本文深入探讨了ZooKeeper Java API的使用方法，包括连接建立、节点操作及Watcher监听机制。通过代码示例展示了如何创建、读取、更新和删除节点，以及如何注册和触发事件监听。
基于 Java API 初探 zookeeper 的使用：
先来简单看一下API的使用：
public class ConnectionDemo {
public static void main(String[] args) {
try {
final CountDownLatch countDownLatch=new CountDownLatch(1);
ZooKeeper zooKeeper=
new ZooKeeper("192.168.254.135:2181," +
"192.168.254.136:2181,192.168.254.137:2181",
4000, new Watcher() {
@Override
public void process(WatchedEvent event) {
if(Event.KeeperState.SyncConnected==event.getState()){
//如果收到了服务端的响应事件，连接成功
countDownLatch.countDown();
}
}
});
System.out.println(zooKeeper.getState());//CONNECTING
countDownLatch.await();
System.out.println(zooKeeper.getState());//CONNECTED
//添加节点
zooKeeper.create("/zk-wuzz","0".getBytes(),ZooDefs.Ids.OPEN_ACL_UNSAFE,CreateMode.PERSISTENT);
Thread.sleep(1000);
Stat stat=new Stat();
//得到当前节点的值
byte[] bytes=zooKeeper.getData("/zk-wuzz",null,stat);
System.out.println(new String(bytes)); // 0
//修改节点值
zooKeeper.setData("/zk-wuzz","1".getBytes(),stat.getVersion());
//得到当前节点的值
byte[] bytes1=zooKeeper.getData("/zk-wuzz",null,stat);
System.out.println(new String(bytes1)); // 1
zooKeeper.delete("/zk-wuzz",stat.getVersion());
zooKeeper.close();
System.in.read();
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
}
}
}
事件机制：
Watcher 监听机制是 Zookeeper 中非常重要的特性，我们基于 zookeeper 上创建的节点，可以对这些节点绑定监听事件，比如可以监听节点数据变更、节点删除、子节点状态变更等事件，通过这个事件机制，可以基于 zookeeper实现分布式锁、集群管理等功能。
watcher 特性：当数据发生变化的时候， zookeeper 会产生一个 watcher 事件，并且会发送到客户端。但是客户端只会收到一次通知。如果后续这个节点再次发生变化，那么之前设置 watcher 的客户端不会再次收到消息。（watcher 是一次性的操作）。 可以通过循环监听去达到永久监听效果。
如何注册事件机制：
通过这三个操作来绑定事件 ：getData、Exists、getChildren
如何触发事件？ 凡是事务类型的操作，都会触发监听事件。create /delete /setData,来看以下代码简单实现
public class WatcherDemo {
public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
final CountDownLatch countDownLatch=new CountDownLatch(1);
final ZooKeeper zooKeeper=
new ZooKeeper("192.168.254.135:2181," +
"192.168.254.136:2181,192.168.254.137:2181",
4000, new Watcher() {
@Override
public void process(WatchedEvent event) {
System.out.println("默认事件： "+event.getType());
if(Event.KeeperState.SyncConnected==event.getState()){
//如果收到了服务端的响应事件，连接成功
countDownLatch.countDown();
}
}
});
countDownLatch.await();
zooKeeper.create("/zk-wuzz","1".getBytes(),
ZooDefs.Ids.OPEN_ACL_UNSAFE,CreateMode.PERSISTENT);
//exists getdata getchildren
//通过exists绑定事件
Stat stat=zooKeeper.exists("/zk-wuzz", new Watcher() {
@Override
public void process(WatchedEvent event) {
System.out.println(event.getType()+"->"+event.getPath());
try {
//再一次去绑定事件 ,但是这个走的是默认事件
zooKeeper.exists(event.getPath(),true);
