Criteria criteria = ...
criteria.setMaxResults(n);
criteria.addOrder(Order.asc("rand()")); // <-- something like this
return criteria.list();

package xy;
import org.hibernate.Criteria;
import org.hibernate.HibernateException;
import org.hibernate.criterion.CriteriaQuery;
import org.hibernate.criterion.Order;

public class OrderRandom extends Order
{
   protected final static String RANDOM = "random()"; //random() generates a random number on PostgreSQL
   private static final long serialVersionUID = 3815371067383546907L;

   public OrderRandom()
   {
      super(null,true);
   }

   @Override
   public String toSqlString(Criteria arg0, CriteriaQuery arg1) throws HibernateException
   {
      return this.toString();
   }
   
   @Override
   public String toString()
   {
      return RANDOM;
   }
}

...
criteria.addOrder(new OrderRandom());
...