code: public static AVIMClient getInstance(String clientId) {
      if (StringUtil.isEmpty(clientId)) {
        return null;
      }
      AVIMClient client = clients.get(clientId);
      if (null == client) {
        client = new AVIMClient(clientId);
        AVIMClient elderClient = clients.putIfAbsent(clientId, client);
        if (null != elderClient) {
          client = elderClient;
        }
      }
      return client;
    }
docs: get AVIMClient instance by clientId.
  @param clientId
  @return