code: public static void register(Class<? extends Event> eventClass, Subscriber subscriber) {
          CopyOnWriteArraySet<Subscriber> set = SUBSCRIBER_MAP.get(eventClass);
          if (set == null) {
              set = new CopyOnWriteArraySet<Subscriber>();
              CopyOnWriteArraySet<Subscriber> old = SUBSCRIBER_MAP.putIfAbsent(eventClass, set);
              if (old != null) {
                  set = old;
              }
          }
          set.add(subscriber);
          if (LOGGER.isDebugEnabled()) {
              LOGGER.debug("Register subscriber: {} of event: {}.", subscriber, eventClass);
          }
      }
docs: 注册一个订阅者
  
  @param eventClass 事件类型
  @param subscriber 订阅者