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
//通过修改的事务类型操作来触发监听事件
stat=zooKeeper.setData("/zk-wuzz","2".getBytes(),stat.getVersion());
Thread.sleep(1000);
zooKeeper.delete("/zk-wuzz",stat.getVersion());
System.in.read();
}
}
以上就是 Watcher 的简单实现操作。接下来浅析一下这个 Watcher 实现的流程。
watcher 事件类型：
//org.apache.zookeeper.Watcher.Event.EventType
public enum EventType {
None (-1), // 客户端连接状态发生变化的时候 会受到none事件
NodeCreated (1), // 节点创建事件
NodeDeleted (2), // 节点删除事件
NodeDataChanged (3), // 节点数据变化
NodeChildrenChanged (4); // 子节点被创建 删除触发该事件
}
事件的实现原理：
client 端连接后会注册一个事件，然后客户端会保存这个事件，通过zkWatcherManager 保存客户端的事件注册，通知服务端 Watcher 为 true，然后服务端会通过WahcerManager 会绑定path对应的事件。如下图：
接下去通过源码层面去熟悉一下这个 Watcher 的流程。由于我们demo 是通过exists 来注册事件，那么我们就通过 exists 来作为入口。
// org.apache.zookeeper.ZooKeeper
public Stat exists(final String path, Watcher watcher)
throws KeeperException, InterruptedException
{
final String clientPath = path;
PathUtils.validatePath(clientPath);
// the watch contains the un-chroot path
WatchRegistration wcb = null; // 存储事件
if (watcher != null) { // 实例化监听事件
wcb = new ExistsWatchRegistration(watcher, clientPath);
}
final String serverPath = prependChroot(clientPath);
RequestHeader h = new RequestHeader();// 构建请求
h.setType(ZooDefs.OpCode.exists);
ExistsRequest request = new ExistsRequest(); // 构建请求
request.setPath(serverPath);
request.setWatch(watcher != null);
SetDataResponse response = new SetDataResponse();//构建响应对象
// 发送请求
ReplyHeader r = cnxn.submitRequest(h, request, response, wcb);
if (r.getErr() != 0) {
if (r.getErr() == KeeperException.Code.NONODE.intValue()) {
return null;
}
throw KeeperException.create(KeeperException.Code.get(r.getErr()),
clientPath);
}
return response.getStat().getCzxid() == -1 ? null : response.getStat();
}
接下去先看一下请求提交：
public ReplyHeader submitRequest(RequestHeader h, Record request,
Record response, WatchRegistration watchRegistration)
throws InterruptedException {
ReplyHeader r = new ReplyHeader();
//组装 请求包packet
Packet packet = queuePacket(h, r, request, response, null, null, null,
null, watchRegistration);
synchronized (packet) {
while (!packet.finished) {
packet.wait();
}
}
return r;
}
queuePacket：
Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
Record response, AsyncCallback cb, String clientPath,
String serverPath, Object ctx, WatchRegistration watchRegistration)
{
Packet packet = null;
// Note that we do not generate the Xid for the packet yet. It is
// generated later at send-time, by an implementation of ClientCnxnSocket::doIO(),
// where the packet is actually sent.
