Spring @Qualifier Annotation with example

Category : Interview Questions | Sub Category : Spring Boot Interview Questions | By Prasad Bonam Last updated: 2023-08-14 11:14:30 Viewed : 565

In Spring, the @Qualifier annotation is used to resolve ambiguities when you have multiple beans of the same type and you want to specify which bean should be injected. This is especially useful when you have a circular dependency situation or when there are multiple candidates for autowiring.

Here is an example of how to use the @Qualifier annotation in Spring:

Suppose you have an interface Animal and two implementations of that interface: Dog and Cat.

javaCopy code
public interface Animal {
    void makeSound();
}

@Component
@Qualifier("dog")
public class Dog implements Animal {
    @Override
    public void makeSound() {
        System.out.println("Woof!");
    }
}

@Component
@Qualifier("cat")
public class Cat implements Animal {
    @Override
    public void makeSound() {
        System.out.println("Meow!");
    }
}

Now, lets say you have a class PetOwner that needs to inject an Animal bean, but you have multiple candidates available. You can use the @Qualifier annotation to specify which bean you want to inject:

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@Component
public class PetOwner {
    private final Animal animal;

    @Autowired
    public PetOwner(@Qualifier("dog") Animal animal) {
        this.animal = animal;
    }

    public void playWithPet() {
        animal.makeSound();
    }
}

In this example, the @Qualifier("dog") annotation is used on the constructor parameter of the PetOwner class to indicate that you want to inject the Dog bean specifically.

If you didn`t use the @Qualifier annotation, Spring would not know which bean to inject and would likely result in an error due to the ambiguity.

Remember that the string you provide to @Qualifier should match the value specified in the @Qualifier annotation on the beans you want to distinguish. Using @Qualifier helps Spring to unambiguously identify the bean to be injected, especially when multiple candidates of the same type are available in the application context.

The @Qualifier annotation in Spring is used to specify which bean should be injected when there are multiple beans of the same type in the context. It helps resolve ambiguity when the container cannot determine which bean to inject based solely on type information. Here is an example of how to use the @Qualifier annotation:

Suppose you have an interface PaymentService and two implementations of it: CreditCardPaymentService and PayPalPaymentService. You want to inject one of these implementations into another bean, and you have multiple beans of the same type. Here is how you can use @Qualifier:

1. Define the PaymentService interface and its implementations:

javaCopy code
public interface PaymentService {
    void processPayment();
}

@Service
public class CreditCardPaymentService implements PaymentService {
    public void processPayment() {
        // Credit card payment logic
    }
}

@Service
public class PayPalPaymentService implements PaymentService {
    public void processPayment() {
        // PayPal payment logic
    }
}

2. In the class where you want to inject the PaymentService, use the @Autowired annotation with @Qualifier to specify which implementation to inject:

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@Service
public class OrderService {
    private final PaymentService paymentService;

    @Autowired
    public OrderService(@Qualifier("creditCardPaymentService") PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    // ...
}

In this example, the @Qualifier("creditCardPaymentService") annotation explicitly tells Spring which bean to inject. Here, "creditCardPaymentService" corresponds to the bean name defined in the Spring context. If you dont use @Qualifier, Spring might not know which bean to inject, especially when there are multiple beans of the same type.

3. In your Spring configuration, you should have the following annotations to enable component scanning and create the beans:

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@SpringBootApplication
@ComponentScan(basePackages = "com.example")
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    } 
}}

In this example, the @ComponentScan annotation scans the specified package for components (such as the PaymentService implementations) to register them with the Spring context.

By using the @Qualifier annotation, you can provide a clear indication to Spring about which bean to inject when dealing with multiple beans of the same type. This helps avoid ambiguity and ensures that the correct bean is injected based on your applications requirements.