code: public static AVIMClient getInstance(AVUser user) {
      if (null == user) {
        return null;
      }
      String clientId = user.getObjectId();
      String sessionToken = user.getSessionToken();
      if (StringUtil.isEmpty(clientId) || StringUtil.isEmpty(sessionToken)) {
        return null;
      }
      AVIMClient client = getInstance(clientId);
      client.userSessionToken = sessionToken;
      return client;
    }
docs: get AVIMClient instance by AVUser
  @param user
  @return