title: PrimeFaces page update after async thread finish job
body: <p>I've tried to write some online calendar generator which generates print ready PDF calendars based on user images. The problem is that generating preview of this calendar is taking a lot of time, mainly since it's done in a different thread.
  The pages looks like:</p>
  
  <ul>
  <li>Page one: image uploading </li>
  <li>Page two: summary and calendar preview</li>
  </ul>
  
  <p>The preview starts to generate after images are uploaded on page one and can take 15-20 sec. By that time, the user can go to page two, where the "preview is being prepared" message is displayed.</p>
  
  <p>I'm using the callback mechanism to access session scoped bean to update item on page two, unfortunately when I trying to get RequestContext or FacesContext.getPartialViewContext() I get a null pointer exception.</p>
  
  <p>The question is: is there any way to solve that and update page component after the thread finises it's job?</p>