What about participating in JSR-250?
You can suggest some more annotations to it:
1. @Mandatory (I think it's better than @NotNull/@NonNull because it can
be mapped to XML Schema "mandatory" constraint) and @Optional (instead
2. @LowerXXXBound(XXX)/@UpperXXXBound, where XXX is one of: Int, Long,
Float, Double, Short or Size (to define value ranges, string length
constraints and array and collection sizes).
3. @Invariant(String expression) for constraints that can be validated
by plugins and user-defined validators.
And package will be, of course, javax.annotation.