Question

In my project there’s a common base class that all client classes extend. This has an @Autowired field that needs to be injected by Hibernate. These are all grouped together in another class that has an @Autowired collection of the base class.

In order to reduce boilerplate for client code I’m trying to get @Component inherited. With @Component not doing this by default (apparently it used to though), I created this workaround annotation

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Component
@Inherited
public @interface InheritedComponent {
}

… and annotated the base class with it. Its not pretty but I hoped it would work. Unfortunately it didn’t, which really confuses me as @Inherited should make it work

Is there any other way to get @Component inherited? Or do I just have to say that any class that extends the base class needs this boilerplate?

Answer 1

The problem is that the Component annotation type itself needs to be marked with @Inherited.

Your @InheritedComponent annotation type is correctly inherited by any classes that extend a superclass which is marked with @InheritedComponent – but it does not inherit @Component. This is because you have @Component on the annotation, not the parent type.

An example:

public class InheritedAnnotationTest {

    @InheritedComponent
    public static class BaseComponent {
    }

    public static class SubClass extends BaseComponent {
    }

    public static void main(String[] args) {
        SubClass s = new SubClass();

        for (Annotation a : s.getClass().getAnnotations()) {
            System.out.printf("%s has annotation %s\n", s.getClass(), a);
        }
    }
}

Output:

class brown.annotations.InheritedAnnotationTest$SubClass has annotation @brown.annotations.InheritedComponent()

In other words, when resolving what annotations a class has, the annotations of the annotations are not resolved – they do not apply to the class, only the annotation (if that makes sense).