import org.apache.hadoop.hbase.EmptyWatcher; //导入依赖的package包/类
/**
 * Waits for HBase installation's base (parent) znode to become available.
 * @throws IOException on ZK errors
 */
public static void waitForBaseZNode(Configuration conf) throws IOException {
  LOG.info("Waiting until the base znode is available");
  String parentZNode = conf.get(HConstants.ZOOKEEPER_ZNODE_PARENT,
      HConstants.DEFAULT_ZOOKEEPER_ZNODE_PARENT);
  ZooKeeper zk = new ZooKeeper(ZKConfig.getZKQuorumServersString(conf),
      conf.getInt(HConstants.ZK_SESSION_TIMEOUT,
      HConstants.DEFAULT_ZK_SESSION_TIMEOUT), EmptyWatcher.instance);

  final int maxTimeMs = 10000;
  final int maxNumAttempts = maxTimeMs / HConstants.SOCKET_RETRY_WAIT_MS;

  KeeperException keeperEx = null;
  try {
    try {
      for (int attempt = 0; attempt < maxNumAttempts; ++attempt) {
        try {
          if (zk.exists(parentZNode, false) != null) {
            LOG.info("Parent znode exists: " + parentZNode);
            keeperEx = null;
            break;
          }
        } catch (KeeperException e) {
          keeperEx = e;
        }
        Threads.sleepWithoutInterrupt(HConstants.SOCKET_RETRY_WAIT_MS);
      }
    } finally {
      zk.close();
    }
  } catch (InterruptedException ex) {
    Thread.currentThread().interrupt();
  }

  if (keeperEx != null) {
    throw new IOException(keeperEx);
  }
}