zookeeper 回调和Watcher
ZooKeeper客户端可以对指定节点设置指定Watcher，当服务器指定节点发生变化是，客户端会收到服务器的通知，然后客户端可以执行相应Watcher的代码。
默认ZooKeeper内置了一个watcher,用于打印收到的服务器的通知。
源码ZooKeeperMain.Watcher：
protected void connectToZK(String newHost) throws InterruptedException, IOException {
if (zk != null && zk.getState().isAlive()) {
zk.close();
}
host = newHost;
zk = new ZooKeeper(host,
Integer.parseInt(cl.getOption("timeout")),
new MyWatcher());
}
private class MyWatcher implements Watcher {
public void process(WatchedEvent event) {
if (getPrintWatches()) {
ZooKeeperMain.printMessage("WATCHER::");
ZooKeeperMain.printMessage(event.toString());
}
}
}
在获取子节点、获取数据、获取状态可以设置Watcher，该Watcher会被存储到Packet包中，当Packet包收到响应时注册该Watcher，当收到服务器notification时，执行Watcher代码。
源码ZooKeeper.getChildren:
public List<String> getChildren(final String path, Watcher watcher)
throws KeeperException, InterruptedException
{
//将watcher封装成childwatcher
WatchRegistration wcb = null;
if (watcher != null) {
wcb = new ChildWatchRegistration(watcher, clientPath);
} final String serverPath = prependChroot(path);
RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.getChildren);
GetChildrenRequest request = new GetChildrenRequest();
request.setPath(serverPath);
request.setWatch(watcher != null);
GetChildrenResponse response = new GetChildrenResponse();
ReplyHeader r = cnxn.submitRequest(h, request, response, wcb);
if (r.getErr() != 0) {
throw KeeperException.create(KeeperException.Code.get(r.getErr()),
clientPath);
}
return response.getChildren();
}
源码ClientCnxn.submitRequest:
public ReplyHeader submitRequest(RequestHeader h, Record request,
Record response, WatchRegistration watchRegistration)
throws InterruptedException {
ReplyHeader r = new ReplyHeader();
//watchRegistration被封装到Packet中
Packet packet = queuePacket(h, r, request, response, null, null, null,
null, watchRegistration);
synchronized (packet) {
while (!packet.finished) {
packet.wait();
}
}
return r;
}
源码ClientCnxn.queuePacket:
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) {
if (h.getType() != OpCode.ping && h.getType() != OpCode.auth) {
h.setXid(getXid());
}
//watchRegistration被封装到Packet中
packet = new Packet(h, r, request, response, null,
watchRegistration);
packet.cb = cb;
packet.ctx = ctx;
packet.clientPath = clientPath;
packet.serverPath = serverPath;
if (!zooKeeper.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);
}
} sendThread.wakeup();
return packet;
}
当Packet包收到响应时注册该Watcher，源码ClientCnxn.finishPacket：
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);
}
}
当收到服务器notification时，执行Watcher代码，源码ClientCnxn.EventThread：
private void processEvent(Object event) {
if (event instanceof WatcherSetEventPair) {
//执行watcher
WatcherSetEventPair pair = (WatcherSetEventPair) event;
for (Watcher watcher : pair.watchers) {
try {
watcher.process(pair.event);
} catch (Throwable t) {
LOG.error("Error while calling watcher ", t);
}
}
}
}
在删除节点、创建节点、获取子节点、设置数据、获取数据、获取权限、设置权限等异步操作时，可以设置CallBack回调函数，该回调对象会被存储到Packet包中，当Packet包收到响应时执行CallBack代码。
源码ZooKeeper.getChildren:
public void getChildren(final String path, Watcher watcher,
ChildrenCallback cb, Object ctx)
{
final String clientPath = path;
WatchRegistration wcb = null;
if (watcher != null) {
wcb = new ChildWatchRegistration(watcher, clientPath);
} final String serverPath = prependChroot(clientPath); RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.getChildren);
GetChildrenRequest request = new GetChildrenRequest();
request.setPath(serverPath);
request.setWatch(watcher != null);
GetChildrenResponse response = new GetChildrenResponse();
cnxn.queuePacket(h, new ReplyHeader(), request, response, cb,
clientPath, serverPath, ctx, wcb);
}
源码ClientCnxn.queuePacket:
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) {
if (h.getType() != OpCode.ping && h.getType() != OpCode.auth) {
h.setXid(getXid());
}
packet = new Packet(h, r, request, response, null,
watchRegistration);
packet.cb = cb;
packet.ctx = ctx;
packet.clientPath = clientPath;
packet.serverPath = serverPath;
if (!zooKeeper.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);
}
} sendThread.wakeup();
return packet;
}
源码ClientCnxn.EventThread：
private void processEvent(Object event) {
try {
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);
}
}
}
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