title: How Spring Security get currently logged in user in concept?
body: <p>I get the currently logged in user by 
  <code>SecurityContextHolder.getContext().getAuthentication()</code> in server side and do some logging on users.</p>
  
  <p>Here is the question:
  Suppose I have three user logged in.
  How the server side can identify the user just simply calling <code>SecurityContextHolder.getContext().getAuthentication();</code> ?</p>
  
  <p>Thanks for your reply.</p>