class A {

  private B reference1;
}

abstract class B {
}

class B1 extends B {

  private SOME_TYPE reference2;
}

class B2 extends B {
}

TypedQuery<A> q = em.createNamedQuery("select e from A e", A.class);

LoadGroup lg = new LoadGroup();
lg.addAttribute("reference1.reference2");
q.setHint(QueryHints.LOAD_GROUP, lg);

q.getSingleResult();