form action="PlaylistServlet" method="get">
<input type = "text" name ="userId" placeholder="userName">
<input type="password" name="password" placeholder="Password">
<input type="submit" value="submit">
</form>

protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    Users user1 = new Users();
    String userName = request.getParameter("userName");
    String password = request.getParameter("password");
    SetData sd = new SetData();
    Users us = sd.get(userName); //get method in service class. Line 38
    String passwordDB = us.getPassword();// line 39
    if (user1.checkPassword(password, passwordDB)) {
        response.sendRedirect("UserLogin.html");
    }
}

public Users get(String userName) {

    Users us = null;
    SessionFactory sf = null;
    Session session1 = null;
    try {
        sf = new Configuration().configure("/hibernate.cfg.xml").buildSessionFactory();
        session1 = sf.openSession();
        Transaction tx = session1.beginTransaction();

        us = (Users) session1.get(Users.class,userName); // line 64
        tx.commit();
        session1.close();
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        sf.close();
    }
    return us;

}

java.lang.IllegalArgumentException: id to load is required for loading
at com.marv.service.SetData.get(SetData.java:64)
at com.marv.service.PlaylistServlet.doGet(PlaylistServlet.java:38)

java.lang.NullPointerException
at com.marv.service.PlaylistServlet.doGet(PlaylistServlet.java:39)

session1.get(Users.class,userName);