zookeeper底层通过NIO进行网络传输，如果对NIO不是很熟悉，先参见java NIO。下面我们来逐步实现基于NIO的zookeeper实现，首先我们要定义应用层的通信协议。

传输协议

　　请求协议格式：

 

字段

长度

说明

 

len

4

请求长度，解决半包粘包

请求头 

xid

4

请求在客户端的id，用于唯一标识和保证响应顺序

type

4

请求类型code

请求体

request

 

不同的请求类型，有不同的请求体结构

　　响应协议格式：

 

字段

长度

解释

 

len

4

响应长度，解决半包粘包

响应头

xid

4

包序号，唯一标识一个包，通过该标识找到对应的客户端Packet对象

zxid

8

服务端事务处理id

err

4

返回状态code

响应体

response

 

不同的响应类型有不同的响应体

　　请求和响应体类型：

List children

请求code

请求头类型

说明

请求格式

字段说明

响应格式

字段说明

0

notification

 

 

 

 

 

1

create

创建节点

String path

路径

String path

路径

byte[] data

节点值

List acl

acl值

int flags

节点类型

2

delete

删除节点

String path

路径

无 

 

int version

版本

3

exists

是否存在指定节点

String path

路径

 Stat stat

节点状态  

boolean watch

是否有watch

4

getData

获取节点数据

String path

路径

Stat stat

节点状态  

boolean watch

是否有watch

byte[] data

节点数据

5

setData

设置节点数据

 String path

 路径

 Stat stat

节点状态  

 byte[] data

 数据

int version

版本

6

getACL

获取节点权限

String path

路径

 List acl

 权限

Stat stat

节点状态 

7 

setACL 

设置节点权限  

String path

路径 

Stat stat

节点状态   

List acl

权限

int version

版本

8 

getChildren 

获取子节点 

String path

 路径

 List children

子节点名

boolean watch

是否有watch

9

sync

同步路径

String path

路径

String path

路径

11

ping

 

 

 

 

 

12

getChildren2

 

 

 

 

 

100

auth

 

 

 

 

 

101

setWatches

 

 

 

 

 

-10

createSession

 

 

 

 

 

-11

closeSession

 

 

 

 

 

－1

error

 

 

 

 

　　1.将请求封装成Packet对象放入outgoingQueue队列中。

　　2.有一个收发送线程会从outgoingQueue队列中取出一个可发送的Packet对象，并发送序列化信息，然后把该Packet放入到pendingQueue队列中。

　　3.收发线程会接收服务端响应，反序列号出结果数据，然后在pendingQueue中找到对应的Packet，设置结果，将Packet放入到waitingEvents队列中。

　　4.有一个事件线程，会不断从waitingEvents队列中取出一个Packet，并调用响应的callback方法。

发送packet包

Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
Record response, AsyncCallback cb, String clientPath,
String serverPath, Object ctx, WatchRegistration watchRegistration)
{

    Packet packet = null;  
    synchronized (outgoingQueue) {  
        //设置一个全局唯一的id，作为数据包的id  
        if (h.getType() != OpCode.ping && h.getType() != OpCode.auth) {  
            h.setXid(getXid());  
        }  
        //将请求头，请求体，返回结果，watcher等封装成数据包。  
        packet = new Packet(h, r, request, response, null,  
                watchRegistration);  
        packet.cb = cb;  
        packet.ctx = ctx;  
        packet.clientPath = clientPath;  
        packet.serverPath = serverPath;  
        //将数据包添加到outgoing队列中。  
        outgoingQueue.add(packet);  
    }  
    sendThread.wakeup();  
    return packet;  
}

发送线程主流程（ClientCnxn.SendThread.run）：

class SendThread extends Thread {
SelectionKey sockKey;
private final Selector selector = Selector.open();
public void run() {
while (zooKeeper.state.isAlive()) {
//建立连接
startConnect();
//获取注册通道
selector.select(1000);
Set selected;
synchronized (this) {
selected = selector.selectedKeys();
}
for (SelectionKey k : selected) {
SocketChannel sc = ((SocketChannel) k.channel());
if ((k.readyOps() & SelectionKey.OP_CONNECT) != 0) {
//建立连接
if (sc.finishConnect()) {
primeConnection(k);
}
//读写数据
} else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
doIO();
}
}
}
try {
selector.close();
} catch (IOException e) {
LOG.warn("Ignoring exception during selector close", e);
}
}

  //通过nio建立连接  
    private void startConnect() throws IOException {  
        //从服务器列表中获取一台服务器地址  
        InetSocketAddress addr = serverAddrs.get(nextAddrToTry);  
        nextAddrToTry++;  
        if (nextAddrToTry == serverAddrs.size()) {  
            nextAddrToTry = 0;  
        }  
        //通过nio注册  
        SocketChannel sock;  
        sock = SocketChannel.open();  
        sock.configureBlocking(false);  
        sock.socket().setSoLinger(false, -1);  
        sock.socket().setTcpNoDelay(true);  
        try {  
            sockKey = sock.register(selector, SelectionKey.OP\_CONNECT);  
        } catch (IOException e) {  
            sock.close();  
            throw e;  
        }  
        //初始化缓存  
        lenBuffer.clear();  
        incomingBuffer = lenBuffer;  
    }  

