mám dataTable svázaný s vector proměnnou, takže do ní tahám data přes  public Vector
managed bean:
<Room> getAllRooms() {
        if(allRooms == null){
            allRooms = roomSBLocal.GetAllRooms();
        }
        return allRooms;
    }
session bean:
@Override
    public Vector<Room> GetAllRooms() {
        Vector<Room> rooms = getRoomDAO().GetAllRooms();
        return rooms;
    }

DAO:
public static String QUERY_SELECT_ALL_ROOMS = "SELECT r FROM Room r";

@Override
    public Vector<Room> GetAllRooms() {
        return (Vector<Room>)em.createQuery(QUERY_SELECT_ALL_ROOMS).getResultList();
    }