Custom order by string parameter in HQL

Use a criteria query.

List<User> users = session.createCriteria(User.class)
    .add(Restrictions.isNotNull("joined"))
    .addOrder(Order.desc(order))
    .list();

---

Or, using HQL:

Query query = session.createQuery("from User as user where user.joined!=null order by user." + order + " desc");