private final ConcurrentMap<String /* group name */, ConcurrentMap<Channel, ClientChannelInfo>> groupChannelTable =
        new ConcurrentHashMap<>();
...
public Channel getAvailableChannel(String groupId) {
        if (groupId == null) {
            return null;
        }
        List<Channel> channelList;
        ConcurrentMap<Channel, ClientChannelInfo> channelClientChannelInfoHashMap = groupChannelTable.get(groupId);
        if (channelClientChannelInfoHashMap != null) {
            channelList = new ArrayList<>(channelClientChannelInfoHashMap.keySet());
        } else {
            log.warn("Check transaction failed, channel table is empty. groupId={}", groupId);
            return null;
        }

        int size = channelList.size();
        if (0 == size) {
            log.warn("Channel list is empty. groupId={}", groupId);
            return null;
        }

        Channel lastActiveChannel = null;

        int index = positiveAtomicCounter.incrementAndGet() % size;
        Channel channel = channelList.get(index);
        int count = 0;
        boolean isOk = channel.isActive() && channel.isWritable();
        while (count++ < GET_AVAILABLE_CHANNEL_RETRY_COUNT) {
            if (isOk) {
                return channel;
            }
            if (channel.isActive()) {
                lastActiveChannel = channel;
            }
            index = (++index) % size;
            channel = channelList.get(index);
            isOk = channel.isActive() && channel.isWritable();
        }

        return lastActiveChannel;
    }