Elements of annotation in Java can be array of different annotations?

Question

I'm trying to create a custom annotation in Java and the question is if it's possible to create custom annotation with element of type "Array of annotations" and each element in this array will be from different type.

For example:

@MyCustomArrayOfAnnotation({@MyCustomAnnotationType1, @MyCustomAnnotationType2})

If it's possible, how can I declare this custom annotation?

Why do you need an array? I think you are overthinking some problem and it can be done much easier. — Fenio
– Fenio, Commented Feb 11, 2018 at 10:27

mernst · Accepted Answer · 2018-02-11 16:06:25Z

This is not possible.

The Java Language Specification, section 9.6.1, states that annotation elements/members may be of the following types:

A primitive type
String
Class
An enum type
An annotation type
An array type whose component type is one of the preceding types.

This shows that an annotation may contain an array of annotations.

However, there is no subtyping among annotations. Therefore, an annotation can only contain a homogeneous array, not a heterogeneous one.

An example of a homogeneous array (which is allowed) is

@MyCustomArrayOfAnnotation({@MyCustomAnnotationType1("a"), @MyCustomAnnotationType1("b")})

An example of a heterogeneous array (which is not allowed) is your example:

@MyCustomArrayOfAnnotation({@MyCustomAnnotationType1, @MyCustomAnnotationType2})

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