AutoWire Need for Disambiguation

• The @Autowired annotation alone isn’t enough for Spring to understand which bean to inject.
• By default, Spring resolves autowired entries by type.
• If more than one bean of the same type is available in the container, the framework will throw NoUniqueBeanDefinitionException
• two possible candidates exist for Spring to inject as bean collaborators:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
@Component("fooFormatter")
public class FooFormatter implements Formatter {
 
    public String format() {
        return "foo";
    }
}

@Component("barFormatter")
public class BarFormatter implements Formatter {
 
    public String format() {
        return "bar";
    }
}

@Component
public class FooService {
     
    @Autowired
    private Formatter formatter;
}

• If we try to load FooService into our context, the Spring framework will throw a NoUniqueBeanDefinitionException.

@Qualifier Annotation

• solve the problem by including the @Qualifier annotation to indicate which bean we want to use:

1
2
3
4
5
6
public class FooService {
     
    @Autowired
    @Qualifier("fooFormatter")
    private Formatter formatter;
}

• the qualifier name to be used is the one declared in the @Component annotation.

• could have also used the @Qualifier annotation on the Formatter implementing classes, instead of specifying the names in their @Component annotations, to obtain the same effect:

1
2
3
4
5
6
7
8
9
10
11
@Component
@Qualifier("fooFormatter")
public class FooFormatter implements Formatter {
    //...
}

@Component
@Qualifier("barFormatter")
public class BarFormatter implements Formatter {
    //...
}

@Qualifier vs @Primary

• The @Primary annotation is useful when we want to specify which bean of a certain type should be injected by default.
• If we require the other bean at some injection point, we would need to specifically indicate it. We can do that via the @Qualifier annotation.
• if both the @Qualifier and @Primary annotations are present, then the @Qualifier annotation will have precedence.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
@Configuration
public class Config {
 
    @Bean
    public Employee johnEmployee() {
        return new Employee("John");
    }
 
    @Bean
    @Primary
    public Employee tonyEmployee() {
        return new Employee("Tony");
    }
}

Reference

• https://www.baeldung.com/spring-qualifier-annotation