}

处理读写主流程,主要是nio操作（ClientCnxn.SendThread.doIO）：

boolean doIO() throws InterruptedException, IOException {
boolean packetReceived = false;
//获取socketchannel
SocketChannel sock = (SocketChannel) sockKey.channel();
//如果可读
if (sockKey.isReadable()) {
//读取数据到缓存中
int rc = sock.read(incomingBuffer);
//直到缓存读满
if (!incomingBuffer.hasRemaining()) {
//重置缓存
incomingBuffer.flip();
//如果读取的是长度信息，读取长度信息，并且分配相应缓存
if (incomingBuffer == lenBuffer) {
int len = incomingBuffer.getInt();
incomingBuffer = ByteBuffer.allocate(len);
} else if (!initialized) {
//如果是connect命令的返回值，获取session，timeout等相关信息
readConnectResult();
enableRead();
lenBuffer.clear();
//重置缓存
incomingBuffer = lenBuffer;
initialized = true;
} else {
//读取数据内容
readResponse();
//重置缓存
lenBuffer.clear();
incomingBuffer = lenBuffer;
packetReceived = true;
}
}
}
//如果是写
if (sockKey.isWritable()) {
synchronized (outgoingQueue) {
if (!outgoingQueue.isEmpty()) {
//从outgoingQueue队列中拿数据包写入通道
ByteBuffer pbb = outgoingQueue.getFirst().bb;
sock.write(pbb);
if (!pbb.hasRemaining()) {
sentCount++;
Packet p = outgoingQueue.removeFirst();
if (p.header != null
&& p.header.getType() != OpCode.ping
&& p.header.getType() != OpCode.auth) {
pendingQueue.add(p);
}
}
}
}
}
if (outgoingQueue.isEmpty()) {
disableWrite();
} else {
enableWrite();
}
return packetReceived;
}

处理返回结果，xid＝－2为ping命令的返回结果；xid＝-4为auth命令；xid＝－1为服务器发送的notification；其他命令返回结果。

void readResponse() throws IOException {
//对返回数据进行反序列化
ByteBufferInputStream bbis = new ByteBufferInputStream(
incomingBuffer);
BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
ReplyHeader replyHdr = new ReplyHeader();
replyHdr.deserialize(bbia, "header");
//根据返回头信息，封装想要的事件，放入事件队列中，交给eventthread处理
//向消息队列放入验证失败消息
if (replyHdr.getXid() == -4) {
// -4 is the xid for AuthPacket
if(replyHdr.getErr() == KeeperException.Code.AUTHFAILED.intValue()) {
zooKeeper.state = States.AUTH_FAILED;
eventThread.queueEvent( new WatchedEvent(Watcher.Event.EventType.None,
Watcher.Event.KeeperState.AuthFailed, null) );
}
return;
}
//
if (replyHdr.getXid() == -1) {
WatcherEvent event = new WatcherEvent();
event.deserialize(bbia, "response");
if (chrootPath != null) {
String serverPath = event.getPath();
if(serverPath.compareTo(chrootPath)==0)
event.setPath("/");
else
event.setPath(serverPath.substring(chrootPath.length()));
}
WatchedEvent we = new WatchedEvent(event);
eventThread.queueEvent( we );
return;
}
//反序列化返回结果
Packet packet = null;
synchronized (pendingQueue) {
packet = pendingQueue.remove();
}
try {
packet.replyHeader.setXid(replyHdr.getXid());
packet.replyHeader.setErr(replyHdr.getErr());
packet.replyHeader.setZxid(replyHdr.getZxid());
if (replyHdr.getZxid() > 0) {
lastZxid = replyHdr.getZxid();
}
if (packet.response != null && replyHdr.getErr() == 0) {
packet.response.deserialize(bbia, "response");
}
} finally {
finishPacket(packet);
}
}

private void finishPacket(Packet p) {
if (p.watchRegistration != null) {
p.watchRegistration.register(p.replyHeader.getErr());
}

    if (p.cb == null) {  
        synchronized (p) {  
            p.finished = true;  
            p.notifyAll();  
        }  
    } else {  
        p.finished = true;  
        eventThread.queuePacket(p);  
    }  
}

事件线程主要是处理回调函数（ClientCnxn.EventThread.run）：

public void run() {
try {
isRunning = true;
while (true) {
//从队列中获取事件
Object event = waitingEvents.take();
if (event == eventOfDeath) {
wasKilled = true;
} else {
//处理事件
processEvent(event);
}
if (wasKilled)
synchronized (waitingEvents) {
if (waitingEvents.isEmpty()) {
isRunning = false;
break;
}
}
}
} catch (InterruptedException e) {
LOG.error("Event thread exiting due to interruption", e);
}
LOG.info("EventThread shut down");
}

