code: public List<String> getCustomAttributes() throws OAuthSystemException, OAuthProblemException, URISyntaxException {
  		cleanError();
  		prepareToken();
  
  		URIBuilder url = new URIBuilder(settings.getURL(Constants.GET_CUSTOM_ATTRIBUTES_URL));
  
  		OneloginURLConnectionClient httpClient = new OneloginURLConnectionClient();
  		OAuthClient oAuthClient = new OAuthClient(httpClient);
  		OAuthClientRequest bearerRequest = new OAuthBearerClientRequest(url.toString())
  			.buildHeaderMessage();
  
  		Map<String, String> headers = getAuthorizedHeader();
  		bearerRequest.setHeaders(headers);
  
  		List<String> customAttributes = null;
  		OneloginOAuthJSONResourceResponse oAuthResponse = oAuthClient.resource(bearerRequest, OAuth.HttpMethod.GET, OneloginOAuthJSONResourceResponse.class);
  		if (oAuthResponse.getResponseCode() == 200) {
  			customAttributes = oAuthResponse.getValuesFromData();
  		} else {
  			error = oAuthResponse.getError();
  			errorDescription = oAuthResponse.getErrorDescription();
  		}
  		return customAttributes;
  	}
docs: Gets a list of all custom attribute fields (also known as custom user fields) that have been defined for OL account.
  
  @return List of custom attribute fields
  
  @throws OAuthSystemException - if there is a IOException reading parameters of the httpURLConnection
  @throws OAuthProblemException - if there are errors validating the OneloginOAuthJSONResourceResponse and throwOAuthProblemException is enabled
  @throws URISyntaxException - if there is an error when generating the target URL at the URIBuilder constructor
  
  @see <a target="_blank" href="https://developers.onelogin.com/api-docs/1/users/get-custom-attributes">Get Custom Attributes documentation</a>