dubbo 小试：连接Zookeeper 超时

前言

dubbo 利用 curator 去连接 zookeeper 的时候，会需要比较长的时间，超过默认的(dubbo:registry:timeout)，时间有点过长，应该是一个bug。暂时的解决方法是：把参数延长。

显示

Caused by: java.lang.IllegalStateException: zookeeper not connected
	at org.apache.dubbo.remoting.zookeeper.curator.CuratorZookeeperClient.<init>(CuratorZookeeperClient.java:80) ~[dubbo-2.7.5.jar:2.7.5]
	... 38 common frames omitted

网络抓包分析

在Dubbo消费者或提供者向zookeeper注册自己的时候，发起网络连接前会停顿很久。（红框部分是用于共享计算机名字和IP的报文，与本次讨论无关）。

初步结论：zookeeper 的客户端连接前有被阻塞的嫌疑

// ClientCnxn.java:930
           setName(getName().replaceAll("\\(.*\\)",
                    "(" + addr.getHostName() + ":" + addr.getPort() + ")"));
                    

// 
2020-03-24 21:55:44.688  INFO 2824 --- [68.23.111:2181)] org.apache.zookeeper.ClientCnxn          : Opening socket connection to server 192.168.23.111/192.168.23.111:2181.

最后问题定位到：

// InetAddress.java
 private static String getHostFromNameService(InetAddress addr, boolean check) {
        String host = null;
        for (NameService nameService : nameServices) {
            try {
                // first lookup the hostname
                // nameService 获取名称时过慢
                host = nameService.getHostByAddr(addr.getAddress());

                /* check to see if calling code is allowed to know
                 * the hostname for this IP address, ie, connect to the host
                 */
                if (check) {
                    SecurityManager sec = System.getSecurityManager();
                    if (sec != null) {
                        sec.checkConnect(host, -1);
                    }
                }