title: How to check if a session is invalid
body: <p>How to check if a session is invalid or not?
  There is no method in the <a href="http://java.sun.com/products/servlet/2.2/javadoc/javax/servlet/http/HttpSession.html" rel="noreferrer">API</a>.</p>
  
  <p>Is it the same as <code>isNew()</code>? And what is the difference if not?</p>