public abstract class AbstractZookeeperTransporter implements ZookeeperTransporter {
    //缓存 zk client，key为zk地址
    private final Map<String, ZookeeperClient> zookeeperClientMap = new ConcurrentHashMap<>();

    
    @Override
    public ZookeeperClient connect(URL url) {
        ZookeeperClient zookeeperClient;
        List<String> addressList = getURLBackupAddress(url);
        // 这个地方会走缓存
        if ((zookeeperClient = fetchAndUpdateZookeeperClientCache(addressList)) != null && zookeeperClient.isConnected()) {
            logger.info("find valid zookeeper client from the cache for address: " + url);
            return zookeeperClient;
        }
        。。。
    }

    ZookeeperClient fetchAndUpdateZookeeperClientCache(List<String> addressList) {

        ZookeeperClient zookeeperClient = null;
        for (String address : addressList) {
            if ((zookeeperClient = zookeeperClientMap.get(address)) != null && zookeeperClient.isConnected()) {
                break;
            }
        }
        if (zookeeperClient != null && zookeeperClient.isConnected()) {
            //将 zookeeperClient 放入 zookeeperClientMap 缓存中
            writeToClientMap(addressList, zookeeperClient);
        }
        return zookeeperClient;
    }
    void writeToClientMap(List<String> addressList, ZookeeperClient zookeeperClient) {
        for (String address : addressList) {
            // 使用 address 作为key
            zookeeperClientMap.put(address, zookeeperClient);
        }
    }
}