code: @Override
  	public HttpService addingService(
  			final ServiceReference<HttpService> serviceReference) {
  		LOG.debug("HttpService available {}", serviceReference);
  		lock.lock();
  		HttpService addedHttpService = null;
  		try {
  			if (httpService != null) {
  				return super.addingService(serviceReference);
  			}
  			httpService = super.addingService(serviceReference);
  			addedHttpService = httpService;
  		} finally {
  			lock.unlock();
  		}
  		try {
  			listener.available(addedHttpService);
  		} catch (Exception ignore) { // CHECKSTYLE:SKIP
  			LOG.error("Cannot register", ignore);
  		}
  		return addedHttpService;
  	}
docs: Gets the service if one is not already available and notify listeners.
  
  @see org.osgi.util.tracker.ServiceTracker#addingService(org.osgi.framework.ServiceReference)