code: public HystrixEventNotifier getEventNotifier() {
          if (notifier.get() == null) {
              // check for an implementation from Archaius first
              Object impl = getPluginImplementation(HystrixEventNotifier.class);
              if (impl == null) {
                  // nothing set via Archaius so initialize with default
                  notifier.compareAndSet(null, HystrixEventNotifierDefault.getInstance());
                  // we don't return from here but call get() again in case of thread-race so the winner will always get returned
              } else {
                  // we received an implementation from Archaius so use it
                  notifier.compareAndSet(null, (HystrixEventNotifier) impl);
              }
          }
          return notifier.get();
      }
docs: Retrieve instance of {@link HystrixEventNotifier} to use based on order of precedence as defined in {@link HystrixPlugins} class header.
  <p>
  Override default by using {@link #registerEventNotifier(HystrixEventNotifier)} or setting property (via Archaius): <code>hystrix.plugin.HystrixEventNotifier.implementation</code> with the full classname to
  load.
  
  @return {@link HystrixEventNotifier} implementation to use