synchronized (outgoingQueue) {//将请求包加入队列
packet = new Packet(h, r, request, response, watchRegistration);
packet.cb = cb;
packet.ctx = ctx;
packet.clientPath = clientPath;
packet.serverPath = serverPath;
if (!state.isAlive() || closing) {
conLossPacket(packet);
} else {
// If the client is asking to close the session then
// mark as closing
if (h.getType() == OpCode.closeSession) {
closing = true;
}
outgoingQueue.add(packet);
}
}
// 通过ClientCnxnSocketNIO的 selector 多路复用去执行异步通信
sendThread.getClientCnxnSocket().wakeupCnxn();
return packet;
}
看到这里，发现只是发送了数据，那哪里触发了对 outgoingQueue 队列的消息进行消费。再把组装的packeet 放入队列的时候用到的 cnxn.submitRequest(h, request, response, wcb);这个cnxn 是哪里来的呢？ 再看 zookeeper的构造函数，我们可以看到如下代码：
public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher,
boolean canBeReadOnly)
throws IOException
{
LOG.info("Initiating client connection, connectString=" + connectString
+ " sessionTimeout=" + sessionTimeout + " watcher=" + watcher);
// 配置事件监听
watchManager.defaultWatcher = watcher;
ConnectStringParser connectStringParser = new ConnectStringParser(
connectString);
HostProvider hostProvider = new StaticHostProvider(
connectStringParser.getServerAddresses());
// 构造 cnxn ClientCnxn
cnxn = new ClientCnxn(connectStringParser.getChrootPath(),
hostProvider, sessionTimeout, this, watchManager,
getClientCnxnSocket(), canBeReadOnly);
cnxn.start();
}
new ClientCnxn(connectStringParser.getChrootPath(),hostProvider, sessionTimeout, this, watchManager, getClientCnxnSocket(), canBeReadOnly); 创建 SendThread，eventThread两个线程并启动线程：
public ClientCnxn(String chrootPath, HostProvider hostProvider, int sessionTimeout, ZooKeeper zooKeeper,
ClientWatchManager watcher, ClientCnxnSocket clientCnxnSocket,
long sessionId, byte[] sessionPasswd, boolean canBeReadOnly) {
this.zooKeeper = zooKeeper;
this.watcher = watcher;
this.sessionId = sessionId;
this.sessionPasswd = sessionPasswd;
this.sessionTimeout = sessionTimeout;
this.hostProvider = hostProvider;
this.chrootPath = chrootPath;
connectTimeout = sessionTimeout / hostProvider.size();
readTimeout = sessionTimeout * 2 / 3;
readOnly = canBeReadOnly;
sendThread = new SendThread(clientCnxnSocket);
eventThread = new EventThread();
}
找到内部类 SendThread 的 run 方法：可以在方法内部找到一个方法 clientCnxnSocket.doTransport(to, pendingQueue, outgoingQueue, ClientCnxn.this); ZooKeeper通过获取ZOOKEEPER_CLIENT_CNXN_SOCKET变量构造了一个ClientCnxnSocket对象，默认情况下是ClientCnxnSocketNIO类。这个方法就是传输的核心方法。SendThread通过doTransport将Packet发送给Server,并通过readResponse获取结果，解析成一个Event，再将Event加入EventThread的队列中等待执行。EventThread通过processEvent消费队列中的Event事件。SendThread 的主要作用除了将Packet包发送给Server之外，还负责维持Client和Server之间的心跳，确保 session 存活。SendThread是一个线程类，因此我们进入其run()方法，看看他的启动流程。
public void run() {
clientCnxnSocket.introduce(this,sessionId);
clientCnxnSocket.updateNow();
clientCnxnSocket.updateLastSendAndHeard();
int to;
long lastPingRwServer = Time.currentElapsedTime();
final int MAX_SEND_PING_INTERVAL = 10000; //10 seconds
InetSocketAddress serverAddress = null;
while (state.isAlive()) {
try {
if (!clientCnxnSocket.isConnected()) {
if(!isFirstConnect){
try {
Thread.sleep(r.nextInt(1000));
} catch (InterruptedException e) {
LOG.warn("Unexpected exception", e);
}
}
// don't re-establish connection if we are closing
if (closing || !state.isAlive()) {
break;
}
if (rwServerAddress != null) {
serverAddress = rwServerAddress;
rwServerAddress = null;
} else {
serverAddress = hostProvider.next(1000);
}
// 启动连接
startConnect(serverAddress);
clientCnxnSocket.updateLastSendAndHeard();
}
if (state.isConnected()) {// 连接成功
// determine whether we need to send an AuthFailed event.
