code: @RequestMapping(value="/{providerId}", method=RequestMethod.GET, params="oauth_token")
  	public RedirectView oauth1Callback(@PathVariable String providerId, NativeWebRequest request) {
  		try {
  			OAuth1ConnectionFactory<?> connectionFactory = (OAuth1ConnectionFactory<?>) connectionFactoryLocator.getConnectionFactory(providerId);
  			Connection<?> connection = connectSupport.completeConnection(connectionFactory, request);
  			return handleSignIn(connection, connectionFactory, request);
  		} catch (Exception e) {
  			logger.error("Exception while completing OAuth 1.0(a) connection: ", e);
  			return redirect(URIBuilder.fromUri(signInUrl).queryParam("error", "provider").build().toString());
  		}
  	}
docs: Process the authentication callback from an OAuth 1 service provider.
  Called after the member authorizes the authentication, generally done once by having he or she click "Allow" in their web browser at the provider's site.
  Handles the provider sign-in callback by first determining if a local user account is associated with the connected provider account.
  If so, signs the local user in by delegating to {@link SignInAdapter#signIn(String, Connection, NativeWebRequest)}
  If not, redirects the user to a signup page to create a new account with {@link ProviderSignInAttempt} context exposed in the HttpSession.
  @param providerId the provider ID to authorize against
  @param request the request
  @return a RedirectView to the provider's authorization page or to the application's signin page if there is an error
  @see ProviderSignInAttempt
  @see ProviderSignInUtils