code: public static void registerMessageHandler(Class<? extends AVIMMessage> clazz,
                                              MessageHandler<?> handler) {
      Set<MessageHandler> handlerSet = new CopyOnWriteArraySet<>();
  
      Set<MessageHandler> set = messageHandlerRepository.putIfAbsent(clazz, handlerSet);
      if (set != null) {
        handlerSet = set;
      }
      handlerSet.add(handler);
    }
docs: 注册特定消息格式的处理单元
  
  @param clazz 特定的消息类
  @param handler