What is the most common place to set the annotations, on the field or on the get- method?
On the field looks better, the validation info is more grouped together, but on interfaces it will be on the method anyway.
What about performance, will it take more to validate a property annotated on the field?

Bart

There is no performance impact, so it's more a matter of taste. When I'm lazy I put them on fields, when I'm not, I put them on properties to benefit from the encapsulation.