Subtyping for Annotations

Last night, I managed to change the Java compiler to parse and encode annotations with inheritance. I’ve complained about this shortcoming many times already: Even though subtyping (inheritance) is one of the core concepts in Java, and even though annotations are represented using (slightly enhanced) interfaces, which support inheritance, Java disallows inheritance for annotations. I never quite understood why.

I’ve now created a modified compiler, based on the LAPT-javac compiler I wrote a while back. The new compiler is called xajavac, for “eXtended Annotation javac”, and supports annotations on local variables, like LAPT-javac, and subtyping for annotations.

Now I can write the following:

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@interface InvariantAnnotation { }
@interface OnlyThreadWithName extends InvariantAnnotation {
String value();
}
@interface OnlyEventThread extends InvariantAnnotation { }
@interface Or extends InvariantAnnotation {
InvariantAnnotation value();
}

@Or({@OnlyThreadWithName("main"),
@OnlyEventThread})
void foo() { ... }

I’m now working on providing an API that supports access to these annotations at runtime.