throwing a <code>ServletException</code> when the login mechanism configured for the
 <code>ServletContext</code> supports username password validation, and when, at the time of the call to login, the
 identity of the caller of the request had not been established (i.e, all of <code>getUserPrincipal</code>,
 <code>getRemoteUser</code>, and <code>getAuthType</code> return null), and when validation of the provided
 credentials is successful. Otherwise, this method throws a <code>ServletException</code> as de