What is @Qualifier and when do you need it? @Qualifier क्या है और इसकी ज़रूरत कब पड़ती है?

When multiple beans of the same type exist in the container, @Autowired alone can't determine which one to inject, causing a NoUniqueBeanDefinitionException. @Qualifier("beanName") resolves the ambiguity by specifying exactly which bean implementation should be wired in, matched against the bean's name or an explicit qualifier value.

An alternative to @Qualifier is marking one implementation with @Primary so it becomes the default choice whenever multiple candidates exist, without needing to qualify every injection point — @Qualifier is preferred when you need to select different implementations at different injection points.

public interface NotificationService { void send(String msg); }

@Service("emailNotification")
public class EmailNotificationService implements NotificationService { ... }

@Service("smsNotification")
public class SmsNotificationService implements NotificationService { ... }

@Service
public class OrderService {
    public OrderService(@Qualifier("emailNotification") NotificationService service) { ... }
}