code: public Boolean verifySubscriberRequestedSubscription(SubscriptionRequest sr) throws SubscriptionOriginVerificationException {
  
          LOG.info("(Hub) -> Subscriber, sending notification to verify the origin of the subscription {}.", sr.getCallback());
  
          SubscriptionConfirmationRequest sc = new SubscriptionConfirmationRequest(sr.getMode(),
                  sr.getTopic(), "challenge", "0");
  
          URI uri;
          try {
              uri = new URIBuilder(sr.getCallback()).setParameters(sc.toRequestParameters()).build();
          } catch (URISyntaxException e) {
              throw new SubscriptionOriginVerificationException("URISyntaxException while sending a confirmation of subscription", e);
          }
  
          HttpGet httpGet = new HttpGet(uri);
  
          CloseableHttpClient httpclient = HttpClients.createDefault();
  
          CloseableHttpResponse response;
          try {
              response = httpclient.execute(httpGet);
          } catch (IOException e) {
              throw new SubscriptionOriginVerificationException("IOException while sending a confirmation of subscription", e);
          }
  
          LOG.info("Subscriber replied with the http code {}.", response.getStatusLine().getStatusCode());
  
          Integer returnedCode = response.getStatusLine().getStatusCode();
  
          // if code is a success code return true, else false
          return (199 < returnedCode) && (returnedCode < 300);
      }
docs: Output method that sends a subscription confirmation for the subscriber to avoid DoS attacks, or false subscription.
  
  @param sr
  @return True case the subscription was confirmed, or False otherwise
  @throws org.ow2.chameleon.fuchsia.push.base.hub.exception.SubscriptionOriginVerificationException