if (zooKeeperSaslClient != null) {
boolean sendAuthEvent = false;
if (zooKeeperSaslClient.getSaslState() == ZooKeeperSaslClient.SaslState.INITIAL) {
try {
zooKeeperSaslClient.initialize(ClientCnxn.this);
} catch (SaslException e) {
LOG.error("SASL authentication with Zookeeper Quorum member failed: " + e);
state = States.AUTH_FAILED;
sendAuthEvent = true;
}
}
KeeperState authState = zooKeeperSaslClient.getKeeperState();
if (authState != null) {
if (authState == KeeperState.AuthFailed) {
// An authentication error occurred during authentication with the Zookeeper Server.
state = States.AUTH_FAILED;
sendAuthEvent = true;
} else {
if (authState == KeeperState.SaslAuthenticated) {
sendAuthEvent = true;
}
}
}
if (sendAuthEvent == true) {
eventThread.queueEvent(new WatchedEvent(
Watcher.Event.EventType.None,
authState,null));
}
}
to = readTimeout - clientCnxnSocket.getIdleRecv();
} else {
to = connectTimeout - clientCnxnSocket.getIdleRecv();
}
if (to <= 0) {
String warnInfo;
warnInfo = "Client session timed out, have not heard from server in "
+ clientCnxnSocket.getIdleRecv()
+ "ms"
+ " for sessionid 0x"
+ Long.toHexString(sessionId);
LOG.warn(warnInfo);
throw new SessionTimeoutException(warnInfo);
}
if (state.isConnected()) {
//1000(1 second) is to prevent race condition missing to send the second ping
//also make sure not to send too many pings when readTimeout is small
int timeToNextPing = readTimeout / 2 - clientCnxnSocket.getIdleSend() -
((clientCnxnSocket.getIdleSend() > 1000) ? 1000 : 0);
//send a ping request either time is due or no packet sent out within MAX_SEND_PING_INTERVAL
if (timeToNextPing <= 0 || clientCnxnSocket.getIdleSend() > MAX_SEND_PING_INTERVAL) {// 发送心跳包
sendPing();
clientCnxnSocket.updateLastSend();
} else {
if (timeToNextPing < to) {
to = timeToNextPing;
}
}
}
// If we are in read-only mode, seek for read/write server
if (state == States.CONNECTEDREADONLY) {
long now = Time.currentElapsedTime();
int idlePingRwServer = (int) (now - lastPingRwServer);
if (idlePingRwServer >= pingRwTimeout) {
lastPingRwServer = now;
idlePingRwServer = 0;
pingRwTimeout =
Math.min(2*pingRwTimeout, maxPingRwTimeout);
pingRwServer();
}
to = Math.min(to, pingRwTimeout - idlePingRwServer);
}
// 将指令发送给Server
clientCnxnSocket.doTransport(to, pendingQueue, outgoingQueue, ClientCnxn.this);
} catch (Throwable e) {
if (closing) {
if (LOG.isDebugEnabled()) {
// closing so this is expected
LOG.debug("An exception was thrown while closing send thread for session 0x"
+ Long.toHexString(getSessionId())
+ " : " + e.getMessage());
}
break;
} else {
// this is ugly, you have a better way speak up
if (e instanceof SessionExpiredException) {
LOG.info(e.getMessage() + ", closing socket connection");
} else if (e instanceof SessionTimeoutException) {
LOG.info(e.getMessage() + RETRY_CONN_MSG);
} else if (e instanceof EndOfStreamException) {
LOG.info(e.getMessage() + RETRY_CONN_MSG);
} else if (e instanceof RWServerFoundException) {
LOG.info(e.getMessage());
} else if (e instanceof SocketException) {
LOG.info("Socket error occurred: {}: {}", serverAddress, e.getMessage());
} else {
LOG.warn("Session 0x{} for server {}, unexpected error{}",
Long.toHexString(getSessionId()),
serverAddress,
RETRY_CONN_MSG,
e);
}
cleanup();
if (state.isAlive()) {
eventThread.queueEvent(new WatchedEvent(
Event.EventType.None,
Event.KeeperState.Disconnected,
null));
}
clientCnxnSocket.updateNow();
clientCnxnSocket.updateLastSendAndHeard();
}
}
}
cleanup();
clientCnxnSocket.close();
if (state.isAlive()) {
eventThread.queueEvent(new WatchedEvent(Event.EventType.None,
