What is Annotation#annotationType() good for?

Question

The interface Annotation specifies a method

Class<? extends Annotation> annotationType()

which has a zero-information¹ javadoc. I wonder what it is good for. All I could find is this question, but it's not actually needed there (as the two top answers below the accepted one shows).

It allows us to use

a.annotationType().equals(MyAnnotation.class)

but

a instanceof MyAnnotation

does the same job... except when a is an instance of a class implementing multiple annotations - but have anyone ever seen such a beast?

If a is an instance of class A implements MyAnnotation, YourAnnotation, then the two above tests are not equivalent, but the question is which one is better...

^{(*) It's like stating that getX() returns x.}

There is now [JDK-8245620](https://bugs.openjdk.java.net/browse/JDK-8245620) which requests improving the documentation — Marcono1234, May 23 '20 at 17:44
JDK-8245620 is now marked as "Won't Fix" for JDK 15, but is fixed in JDK 16. — tom_mai78101, Aug 24 '21 at 20:18

score 13 · Accepted Answer · answered Mar 29 '16 at 19:56

13

It's like getClass(), except due to the way annotations are implemented at runtime, getClass() frequently returns a nonsense type for annotation instances.

annotationType() returns "the right thing" -- the reflective type of the annotation, not some nonsense runtime type.

Compare Enum.getDeclaringClass(), which returns the "right thing" for an enum constant, whereas myEnumConstant.getClass() may return a synthetic subtype of the enum.

answered Mar 29 '16 at 19:56

Louis Wasserman

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Now I understand... comparison to the `Enum` helped. If I ever want a `Map, Whatever>`, then the "real" class is needed. – maaartinus Mar 29 '16 at 20:10
Yup. I frequently pass around `ClassToInstanceMap`, for example. – Louis Wasserman Mar 29 '16 at 20:16

score 1 · Answer 2 · answered Mar 29 '16 at 20:07

One reason to use annotationType() is if your annotations are implemented using Proxy classes. Object.getClass() would return the proxy class whereas you may want the underlying "real" implementation class.

There are various examples of this, particularly when dependency injection or other heavyweight reflection is involved.

What is Annotation#annotationType() good for?

2 Answers2