How is the @Qualifier annotation used and in what cases is it applied when injecting dependencies in Spring?

The @Qualifier annotation in Spring is used to specify which exact bean should be injected when there are multiple candidates of the same type in the context.

When you have multiple implementations of an interface or class, and you use @Autowired, Spring cannot unambiguously select the required bean and will throw an exception. To resolve this, @Qualifier is used.

Example:

@Component
public class ServiceA implements MyService {}

@Component
public class ServiceB implements MyService {}

@Component
public class Client {
    private final MyService myService;

    @Autowired
    public Client(@Qualifier("serviceA") MyService myService) {
        this.myService = myService;
    }
}

Here, @Qualifier("serviceA") indicates that the bean named serviceA should be injected.

@Qualifier is used:

• when there are multiple beans of the same type;
• to explicitly specify the required bean;
• to improve readability and control over dependency injection.