Use cases for implementing annotations

Question

What are valid use cases for implementing annotations?

When designing primarily annotation based configuration systems I occasionally need to create classes which i

Answer 1

There are no valid user cases for that - compiler just tollerates it since it would be quite messy to forbid it and people who are writing compilers may need the facility on a very rare occasion. If you need to classify annotations check out this article to see how to do it: Why is not possible to extend annotations in Java?

Inagine a poor soul coming after you to maintain and debug that code or another one who needs to write a codegen tool and assumes that annotatuion types are straight or another who just used such annotation not even dreaming what can happend and what to do about it. By the time he discovers that hack and finds a way to eliminate it he's going to die from hernia - or equivalent ailment :-) Annotations are expected to be purely declarative statements, interpreted solely by a codegen tool which runs separately from the annotated code and treats it as data.

Take a fresh look at that code and try to honestly say what's a rational rason for something like:

 public Class annotationType() { 
     return Id.class; 
 } 

and that's stil a small thing compared to that people can put in code.

Annotations are not the place to practice hacking - that's what compiler is trying to convey. Do you know exactly when and how the code in "implementation" of annotation may run? Including CTOR? What is available and what not at that point? What is safe to call? Compiler doesn't - it would take pretty heavy static analysis for a compiler to check actual safety of such hack. So instead it just issues a warning so that when something goes wrong people can't blame compile, VM and everything else.