Say I have an annotation @Annotate and a class A. How can I check whether @Annotate is used on class A and its subclasses only (probably at compile time)?
So, if we have:
/* Example 1 */
public class A {
}
public class B extends A {
}
public class C {
}
How do I check that class A and class B can be annotated by @Annotate but class C is not allowed (and might raise a compile error).
If we go with the decision that this would be checked at compile time:
/* Example 2 */
public class A {
}
@Annotate
public class B extends A {
}
Example 2 will not raise a compile time error because @Annotate is used on a subclass of A. Example 3, however, will raise an compile error because @Annotate is not used on a subclass of A.
/* Example 3 */
@Annotate
public class C {
}
However, this does not have to be checked at compile time in anyway. I just personally thought that it makes sense to do so.
You should write an annotation processor. An annotation processor can generate new Java files and, more relevantly for your case, issue compile-time warnings.
You will invoke it at compile time by running the java compiler like this: javac -processor MyProcessor MyFile.java. If regular javac or your annotation processor issues any warnings, then compilation fails.
Your annotation processor's logic is specific to your @Annotate annotation and is simple: permit the annotation only on classes that subclass A. There are various tutorials on the web about how to write an annetation processor, in addition to the Oracle documentation I linked above.