Question

Here’s my scenario. I have two classes ClassA and ClassB. ClassB inherits ClassA.

I’m using @Component annotation on both of them to make them Spring beans.

@Component
public class ClassA{
    // parent
}

@Component
public class ClassB extends ClassA{
    // child
}

public class ClassC{

    @Autowired
    private ClassA classA;

    public void doSomething(){

    }
}

Now, whenever I try to use @autowired annotation in ClassC to inject ClassA as shown above, I get the following exception.

No unique bean of type [ClassA] is defined: expected single matching bean but found 2: [classA, classB]

I understand that when auto-wiring by type, spring finds two beans that are assignable to ClassA and so it throws an exception.

Is there an easy way to resolve this problem apart from using @Qualifier annotation and auto-wiring by name?

Could I instruct spring to not only check for a bean that is assignable by type but also check to see if it is the exact same class type?

Answer 1

Yes, use the @Primary annotation.

If that is unfeasible and you do not want to use @Qualifier, then you will have to resolve it manually. Something like:

@Autowired
private List<ClassA> candidates;

private ClassA actual;

@PostConstruct
public void postConstruct() {
    for (ClassA candidate : candidates) {
        if (AopProxyUtils.ultimateTargetClass(candidate).equals(ClassA.class)) {
            actual = candidate;
            break;
        }
    }
    org.springframework.util.Assert.notNull(actual , "No bean of exact type ClassA found.");
}