Watcher是什么
在ZooKeeper中，接口类Watcher用于表示一个标准的事件处理器，其定义了事件通知相关的逻辑，包含KeeperState和EventType两个枚举类，分别代表了通知状态和事件类型，同时定义了事件的回调方法：process（WatchedEvent event）
同一个事件类型在不同的通知状态中代表的含义有所不同，表7-3列举了常见的通知状态和事件类型。
|
KeeperState |
EventType |
触发条件 |
说明 |
|
|
None |
客户端与服务端成功建立连接 |
|
|
SyncConnected |
NodeCreated |
Watcher监听的对应数据节点被创建 |
|
|
|
NodeDeleted |
Watcher监听的对应数据节点被删除 |
此时客户端和服务器处于连接状态 |
|
|
NodeDataChanged |
Watcher监听的对应数据节点的数据内容发生变更 |
|
|
|
NodeChildChanged |
Wather监听的对应数据节点的子节点列表发生变更 |
|
|
Disconnected |
None |
客户端与ZooKeeper服务器断开连接 |
此时客户端和服务器处于断开连接状态 |
|
Expired |
Node |
会话超时 |
此时客户端会话失效，通常同时也会受到SessionExpiredException异常 |
|
AuthFailed |
None |
通常有两种情况，1：使用错误的schema进行权限检查 2：SASL权限检查失败 |
通常同时也会收到AuthFailedException异常 |
zookeeper是树的结构存储方式，可以通过节点来保存信息，所以可以监听每个节点的变化，来知道程序如何操作zookeeper的，而Watcher的作用就是告诉监听者，zookeeper节点的变化（创建节点,删除节点,修改节点）
process方法是Watcher接口中的一个回调方法，当ZooKeeper向客户端发送一个Watcher事件通知时，客户端就会对相应的process方法进行回调，从而实现对事件的处理。process方法的定义如下：
abstract public void process(WatchedEvent event);
这个回调方法的定义非常简单，我们重点看下方法的参数定义：WatchedEvent。
WatchedEvent包含了每一个事件的三个基本属性：通知状态（keeperState），事件类型（EventType）和节点路径（path），其数据结构如图7-5所示。ZooKeeper使用WatchedEvent对象来封装服务端事件并传递给Watcher，从而方便回调方法process对服务端事件进行处理。
提到WatchedEvent，不得不讲下WatcherEvent实体。笼统地讲，两者表示的是同一个事物，都是对一个服务端事件的封装。不同的是，WatchedEvent是一个逻辑事件，用于服务端和客户端程序执行过程中所需的逻辑对象，而WatcherEvent因为实现了序列化接口，因此可以用于网络传输。
服务端在生成WatchedEvent事件之后，会调用getWrapper方法将自己包装成一个可序列化的WatcherEvent事件，以便通过网络传输到客户端。客户端在接收到服务端的这个事件对象后，首先会将WatcherEvent还原成一个WatchedEvent事件，并传递给process方法处理，回调方法process根据入参就能够解析出完整的服务端事件了。
需要注意的一点是，无论是WatchedEvent还是WatcherEvent，其对ZooKeeper服务端事件的封装都是机及其简单的。举个例子来说，当/zk-book这个节点的数据发生变更时，服务端会发送给客户端一个“ZNode数据内容变更”事件，客户端只能够接收到如下信
java操作API对Watched事件监听
package com.zk.watcher; import java.util.concurrent.CountDownLatch; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.EventType; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.ZooDefs.Ids; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.data.Stat; public class ZookeeperWatcher implements Watcher { // 集群连接地址 private static final String CONNECT_ADDRES = "127.0.0.1:2181"; // 会话超时时间 private static final int SESSIONTIME = 2000; // 信号量,zk连接异步，用于阻塞，保证所有的操作都在zk创建连接成功以后再执行 private static final CountDownLatch countDownLatch = new CountDownLatch(1); private ZooKeeper zk; public void createConnection(String connectAddres, int sessionTimeOut) { try { zk = new ZooKeeper(connectAddres, sessionTimeOut, this); System.out.println("LOG" + "zk 开始启动连接服务器...."); countDownLatch.await(); } catch (Exception e) { e.printStackTrace(); } } public boolean createPath(String path, String data) { try { this.exists(path, true); // 创建持久节点 this.zk.create(path, data.getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT); System.out.println("LOG" + "节点创建成功, Path:" + path + ",data:" + data); } catch (Exception e) { e.printStackTrace(); return false; } return true; } /** * 判断指定节点是否存在 不存在该节点返回null * * @param path * 节点路径 */ public Stat exists(String path, boolean needWatch) { try { return this.zk.exists(path, needWatch); } catch (Exception e) { e.printStackTrace(); return null; } } public boolean updateNode(String path, String data) throws KeeperException, InterruptedException { exists(path, true); this.zk.setData(path, data.getBytes(), -1); return false; } public void process(WatchedEvent watchedEvent) { // 获取事件状态 KeeperState keeperState = watchedEvent.getState(); // 