Does the getting field name via reflection is an expensive operation that should be avoided?

I'm writing some validation code.

And don't want to declare much constants, so thinking about more dynamic way of how to get a name of properties of a class.

I.e.

class User {
   String firstname;
   String lastname;

   getters/setters ....
}

Is the access via the

User.class.getDeclaredField("firstname").getName();

is an expensive operation and I rather go with constants or some other way?