[java] Spring @Autowired and @Qualifier

Is it auto detected with @Autowired? Is it dependency injection by name when @Qualifier is used? How can we do setter and constructor injection using these annotations?

The @Qualifier annotation is used to resolve the autowiring conflict, when there are multiple beans of same type.

The @Qualifier annotation can be used on any class annotated with @Component or on methods annotated with @Bean. This annotation can also be applied on constructor arguments or method parameters.

Ex:-

public interface Vehicle {
     public void start();
     public void stop();
}

There are two beans, Car and Bike implements Vehicle interface

@Component(value="car")
public class Car implements Vehicle {

     @Override
     public void start() {
           System.out.println("Car started");
     }

     @Override
     public void stop() {
           System.out.println("Car stopped");
     }
 }

@Component(value="bike")
public class Bike implements Vehicle {

     @Override
     public void start() {
          System.out.println("Bike started");
     }

     @Override
     public void stop() {
          System.out.println("Bike stopped");
     }
}

Injecting Bike bean in VehicleService using @Autowired with @Qualifier annotation. If you didn't use @Qualifier, it will throw NoUniqueBeanDefinitionException.

@Component
public class VehicleService {

    @Autowired
    @Qualifier("bike")
    private Vehicle vehicle;

    public void service() {
         vehicle.start();
         vehicle.stop();
    }
}

Reference:- @Qualifier annotation example

You can use @Qualifier along with @Autowired. In fact spring will ask you explicitly select the bean if ambiguous bean type are found, in which case you should provide the qualifier

For Example in following case it is necessary provide a qualifier

@Component
@Qualifier("staff") 
public Staff implements Person {}

@Component
@Qualifier("employee") 
public Manager implements Person {}


@Component
public Payroll {

    private Person person;

    @Autowired
    public Payroll(@Qualifier("employee") Person person){
          this.person = person;
    }

}

EDIT:

In Lombok 1.18.4 it is finally possible to avoid the boilerplate on constructor injection when you have @Qualifier, so now it is possible to do the following:

@Component
@Qualifier("staff") 
public Staff implements Person {}

@Component
@Qualifier("employee") 
public Manager implements Person {}


@Component
@RequiredArgsConstructor
public Payroll {
   @Qualifier("employee") private final Person person;
}

provided you are using the new lombok.config rule copyableAnnotations (by placing the following in lombok.config in the root of your project):

# Copy the Qualifier annotation from the instance variables to the constructor
# see https://github.com/rzwitserloot/lombok/issues/745
lombok.copyableAnnotations += org.springframework.beans.factory.annotation.Qualifier

This was recently introduced in latest lombok 1.18.4.

• The blog post where the issue is discussed in detail
• The original issue on github
• And a small github project to see it in action

NOTE

If you are using field or setter injection then you have to place the @Autowired and @Qualifier on top of the field or setter function like below(any one of them will work)

public Payroll {
   @Autowired @Qualifier("employee") private final Person person;
}

or

public Payroll {
   private final Person person;
   @Autowired
   @Qualifier("employee")
   public void setPerson(Person person) {
     this.person = person;
   } 
}

If you are using constructor injection then the annotations should be placed on constructor, else the code would not work. Use it like below -

public Payroll {

    private Person person;

    @Autowired
    public Payroll(@Qualifier("employee") Person person){
          this.person = person;
    }

}

@Autowired to autowire(or search) by-type
@Qualifier to autowire(or search) by-name
Other alternate option for @Qualifier is @Primary

@Component
@Qualifier("beanname")
public class A{}

public class B{

//Constructor
@Autowired  
public B(@Qualifier("beanname")A a){...} //  you need to add @autowire also 

//property
@Autowired
@Qualifier("beanname")
private A a;

}

//If you don't want to add the two annotations, we can use @Resource
public class B{

//property
@Resource(name="beanname")
private A a;

//Importing properties is very similar
@Value("${property.name}")  //@Value know how to interpret ${}
private String name;
}

more about @value