Spring @Qualifier Annotation Example

Posted by on
In this example, we will see how to use @Qualifier annotation to resolve ambiguous dependencies.
  1. This annotation helps fine-tune annotation-based autowiring. There may be scenarios when we create more than one bean of the same type and want to wire only one of them with a property. This can be controlled using @Qualifier annotation along with the @Autowired annotation.
  2. The @Qualifier is used to resolve ambiguous dependencies i.e, it helps @Autowired annotations to choose one of the dependency.
If there are multiple implementations for single interface then we can use @Qualifier to choose required implementation at runtime.
Let's understand the @Qualifier with detail example.
Let's take a Message Processing Example - a message can be sent in many ways like email, SMS, twitter etc.
Create MessageService interface for multiple message service implementations - EmailService, SMSServiceand TwitterService classes.
public interface MessageService {
    public void sendMsg(String message);
}
Create implementations - EmailService, SMSServiceand TwitterService classes.
public class EmailService implements MessageService{

    public void sendMsg(String message) {
         System.out.println(message);
    }

}
public class TwitterService implements MessageService{

    public void sendMsg(String message) {
        System.out.println(message);
    }

}
public class SMSService implements MessageService{

    public void sendMsg(String message) {
         System.out.println(message);
    }

}
It's time to see the usage of @Qualifier annotation.
public interface MessageProcessor {
    public void processMsg(String message);
}

public class MessageProcessorImpl implements MessageProcessor {

    private MessageService messageService;

    // setter based DI
    @Autowired
    @Qualifier("TwitterService")
    public void setMessageService(MessageService messageService) {
        this.messageService = messageService;
    }
 
    // constructor based DI
    @Autowired
    public MessageProcessorImpl(@Qualifier("TwitterService") MessageService messageService) {
        this.messageService = messageService;
    }
 
    public void processMsg(String message) {
        messageService.sendMsg(message);
    }
}
In the above example, Dependency injected by both setter and constructor so you can use either one of them.
Let's write the java based configuration.
@Configuration
@ComponentScan("com.javadevsguide.springframework.di")
public class AppConfiguration {

    @Bean(name="emailService")
    public MessageService emailService(){
         return new EmailService();
    }
 
    @Bean(name="twitterService")
    public MessageService twitterService(){
        return new TwitterService();
    }
 
    @Bean(name="smsService")
    public MessageService smsService(){
        return new SMSService();
    }
 
    @Bean
    public MessageProcessor messageProcessor(){
        return new MessageProcessorImpl(twitterService());
    }
}
Let's test the example using Spring IOC container that is an ApplicationContext object.
public class TestApplication {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfiguration.class);
        MessageProcessor  userService = applicationContext.getBean(MessageProcessor.class);
        userService.processMsg("twitter message sending ");
    }
}
Output:
twitter message sending

Conclusion

In this example, we have seen how to use @Qualifier annotation to resolve ambiguous dependencies. The implementation of this simple Spring @Qualifier Annotation Example can be found in the GitHub project – this is an Eclipse based project, so it should be easy to import and run as it is.

Spring Framework Related Posts

Comments

Post a Comment