servlet. All servlet containers support basic, form
 and client certificate authentication, and may additionally support digest authentication. If the servlet is not
 authenticated <code>null</code> is returned.

 <p>
 Same as the value of the CGI variable AUTH_TYPE.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>one of the static members BASIC_AUTH, FORM_AUTH, CLIENT_CERT_AUTH, DIGEST_AUTH (suitable for == comparison)
 or the container-specific string indicating the authentication scheme