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ClientCnxnSocket抽象类结构 定义了底层Socket通信接口，默认实现是ClientCnxnSocketNIO

readConnectResult 读取server的connnect的response

readLength 方法 读取buffer长度并给incomingBuffer分配对应大小空间

ClientCnxnSocketNIO 实现

findSendablePacket函数 从outgoingQueue中读取发送的packet

doIO函数 处理读写

doTransport函数

如果连接就绪，调用sendThread连接操作

若读写就绪，调用doIO函数

ClientCnxnSocket

属性

ClientCnxnSocketNIO子类

属性

//nio中的selector
private final Selector selector = Selector.open();
/**
 * nio中的selectionKey
 */
private SelectionKey sockKey;
private SocketAddress localSocketAddress;
private SocketAddress remoteSocketAddress;
方法 
@Override
void connect(InetSocketAddress addr) throws IOException {
    SocketChannel sock = createSock();
    try {
        registerAndConnect(sock, addr);
    } catch (IOException e) {
        LOG.error("Unable to open socket to " + addr);
        sock.close();
        throw e;
    }
//已经连接，但是没有收到resposne
    initialized = false;

    /*
     * Reset incomingBuffer
     */
    lenBuffer.clear();
    incomingBuffer = lenBuffer;
}

client和server主要交互函数
@Override
void doTransport(
    int waitTimeOut,
    Queue pendingQueue,
    ClientCnxn cnxn) throws IOException, InterruptedException {
    selector.select(waitTimeOut);
    Set selected;
    synchronized (this) {
        selected = selector.selectedKeys();
    }
    // Everything below and until we get back to the select is
    // non blocking, so time is effectively a constant. That is
    // Why we just have to do this once, here
    updateNow();
    for (SelectionKey k : selected) {
        SocketChannel sc = ((SocketChannel) k.channel());
        if ((k.readyOps() & SelectionKey.OP_CONNECT) != 0) {
            if (sc.finishConnect()) {
                updateLastSendAndHeard();
                updateSocketAddresses();
                sendThread.primeConnection();
            }
        } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
            doIO(pendingQueue, cnxn);
        }
    }
    if (sendThread.getZkState().isConnected()) {
        if (findSendablePacket(outgoingQueue, sendThread.tunnelAuthInProgress()) != null) {
            enableWrite();
        }
    }
    selected.clear();
}

主要分为读写两种
读：没有初始化完成初始化
读取len再改incomingbuffer分配对应空间
读取对应response
写： 找到可以发送的packet
如果packet的bytebuffer没有创建，那就进行属性添加
bytebuffer写入socketChannel
把Packet从outgingQueue中取出，放入pendingQueue中
/**
 * @throws InterruptedException
 * @throws IOException
 */
void doIO(Queue pendingQueue, ClientCnxn cnxn) throws InterruptedException, IOException {
    SocketChannel sock = (SocketChannel) sockKey.channel();
    if (sock == null) {
        throw new IOException("Socket is null!");
    }
    if (sockKey.isReadable()) {
        int rc = sock.read(incomingBuffer);
        if (rc < 0) {
            throw new EndOfStreamException("Unable to read additional data from server sessionid 0x"
                                           + Long.toHexString(sessionId)
                                           + ", likely server has closed socket");
        }
        if (!incomingBuffer.hasRemaining()) {
            incomingBuffer.flip();
            if (incomingBuffer == lenBuffer) {
                recvCount.getAndIncrement();
                readLength();
            } else if (!initialized) {
                readConnectResult();
                enableRead();
                if (findSendablePacket(outgoingQueue, sendThread.tunnelAuthInProgress()) != null) {
                    // Since SASL authentication has completed (if client is configured to do so),
                    // outgoing packets waiting in the outgoingQueue can now be sent.
                    enableWrite();
                }
                lenBuffer.clear();
                incomingBuffer = lenBuffer;
                updateLastHeard();
                initialized = true;
            } else {
                sendThread.readResponse(incomingBuffer);
                lenBuffer.clear();
                incomingBuffer = lenBuffer;
                updateLastHeard();
            }
        }
    }
    if (sockKey.isWritable()) {
        Packet p = findSendablePacket(outgoingQueue, sendThread.tunnelAuthInProgress());

        if (p != null) {
            updateLastSend();
            // If we already started writing p, p.bb will already exist
            if (p.bb == null) {
                if ((p.requestHeader != null)
                    && (p.requestHeader.getType() != OpCode.ping)
                    && (p.requestHeader.getType() != OpCode.auth)) {
                    p.requestHeader.setXid(cnxn.getXid());
                }
                p.createBB();
            }
            sock.write(p.bb);
            if (!p.bb.hasRemaining()) {
                sentCount.getAndIncrement();
                outgoingQueue.removeFirstOccurrence(p);
                if (p.requestHeader != null
                    && p.requestHeader.getType() != OpCode.ping
                    && p.requestHeader.getType() != OpCode.auth) {
                    synchronized (pendingQueue) {
                        pendingQueue.add(p);
                    }
                }
            }
        }
        if (outgoingQueue.isEmpty()) {
            // No more packets to send: turn off write interest flag.
            // Will be turned on later by a later call to enableWrite(),
            // from within ZooKeeperSaslClient (if client is configured
            // to attempt SASL authentication), or in either doIO() or
            // in doTransport() if not.
            disableWrite();
        } else if (!initialized && p != null && !p.bb.hasRemaining()) {
            // On initial connection, write the complete connect request
            // packet, but then disable further writes until after
            // receiving a successful connection response.  If the
            // session is expired, then the server sends the expiration
            // response and immediately closes its end of the socket.  If
            // the client is simultaneously writing on its end, then the
            // TCP stack may choose to abort with RST, in which case the
            // client would never receive the session expired event.  See
            // http://docs.oracle.com/javase/6/docs/technotes/guides/net/articles/connection_release.html
            disableWrite();
        } else {
            // Just in case
            enableWrite();
        }
    }
}


查询待发送队列中可以发送的packet
private Packet findSendablePacket(LinkedBlockingDeque outgoingQueue, boolean tunneledAuthInProgres) {
   //没有要发送的，返回null
     if (outgoingQueue.isEmpty()) {
        return null;
    }
    // If we've already starting sending the first packet, we better finish
    if (outgoingQueue.getFirst().bb != null || !tunneledAuthInProgres) {
        //取队列第一个进行发送
        return outgoingQueue.getFirst();
    }
    // Since client's authentication with server is in progress,
    // send only the null-header packet queued by primeConnection().
    // This packet must be sent so that the SASL authentication process
    // can proceed, but all other packets should wait until
    // SASL authentication completes.
    //有正在认证处理，发送空请求头包给服务端
    Iterator iter = outgoingQueue.iterator();
    while (iter.hasNext()) {
        Packet p = iter.next();
        if (p.requestHeader == null) {
            // We've found the priming-packet. Move it to the beginning of the queue.
            iter.remove();
            outgoingQueue.addFirst(p);
            return p;
        } else {
            // Non-priming packet: defer it until later, leaving it in the queue
            // until authentication completes.
            LOG.debug("deferring non-priming packet {} until SASL authentation completes.", p);
        }
    }
    return null;
}

发送接收packet图

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文章标题：如何理解zk-client通信层ClientCnxnSocket
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