Event.KeeperState.Disconnected, null));
}
ZooTrace.logTraceMessage(LOG, ZooTrace.getTextTraceLevel(),
"SendThread exited loop for session: 0x"
+ Long.toHexString(getSessionId()));
}
我们看到在请求被组装放入到 outgoingQueue 的时候有如下代码
synchronized (packet) {
while (!packet.finished) {
packet.wait();
}
}
这是在等待数据被包解析。 在doTransport 方法中的doIO发送socket信息之前，先从socket中获取返回数据，通过readResonse进行处理。
void doIO(List<Packet> pendingQueue, LinkedList<Packet> outgoingQueue, 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++;
readLength();
} else if (!initialized) {
readConnectResult();
enableRead();
if (findSendablePacket(outgoingQueue,
cnxn.sendThread.clientTunneledAuthenticationInProgress()) != 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 {
// 读取返回信息
sendThread.readResponse(incomingBuffer);
lenBuffer.clear();
incomingBuffer = lenBuffer;
updateLastHeard();
}
}
}
if (sockKey.isWritable()) {
synchronized(outgoingQueue) {
Packet p = findSendablePacket(outgoingQueue,
cnxn.sendThread.clientTunneledAuthenticationInProgress());
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++;
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();
}
}
}
}
在readReponse中,通过解析数据，我们可以得到WatchedEvent对象，并将其压入EventThread的消息队列，等待分发
void readResponse(ByteBuffer incomingBuffer) throws IOException {
ByteBufferInputStream bbis = new ByteBufferInputStream(
incomingBuffer);
BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
ReplyHeader replyHdr = new ReplyHeader();
replyHdr.deserialize(bbia, "header");
if (replyHdr.getXid() == -2) {
// -2 is the xid for pings
if (LOG.isDebugEnabled()) {
LOG.debug("Got ping response for sessionid: 0x"
+ Long.toHexString(sessionId)
+ " after "
+ ((System.nanoTime() - lastPingSentNs) / 1000000)
+ "ms");
}
return;
}
if (replyHdr.getXid() == -4) {
// -4 is the xid for AuthPacket
if(replyHdr.getErr() == KeeperException.Code.AUTHFAILED.intValue()) {
state = States.AUTH_FAILED;
eventThread.queueEvent( new WatchedEvent(Watcher.Event.EventType.None,
Watcher.Event.KeeperState.AuthFailed, null) );
}
if (LOG.isDebugEnabled()) {
LOG.debug("Got auth sessionid:0x"
+ Long.toHexString(sessionId));
}
return;
}
if (replyHdr.getXid() == -1) {
// -1 means notification
if (LOG.isDebugEnabled()) {
LOG.debug("Got notification sessionid:0x"
+ Long.toHexString(sessionId));
}
WatcherEvent event = new WatcherEvent();
// 序列化
event.deserialize(bbia, "response");
// convert from a server path to a client path
if (chrootPath != null) {
String serverPath = event.getPath();
if(serverPath.compareTo(chrootPath)==0)
event.setPath("/");
else if (serverPath.length() > chrootPath.length())
event.setPath(serverPath.substring(chrootPath.length()));
else {
LOG.warn("Got server path " + event.getPath()
+ " which is too short for chroot path "
+ chrootPath);
}
}
WatchedEvent we = new WatchedEvent(event);
if (LOG.isDebugEnabled()) {
LOG.debug("Got " + we + " for sessionid 0x"
+ Long.toHexString(sessionId));
}
// 压入队列
eventThread.queueEvent( we );
return;
}
// If SASL authentication is currently in progress, construct and
// send a response packet immediately, rather than queuing a
// response as with other packets.
if (clientTunneledAuthenticationInProgress()) {
GetSASLRequest request = new GetSASLRequest();
request.deserialize(bbia,"token");
zooKeeperSaslClient.respondToServer(request.getToken(),
ClientCnxn.this);
return;
}
Packet packet;
synchronized (pendingQueue) {
if (pendingQueue.size() == 0) {
throw new IOException("Nothing in the queue, but got "
+ replyHdr.getXid());
}
packet = pendingQueue.remove();
}
/*
* Since requests are processed in order, we better get a response
* to the first request!
