Javax Annotation Processing: Check if an annotated type is a sub-type of another type

Question

I'm creating an AnnotationProcessor and I need to check if the annotated type is a subtype of an specified class. The annotation:

public @interface Component {
    Class<?> supertype();
}

Example: (correct)

@Component(supertype = MyInterface.class)
public class MyClass implements MyInterface {
   // ...
}

Example: (incorrect, this must not compile because MyClass isn't a subtype of String )

@Component(supertype = String.class)
public class MyClass implements MyInterface {

}

I know that I cannot get the annotated Class because it isn't compiled yet.

Answer 1

You don't mention it in your question, but I assume you're running into TypeMirror issues.

Here's a very old and hacky solution to get to the TypeMirror of your supertype field, and compare it against the type's base class:

@Override
public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
    for (Element element : roundEnvironment.getElementsAnnotatedWith(Component.class)) {
        // TODO: Ensure element is a class before continuing

        TypeElement typeElement = (TypeElement) element;
        Component componentAnnotation = typeElement.getAnnotation(Component.class);
        TypeMirror superType = null;
        try {
            // Hack to quickly get to TypeMirror of the annotation property
            componentAnnotation.supertype();
        } catch (MirroredTypeException mte) {
            superType = mte.getTypeMirror();
        }

        // TODO: superType null check

        Set<TypeMirror> validTypes = Sets.newHashSet(typeElement.getInterfaces());
        validTypes.add(typeElement.getSuperclass());

        if (!validTypes.contains(superType)) {
            // TODO: throw something better
            throw new IllegalArgumentException(typeElement.toString() + " does not implement or inherit from "
                    + superType.toString() + " declared in @Component annotation");
        }
    }

    return true;
}

If I were you I'd look into this a bit more to see if there are better options these days, but if not this will solve your problem.

Answer 2

This version does not rely on exceptions, it uses Element.getAnnotationMirrors which is probably there for this type of use. It's a lot more verbose though:

    @Override
    public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
        for (Element element : roundEnvironment.getElementsAnnotatedWith(Component.class)) {
            // TODO: Ensure element is a class before continuing

            TypeMirror superType = null;

            boolean found = false;
            for (AnnotationMirror mirror : element.getAnnotationMirrors()) {
                TypeElement annotationElement = (TypeElement) mirror.getAnnotationType().asElement();
                if (annotationElement.getQualifiedName().contentEquals(Component.class.getName())) {
                    found = true;

                    for (Map.Entry<? extends ExecutableElement, ? extends AnnotationValue> kv :
                            mirror.getElementValues().entrySet()) {

                        ExecutableElement member = kv.getKey();
                        if (member.getKind() == ElementKind.METHOD &&
                                member.getSimpleName().contentEquals("supertype")) {
                            superType = (TypeMirror) kv.getValue().getValue();
                            break;
                        }
                    }
                }

                if (found) {
                    break;
                }
            }

            // TODO: superType null check

            TypeElement typeElement = (TypeElement) element;

            Set<TypeMirror> validTypes = Sets.newHashSet(typeElement.getInterfaces());
            validTypes.add(typeElement.getSuperclass());

            if (!validTypes.contains(superType)) {
                // TODO: throw something better
                throw new IllegalArgumentException(typeElement.toString() + " does not implement or inherit from "
                        + superType.toString() + " declared in @Component annotation");
            }
        }

        return true;
    }

Answer 3

Use the following code to get the TypeMirror of the class value in the annotation

public Optional<TypeMirror> getClassValueFromAnnotation(Element element, Class<? extends Annotation> annotation, String paramName) {
    for (AnnotationMirror am : element.getAnnotationMirrors()) {
        if (types.isSameType(am.getAnnotationType(), elements.getTypeElement(annotation.getCanonicalName()).asType())) {
            for (Map.Entry<? extends ExecutableElement, ? extends AnnotationValue> entry : am.getElementValues().entrySet()) {
                if (paramName.equals(entry.getKey().getSimpleName().toString())) {
                    AnnotationValue annotationValue = entry.getValue();
                    return Optional.of((DeclaredType) annotationValue.getValue());
                }
            }
        }
    }
    return Optional.empty();
}

then use this code to check if it is a super type of a specific class

 /**
     * A wrapper over {@link Types#isAssignable(TypeMirror, TypeMirror)} which will apply type erasure on the targetClass before calling the wrapped method.
     *
     * @param typeMirror  a {@link javax.lang.model.type.TypeMirror} object.
     * @param targetClass a {@link java.lang.Class} object.
     * @return a boolean.
     */
    public boolean isAssignableFrom(TypeMirror typeMirror, Class<?> targetClass) {
        return types.isAssignable(typeMirror, types.erasure(elements.getTypeElement(targetClass.getCanonicalName()).asType()));
    }