How do Java method annotations work in conjunction with method overriding?

问题

I have a parent class Parent and a child class Child, defined thus:

class Parent {
    @MyAnnotation(\"hello\")
    void foo() {
        // implementation irrelevant
    }
}
class Child {
    @Override
    foo() {
        // implementation irrelevant
    }
}

If I obtain a Method reference to Child::foo, will childFoo.getAnnotation(MyAnnotation.class) give me @MyAnnotation? Or will it be null?

I\'m interested more generally in how or whether annotation works with Java inheritance.

回答1:

Copied verbatim from http://www.eclipse.org/aspectj/doc/released/adk15notebook/annotations.html#annotation-inheritance:

Annotation Inheritance

It is important to understand the rules relating to inheritance of annotations, as these have a bearing on join point matching based on the presence or absence of annotations.

By default annotations are not inherited. Given the following program

        @MyAnnotation
        class Super {
          @Oneway public void foo() {}
        }

        class Sub extends Super {
          public void foo() {}
        }

Then Sub does not have the MyAnnotation annotation, and Sub.foo() is not an @Oneway method, despite the fact that it overrides Super.foo() which is.

If an annotation type has the meta-annotation @Inherited then an annotation of that type on a class will cause the annotation to be inherited by sub-classes. So, in the example above, if the MyAnnotation type had the @Inherited attribute, then Sub would have the MyAnnotation annotation.

@Inherited annotations are not inherited when used to annotate anything other than a type. A type that implements one or more interfaces never inherits any annotations from the interfaces it implements.

回答2:

You found your answer already: there is no provision for method-annotation inheritance in the JDK.

But climbing the super-class chain in search of annotated methods is also easy to implement:

/**
 * Climbs the super-class chain to find the first method with the given signature which is
 * annotated with the given annotation.
 *
 * @return A method of the requested signature, applicable to all instances of the given
 *         class, and annotated with the required annotation
 * @throws NoSuchMethodException If no method was found that matches this description
 */
public Method getAnnotatedMethod(Class<? extends Annotation> annotation,
                                 Class c, String methodName, Class... parameterTypes)
        throws NoSuchMethodException {

    Method method = c.getMethod(methodName, parameterTypes);
    if (method.isAnnotationPresent(annotation)) {
        return method;
    }

    return getAnnotatedMethod(annotation, c.getSuperclass(), methodName, parameterTypes);
}

回答3:

Using Spring Core you can resolve with

AnnotationUtils.java

回答4:

While the answer to the question as asked is that Java's Method.getAnnotation() does not consider overridden methods, sometimes it is useful to find these annotations. Here is a more complete version of Saintali's answer that I'm currently using:

public static <A extends Annotation> A getInheritedAnnotation(
    Class<A> annotationClass, AnnotatedElement element)
{
    A annotation = element.getAnnotation(annotationClass);
    if (annotation == null && element instanceof Method)
        annotation = getOverriddenAnnotation(annotationClass, (Method) element);
    return annotation;
}

private static <A extends Annotation> A getOverriddenAnnotation(
    Class<A> annotationClass, Method method)
{
    final Class<?> methodClass = method.getDeclaringClass();
    final String name = method.getName();
    final Class<?>[] params = method.getParameterTypes();

    // prioritize all superclasses over all interfaces
    final Class<?> superclass = methodClass.getSuperclass();
    if (superclass != null)
    {
        final A annotation =
            getOverriddenAnnotationFrom(annotationClass, superclass, name, params);
        if (annotation != null)
            return annotation;
    }

    // depth-first search over interface hierarchy
    for (final Class<?> intf : methodClass.getInterfaces())
    {
        final A annotation =
            getOverriddenAnnotationFrom(annotationClass, intf, name, params);
        if (annotation != null)
            return annotation;
    }

    return null;
}

private static <A extends Annotation> A getOverriddenAnnotationFrom(
    Class<A> annotationClass, Class<?> searchClass, String name, Class<?>[] params)
{
    try
    {
        final Method method = searchClass.getMethod(name, params);
        final A annotation = method.getAnnotation(annotationClass);
        if (annotation != null)
            return annotation;
        return getOverriddenAnnotation(annotationClass, method);
    }
    catch (final NoSuchMethodException e)
    {
        return null;
    }
}

来源：https://stackoverflow.com/questions/10082619/how-do-java-method-annotations-work-in-conjunction-with-method-overriding

标签

java

inheritance

annotations

overriding