Java中的注释元素可以是不同注释的数组吗？

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?

Answer 1

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})