*/
try {
if (packet.requestHeader.getXid() != replyHdr.getXid()) {
packet.replyHeader.setErr(
KeeperException.Code.CONNECTIONLOSS.intValue());
throw new IOException("Xid out of order. Got Xid "
+ replyHdr.getXid() + " with err " +
+ replyHdr.getErr() +
" expected Xid "
+ packet.requestHeader.getXid()
+ " for a packet with details: "
+ packet );
}
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) {
// stat 信息返回
packet.response.deserialize(bbia, "response");
}
if (LOG.isDebugEnabled()) {
LOG.debug("Reading reply sessionid:0x"
+ Long.toHexString(sessionId) + ", packet:: " + packet);
}
} finally { //设置完
finishPacket(packet);
}
}
在EventThread中通过processEvent对队列中的事件进行消费，并分发给不同的Watcher。设置完调用 finishPacket 进行Watcher 的注册：
private void finishPacket(Packet p) {
if (p.watchRegistration != null) {
// 注册 事件注册到 zkwatchemanager 中
p.watchRegistration.register(p.replyHeader.getErr());
}
if (p.cb == null) {
synchronized (p) {
p.finished = true;
p.notifyAll();
}
} else {
p.finished = true;
eventThread.queuePacket(p);
}
}
watchRegistration，熟悉吗？在组装请求的时候，我们初始化了这个对象，把 watchRegistration 子 类 里 面 的Watcher 实 例 放 到 ZKWatchManager 的existsWatches 中存储起来。调用 ZKWatchManager 进行register ：
public void register(int rc) {
if (shouldAddWatch(rc)) {
Map<String, Set<Watcher>> watches = getWatches(rc);
synchronized(watches) {
Set<Watcher> watchers = watches.get(clientPath);
if (watchers == null) {
watchers = new HashSet<Watcher>();
watches.put(clientPath, watchers);
}
watchers.add(watcher);
}
}
}
看看ZKWatchManager哪里调用注册：
class ExistsWatchRegistration extends WatchRegistration {
public ExistsWatchRegistration(Watcher watcher, String clientPath) {
super(watcher, clientPath);
}
@Override
protected Map<String, Set<Watcher>> getWatches(int rc) {
//就是这里调用watchhManager的existWatchs这个Map里
return rc == 0 ? watchManager.dataWatches : watchManager.existWatches;
}
@Override
protected boolean shouldAddWatch(int rc) {
return rc == 0 || rc == KeeperException.Code.NONODE.intValue();
}
}总 的 来 说 ， 当 使 用 ZooKeeper 构 造 方 法 或 者 使 用getData 、 exists 和 getChildren 三 个 接 口 来 向ZooKeeper 服务器注册 Watcher 的时候，首先将此消息传递给服务端，传递成功后，服务端会通知客户端，然后客户端将该路径和 Watcher 对应关系存储起来备用。
事件触发：
最后处理事件是我们 前边提到的 EventThread 线程去处理的，这里就不做详细分析了，最后执行任务的核心代码是:
private void processEvent(Object event) {
try {
if (event instanceof WatcherSetEventPair) {
// each watcher will process the event
WatcherSetEventPair pair = (WatcherSetEventPair) event;
// 循环遍历 最后调用 process方法处理
for (Watcher watcher : pair.watchers) {
try {
watcher.process(pair.event);
} catch (Throwable t) {
LOG.error("Error while calling watcher ", t);
}
}
}
}
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