Java metaprogramming conundrum: get all annotations that are itself annotated by a given annotation A

Question

Do you think you are a java wizard?

That you are well versed in the secrets of the reflection API?

public @interface @a {}
public @interface @b {}
@Mark public @interface @c {}    
@Mark public @interface @d {}
public @interface @e {}

public Class C
{
    @a @b @c @d @e public void x();
}

public class Solver
{
    public Annotation[] solve(Method m, Class c);
}

You have to write method solve, so that if it is invoked on method C.x() and Mark.class it returns {c, d}.

(This is not homework, is a real programming assignment for a framework metaprogramming framework I am trying to develop)

Answer 1

Actually, I don't get how this is even remotely tricky.

Update, forgot to include the contains function and also made a mistake switching Annotation.getClass() with Annotation.annotationType(). This code works

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
@Mark
public @interface A {}

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface B {}

@Retention(RetentionPolicy.RUNTIME)
@Target(value = ElementType.TYPE)
public @interface Mark {}

public class C {
@A @B public void f() {}
}

public class Solver {
public static boolean  contains(Annotation a, Class<?> targetAnnotation) {
    Class<?> c = a.annotationType();
    Annotation[] cas = c.getAnnotations();
    for (Annotation aa : cas) {
        if (aa.annotationType().equals(targetAnnotation)) {
            return true;
        }
    }
    return false;
}

public static Annotation[] getMarked(Method m) {
    List<Annotation> retVal = new ArrayList<Annotation>();
    for (Annotation a : m.getAnnotations()) {
        if (contains(a.getClass().getAnnotations(), Mark.class) {
            retVal.add(a);
        }
    }
    return retVal.toArray(new Annotation[]{});
}

public static void main(String[] args) throws SecurityException, NoSuchMethodException {
    Annotation[] result = getMarked(C.class.getMethod("f"));    
}
} // solver

Note that this requires that all the annotations be marked with runtime level retention and also that returning Annotation[] is probably not what you want. You probably want to return a Class[] full of the actual types (in my example, A.class)

Answer 2

This is tested to work. It was indeed harder than it should have been.

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface a{}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface b{}

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.ANNOTATION_TYPE})
public @interface Mark{}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Mark
public @interface c{}

public static class D {
    @a @b @c
    public void x() {}
}

public static void main(String[] args) throws Exception {
    Method m = D.class.getMethod("x");

    Collection<Annotation> ret = new HashSet<Annotation>();
    Annotation[] annotations = m.getAnnotations();
    for (Annotation annotation : annotations) {
        Annotation subAnnots = annotation.annotationType().getAnnotation(Mark.class);
        if (subAnnots != null) {
            ret.add(annotation);
        }
    }
    System.out.println(ret);
}

I guess it just begs the question of why annotationType() works, but getClass() does not.