title: Force JavaFx `duration` property to be known
body: <p>When I try to get a <a href="http://docs.oracle.com/javafx/2/api/javafx/scene/media/package-summary.html" rel="nofollow">Media</a> objects duration through the <code>getDuration()</code> method, it returns that it is unknown. I found this forum <a href="https://forums.oracle.com/thread/2287900" rel="nofollow">https://forums.oracle.com/thread/2287900</a> that says that it might not be known immediately.</p>
  
  <p>Yet that forum says its a network issue, but I'm not using a network. So what are other reasons for why the <code>duration</code> might not be known? How do I find it out?</p>