title: HttpServletRequest.getSession(false): When does it return null?
body: <p>I'd like to know when to expect a "null" when calling <a href="http://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequest.html#getSession%28boolean%29" rel="noreferrer">HttpServletRequest.getSession(false)</a></p>
  
  <p>Also: Are there good tutorials on HttpSession? I'd like to get details like: when to invalidate()? what are the consequences? do I need to check if the returned HttpSession is valid? ... &lt;== Answer that in a comment please.</p>