处理回调函数和watcher

private void processEvent(Object event) {
try {
//处理watcher
if (event instanceof WatcherSetEventPair) {
WatcherSetEventPair pair = (WatcherSetEventPair) event;
for (Watcher watcher : pair.watchers) {
try {
watcher.process(pair.event);
} catch (Throwable t) {
LOG.error("Error while calling watcher ", t);
}
}
} else {
//
Packet p = (Packet) event;
int rc = 0;
String clientPath = p.clientPath;
if (p.replyHeader.getErr() != 0) {
rc = p.replyHeader.getErr();
}
if (p.response instanceof ExistsResponse
|| p.response instanceof SetDataResponse
|| p.response instanceof SetACLResponse) {
StatCallback cb = (StatCallback) p.cb;
if (rc == 0) {
if (p.response instanceof ExistsResponse) {
cb.processResult(rc, clientPath, p.ctx,
((ExistsResponse) p.response)
.getStat());
} else if (p.response instanceof SetDataResponse) {
cb.processResult(rc, clientPath, p.ctx,
((SetDataResponse) p.response)
.getStat());
} else if (p.response instanceof SetACLResponse) {
cb.processResult(rc, clientPath, p.ctx,
((SetACLResponse) p.response)
.getStat());
}
} else {
cb.processResult(rc, clientPath, p.ctx, null);
}
} else if (p.response instanceof GetDataResponse) {
DataCallback cb = (DataCallback) p.cb;
GetDataResponse rsp = (GetDataResponse) p.response;
if (rc == 0) {
cb.processResult(rc, clientPath, p.ctx, rsp
.getData(), rsp.getStat());
} else {
cb.processResult(rc, clientPath, p.ctx, null,
null);
}
} else if (p.response instanceof GetACLResponse) {
ACLCallback cb = (ACLCallback) p.cb;
GetACLResponse rsp = (GetACLResponse) p.response;
if (rc == 0) {
cb.processResult(rc, clientPath, p.ctx, rsp
.getAcl(), rsp.getStat());
} else {
cb.processResult(rc, clientPath, p.ctx, null,
null);
}
} else if (p.response instanceof GetChildrenResponse) {
ChildrenCallback cb = (ChildrenCallback) p.cb;
GetChildrenResponse rsp = (GetChildrenResponse) p.response;
if (rc == 0) {
cb.processResult(rc, clientPath, p.ctx, rsp
.getChildren());
} else {
cb.processResult(rc, clientPath, p.ctx, null);
}
} else if (p.response instanceof GetChildren2Response) {
Children2Callback cb = (Children2Callback) p.cb;
GetChildren2Response rsp = (GetChildren2Response) p.response;
if (rc == 0) {
cb.processResult(rc, clientPath, p.ctx, rsp
.getChildren(), rsp.getStat());
} else {
cb.processResult(rc, clientPath, p.ctx, null, null);
}
} else if (p.response instanceof CreateResponse) {
StringCallback cb = (StringCallback) p.cb;
CreateResponse rsp = (CreateResponse) p.response;
if (rc == 0) {
cb.processResult(rc, clientPath, p.ctx,
(chrootPath == null
? rsp.getPath()
: rsp.getPath()
.substring(chrootPath.length())));
} else {
cb.processResult(rc, clientPath, p.ctx, null);
}
} else if (p.cb instanceof VoidCallback) {
VoidCallback cb = (VoidCallback) p.cb;
cb.processResult(rc, clientPath, p.ctx);
}
}
} catch (Throwable t) {
LOG.error("Caught unexpected throwable", t);
}
}

Packet结构

　　包，ClientCnxn内部管理请求内容的模块。由以下几个模块组成：

　　1.RequestHeader header 请求头

　　2.Record request 请求内容

　　3.ByteBuffer bb 实际需要发送的请求内容。

　　4.ReplyHeader replyHeader 响应头

　　5.Record response 响应内容

　　6.String clientPath

　　7.String serverPath

　　8.boolean finished

　　9.AsyncCallback cb

　　10.Object ctx

　　11.WatchRegistration watchRegistration

Packet(RequestHeader header, ReplyHeader replyHeader, Record record,
Record response, ByteBuffer bb,
WatchRegistration watchRegistration) {
this.header = header;
this.replyHeader = replyHeader;
this.request = record;
this.response = response;
if (bb != null) {
this.bb = bb;
} else {
try {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
BinaryOutputArchive boa = BinaryOutputArchive
.getArchive(baos);
boa.writeInt(-1, "len"); // We'll fill this in later
header.serialize(boa, "header");
if (record != null) {
record.serialize(boa, "request");
}
baos.close();
this.bb = ByteBuffer.wrap(baos.toByteArray());
this.bb.putInt(this.bb.capacity() - 4);
this.bb.rewind();
} catch (IOException e) {
LOG.warn("Ignoring unexpected exception", e);
}
}
this.watchRegistration = watchRegistration;
}