#vulnerable code 
                                private void refreshSrvIfNeed() {
        try {

            if (!CollectionUtils.isEmpty(serverList)) {
                LogUtils.LOG.debug("server list provided by user: " + serverList);
                return;
            }

            if (System.currentTimeMillis() - lastSrvRefTime < vipSrvRefInterMillis) {
                return;
            }

            List<String> list = getServerListFromEndpoint();

            if (list.isEmpty()) {
                throw new Exception("Can not acquire vipserver list");
            }

            if (!CollectionUtils.isEqualCollection(list, serversFromEndpoint)) {
                LogUtils.LOG.info("SERVER-LIST", "server list is updated: " + list);
            }

            serversFromEndpoint = list;
            lastSrvRefTime = System.currentTimeMillis();
        } catch (Throwable e) {
            LogUtils.LOG.warn("failed to update server list", e);
        }
    } 
                            #location 15 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void refreshSrvIfNeed() {
        try {

            if (!CollectionUtils.isEmpty(serverList)) {
                LogUtils.LOG.debug("server list provided by user: " + serverList);
                return;
            }

            if (System.currentTimeMillis() - lastSrvRefTime < vipSrvRefInterMillis) {
                return;
            }

            List<String> list = getServerListFromEndpoint();

            if (CollectionUtils.isEmpty(list)) {
                throw new Exception("Can not acquire vipserver list");
            }

            if (!CollectionUtils.isEqualCollection(list, serversFromEndpoint)) {
                LogUtils.LOG.info("SERVER-LIST", "server list is updated: " + list);
            }

            serversFromEndpoint = list;
            lastSrvRefTime = System.currentTimeMillis();
        } catch (Throwable e) {
            LogUtils.LOG.warn("failed to update server list", e);
        }
    }