code: private void checkLink(LinkQueueItem queueItem)
      {
          int responseCode = this.httpChecker.check(queueItem.getLinkReference());
  
          Map<String, LinkState> contentReferences =
              this.linkStateManager.getLinkStates().get(queueItem.getLinkReference());
          if (contentReferences == null) {
              contentReferences = new ConcurrentHashMap<>();
          }
          LinkState state = new LinkState(responseCode, System.currentTimeMillis(), queueItem.getContextData());
          contentReferences.put(queueItem.getContentReference(), state);
          this.linkStateManager.getLinkStates().put(queueItem.getLinkReference(), contentReferences);
  
          // If there's an error, then send an Observation Event so that anyone interested can listen to it.
          if (responseCode < 200 || responseCode > 299) {
              Map<String, Object> eventData = new HashMap<>();
              eventData.put(EVENT_DATA_URL, queueItem.getLinkReference());
              eventData.put(EVENT_DATA_SOURCE, queueItem.getContentReference());
              eventData.put(EVENT_DATA_STATE, state);
              eventData.put(EVENT_DATA_CONTEXTDATA, queueItem.getContextData());
              sendEvent(queueItem.getLinkReference(), eventData);
          }
      }
docs: Perform the HTTP connection and save the result in the {@link LinkStateManager}.
  
  @param queueItem the link to check