code: public void commence(HttpServletRequest req, HttpServletResponse resp, AuthenticationException authException) throws IOException, ServletException {
  
  		String redirectUrl = null;
  		logger.debug("No auth context found. Kicking off a new auth flow.");
  
  		AuthorizationCodeFlow flow = oAuth2Util.newAuthorizationCodeFlow();
  		GenericUrl url = flow.newAuthorizationUrl().setRedirectUri(getCallbackUrl());
  		url.set("approval_prompt", "force");
  		redirectUrl = url.build();
  		redirectStrategy.sendRedirect(req, resp, redirectUrl);
  
  	}
docs: Performs the redirect (or forward) to the login form URL.