在zk叢集啟動的時候，根據選擇的io型別（NIO或者Netty方式）啟動監聽客戶端連線通道，比如NIO方式

    @Override
    public void configure(InetSocketAddress addr, int maxcc) throws IOException {
        configureSaslLogin();

        thread = new ZooKeeperThread(this, "NIOServerCxn.Factory:" + addr);
        thread.setDaemon(true);
        maxClientCnxns = maxcc;
        this
 
.ss = ServerSocketChannel.open();
        ss.socket().setReuseAddress(true);
        LOG.info("binding to port " + addr);
        ss.socket().bind(addr);
        ss.configureBlocking(false);
        ss.register(selector, SelectionKey.OP_ACCEPT);
    }

然後會跑一個執行緒，迴圈去取接收到的客戶端連線事件

    public void run() {
        while (!ss.socket
 
().isClosed()) {
            try {
                selector.select(1000);
                Set<SelectionKey> selected;
                synchronized (this) {
                    selected = selector.selectedKeys();
                }
                ArrayList<SelectionKey> selectedList = new ArrayList<SelectionKey>(
                        selected);
 

                Collections.shuffle(selectedList);
                for (SelectionKey k : selectedList) {
                //表示客戶端有連線請求
                    if ((k.readyOps() & SelectionKey.OP_ACCEPT) != 0) {
                        SocketChannel sc = ((ServerSocketChannel) k
                                .channel()).accept();
                        InetAddress ia = sc.socket().getInetAddress();
                        int cnxncount = getClientCnxnCount(ia);
                        if (maxClientCnxns > 0 && cnxncount >= maxClientCnxns){
                            LOG.warn("Too many connections from " + ia
                                     + " - max is " + maxClientCnxns );
                            sc.close();
                        } else {
                            LOG.info("Accepted socket connection from "
                                     + sc.socket().getRemoteSocketAddress());
                            sc.configureBlocking(false);
                            // 向selector註冊一個讀請求監聽
                            SelectionKey sk = sc.register(selector,
                                    SelectionKey.OP_READ);
                            //建立連線生成一個NIOServerCnxn
                            NIOServerCnxn cnxn = createConnection(sc, sk);
                            sk.attach(cnxn);
                            addCnxn(cnxn);
                        }
                    } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
                        NIOServerCnxn c = (NIOServerCnxn) k.attachment();
                        c.doIO(k);
                    } else {
                        if (LOG.isDebugEnabled()) {
                            LOG.debug("Unexpected ops in select "
                                      + k.readyOps());
                        }
                    }
                }
                selected.clear();
            } catch (RuntimeException e) {
                LOG.warn("Ignoring unexpected runtime exception", e);
            } catch (Exception e) {
                LOG.warn("Ignoring exception", e);
            }
        }
        closeAll();
        LOG.info("NIOServerCnxn factory exited run method");
    }