Checking if an annotation is of a specific type

问题

I am using reflection to see if an annotation that is attached to a property of a class, is of a specific type. Current I am doing:

if("javax.validation.Valid".equals(annotation.annotationType().getName())) {
   ...
}

Which strikes me as a little kludgey because it relies on a string that is a fully-qualified class-name. If the namespace changes in the future, this could cause subtle errors.

I would like to do:

if(Class.forName(annotation.annotationType().getName()).isInstance(
     new javax.validation.Valid()
)) {
   ...
}

But javax.validation.Valid is an abstract class and cannot be instantiated. Is there a way to simulate instanceof (or basically use isInstance) against an interface or an abstract class?

回答1:

Are you just looking for

if (annotation.annotationType().equals(javax.validation.Valid.class)){}

?

回答2:

Or even simpler:

if (annotation instanceof Valid) { /* ... */ }

回答3:

Just for completeness' sake, another possibility is

if (this.getClass().isAnnotationPresent(MyCustomAnnotation.class)) {

回答4:

Ok, I guess I should have done a little more research before posting the question. I discovered that I could use Class.isAssignableFrom(Class<?> cls):

import javax.validation.Valid;

if(Valid.class.isAssignableFrom(annotation.annotationType())) {
   ...
}

This seems to do the job. I'm not sure if there are any caveats to using this approach, though.

回答5:

Since an annotation is just a class, you can simply use an == compare:

if (annotation.annotationType() == Valid.class) { /* ... */ }

来源：https://stackoverflow.com/questions/3348363/checking-if-an-annotation-is-of-a-specific-type