Passing annotation properties to meta-annotations

40
votes

Say I have an annotation with a property:

@Named(name = "Steve")
private Person person

and I want to create a compound annotation with several meta-annotations, including the one that takes a property

@Named
@AnotherAnnotation
@YetAnotherAnnotation
public @interface CompoundAnnotation {

    ...
}

Is there a way that I can pass properties to the compound annotation to one of the meta annotations?

Eg, something like this:

@CompoundAnnotation(name = "Bob")
private Person person;

that is equivalent to, but much more convenient than

@Named(name = "Bob")
@AnotherAnnotation
@YetAnotherAnnotation
private Person person;

Thanks!

PS apologies for my poor choice of an example annotation - I didn't have the javax.inject.@Named annotation in mind, just some arbitrary annotation that has properties.

Thank you everyone for your answers/comments.

It definitely seems to be the case that this is not possible. However, it just happens that there is a simple work-around for my case-in-point, which I will share in case it helps anyone:

I am working with Spring and want to create my own Annotations that have @Component as a meta-annotation, thus being autodetected by component scanning. However, I also wanted to be able to set the BeanName property (corresponding to the value property in @Component) so I could have custom bean names.

Well it turns out that the thoughtful guys at Spring made it possible to do just that - the AnnotationBeanNameGenerator will take the 'value' property of whatever annotation it is passed and use that as the bean name (and of course, by default, it will only get passed annotations that are @Component or have @Component as a meta-annotation). In retrospect this should have been obvious to me from the start - this is how existing annotations with @Component as a meta-annotation, such as @Service and @Registry, can provide bean names.

Hope that is useful to someone. I still think it's a shame that this is not possible more generally though!

2
javaannotations
I don't see how, unless you're adding annotations via byte-code manipulation at load time. (Or a custom annotation processor, I suppose.) Curious to see what all is possible w/o tricks, though. – Dave Newton
Just thinking aloud here, will this not be a workaround if you had a base class annotated with AnotherAnnotation and YAA, and then Person class extended that? Look at Reflections too, maybe you'll get some ideas: code.google.com/p/reflections – maksimov
Related to stackoverflow.com/questions/1624084/… – Gray

2 Answers

47
votes

It is a few years later now, and since you are using Spring, what you are asking for is sort of possible now using the @AliasFor annotation.

For example:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@SpringApplicationConfiguration
@ActiveProfiles("test")
public @interface SpringContextTest {

    @AliasFor(annotation = SpringApplicationConfiguration.class, attribute = "classes")
    Class<?>[] value() default {};

    @AliasFor("value")
    Class<?>[] classes() default {};
}

Now you can annotate your test with @SpringContextTest(MyConfig.class), and the amazing thing is that it actually works the way you would expect.

10
votes

Is there a way that I can pass properties to the compound annotation to one of the meta annotations?

I think the simple answer is "no". There is no way to ask Person what annotations it has on it and get @Named for example.

The more complex answer is that you can chain annotations but you would have to investigate these annotations via reflection. For example, the following works:

@Bar
public class Foo {
    public static void main(String[] args) {
        Annotation[] fooAnnotations = Foo.class.getAnnotations();
        assertEquals(1, fooAnnotations.length);
        for (Annotation annotation : fooAnnotations) {
            Annotation[] annotations =
                annotation.annotationType().getAnnotations();
            assertEquals(2, annotations.length);
            assertEquals(Baz.class, annotations[0].annotationType());
        }
    }

    @Baz
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Bar {
    }

    @Retention(RetentionPolicy.RUNTIME)
    public @interface Baz {
    }
}

However the following statement will return null:

// this always returns null
Baz baz = Foo.class.getAnnotation(Baz.class)

This means that any 3rd party class that is looking for the @Baz annotation won't see it.