code: public String getCallBackUrl(ApiImplementor impl, String site) {
  		String url = site + CALL_BACK_URL + random.nextLong();
  		this.callBacks.put(url, impl);
  		logger.debug("Callback " + url + " registered for " + impl.getClass().getCanonicalName());
  		return url;
  	}
docs: Gets (and registers) a new callback URL for the given implementor and site.
  
  @param impl the implementor that will handle the callback.
  @param site the site that will call the callback.
  @return the callback URL.
  @since 2.0.0
  @see #removeCallBackUrl(String)
  @see #removeCallBackUrls(ApiImplementor)
  @see #removeApiImplementor(ApiImplementor)