Annotation member which holds other annotations?

Question

I want to create a custom annotation (using Java) which would accept other annotations as parameter, something like:

public @interface ExclusiveOr {
    Anno         


        

Answer 1

Depending on the reason why you would want to specify other annotations there are multiple solutions:

An array of instances of a single annotation type

Probably not what you meant in your question, but if you want to specify multiple instances of a single annotation type it's certainly possible:

public @interface Test {
    SomeAnnotation[] value();
}

An array of annotation types instead of instances

If you do not need to specify any parameters on the individual annotations you can just user their class objects instead of instances.

public @interface Test {
    Class[] value();
}

But an enum would of course also do the trick in most situations.

Use multiple arrays

If the set of possible annotation types you want to use is limited, you can create a separate parameter for each one.

public @interface Test {
    SomeAnnotation[] somes() default { };
    ThisAnnotation[] thiss() default { };
    ThatAnnotation[] thats() default { };
}

Giving a default value to each member makes it possible to only specify arrays for the types you need.