code: @Nullable
    public static X509Certificate [] getRequestClientCertificates (@Nonnull final HttpServletRequest aHttpRequest)
    {
      return _getRequestAttr (aHttpRequest, SERVLET_ATTR_CLIENT_CERTIFICATE, X509Certificate [].class);
    }
docs: Get the client certificates provided by a HTTP servlet request.
  
  @param aHttpRequest
  The HTTP servlet request to extract the information from. May not be
  <code>null</code>.
  @return <code>null</code> if the passed request does not contain any client
  certificate