How do I get and set an object in session scope in JSF?

When you call the method FacesContext.getCurrentInstance() it will return the current thread but in case of P.S.V.M there will be no thread running in application context. So you get a NPE.

Better use something like this:

public String checker() {
    SessionFactory factory = HibernateUtil.getSessionFactory();
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Query q = session.createQuery("from UserLogin where UserId='"
            + uid.getUserId() + "'and Pswd='" + uid.getPswd()
            + "'and RoleId='" + uid.getRoleId() + "'");
    setUid((UserLogin) q.uniqueResult());

    System.out.println("upto here every thing is workind properly");

    if (uid != null) {
        FacesContext context = FacesContext.getCurrentInstance();

        HttpServletRequestrequest = (HttpServletRequest) context
                .getExternalContext().getRequest();
        HttpSession appsession = request.getSession(true);

        if (appsession.isNew() == false) {
            appsession.invalidate();
            appsession = request.getSession(true);
        }
        context.getExternalContext().getSessionMap().put("userbean", uid);
        session.close();
        return uid.getRoleId();
    } else
        return "invalid";
}

and put it into a session bean. You can use the code to validate users.