {
    // implementation
}

Thus, Spring requires explicit indication of which bean to use if there are multiple for the same type.