获取事件类型 EventType eventType = watchedEvent.getType(); // zk 路径 String path = watchedEvent.getPath(); // 判断是否建立连接 if (KeeperState.SyncConnected == keeperState) { // 如果当前状态已经连接上了 SyncConnected：连接，AuthFailed：认证失败,Expired:失效过期, // ConnectedReadOnly:连接只读,Disconnected:连接失败 if (EventType.None == eventType) { // 如果建立建立成功,让后程序往下走 System.out.println("LOG" + "zk 建立连接成功!"); countDownLatch.countDown(); } else if (EventType.NodeCreated == eventType) { System.out.println("LOG" + "事件通知,新增node节点" + path); } else if (EventType.NodeDataChanged == eventType) { System.out.println("LOG" + "事件通知,当前node节点" + path + "被修改...."); } else if (EventType.NodeDeleted == eventType) { System.out.println("LOG" + "事件通知,当前node节点" + path + "被删除...."); } } } public static void main(String[] args) throws KeeperException, InterruptedException { ZookeeperWatcher zkClientWatcher = new ZookeeperWatcher(); zkClientWatcher.createConnection(CONNECT_ADDRES, SESSIONTIME); zkClientWatcher.createPath("/createNode", "pa-644064"); // 修改createNode节点 zkClientWatcher.updateNode("/createNode", "7894561"); } }
SLF4J: Class path contains multiple SLF4J bindings. SLF4J: Found binding in [jar:file:/E:/developer/mvnResponsity/ch/qos/logback/logback-classic/1.1.11/logback-classic-1.1.11.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: Found binding in [jar:file:/E:/developer/mvnResponsity/org/slf4j/slf4j-log4j12/1.7.26/slf4j-log4j12-1.7.26.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation. SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder] 14:11:26.432 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:zookeeper.version=3.4.14-4c25d480e66aadd371de8bd2fd8da255ac140bcf, built on 03/06/2019 16:18 GMT 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:host.name=NNH1J2ASSQP10BA 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.version=1.8.0_172 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.vendor=Oracle Corporation 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.home=E:Javajdk1.8.0_172jre 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.class.path=E:Javajdk1.8.0_172jrelib esources.jar;E:Javajdk1.8.0_172jrelib t.jar;E:Javajdk1.8.0_172jrelibjsse.jar;E:Javajdk1.8.0_172jrelibjce.jar;E:Javajdk1.8.0_172jrelibcharsets.jar;E:Javajdk1.8.0_172jrelibjfr.jar;E:Javajdk1.8.0_172jrelibextaccess-bridge-64.jar;E:Javajdk1.8.0_172jrelibextcldrdata.jar;E:Javajdk1.8.0_172jrelibextdnsns.jar;E:Javajdk1.8.0_172jrelibextjaccess.jar;E:Javajdk1.8.0_172jrelibextjfxrt.jar;E:Javajdk1.8.0_172jrelibextlocaledata.jar;E:Javajdk1.8.0_172jrelibext ashorn.jar;E:Javajdk1.8.0_172jrelibextsunec.jar;E:Javajdk1.8.0_172jrelibextsunjce_provider.jar;E:Javajdk1.8.0_172jrelibextsunmscapi.jar;E:Javajdk1.8.0_172jrelibextsunpkcs11.jar;E:Javajdk1.8.0_172jrelibextzipfs.jar;E:codeservletjava-zk argetclasses;E:codeservletjava-zklibzookeeper-3.4.14.jar;E:developermvnResponsityorgspringframeworkootspring-boot-starter1.5.21.RELEASEspring-boot-starter-1.5.21.RELEASE.jar;E:developermvnResponsityorgspringframeworkootspring-boot1.5.21.RELEASEspring-boot-1.5.21.RELEASE.jar;E:developermvnResponsityorgspringframeworkspring-context4.3.24.RELEASEspring-context-4.3.24.RELEASE.jar;E:developermvnResponsityorgspringframeworkspring-aop4.3.24.RELEASEspring-aop-4.3.24.RELEASE.jar;E:developermvnResponsityorgspringframeworkspring-beans4.3.24.RELEASEspring-beans-4.3.24.RELEASE.jar;E:developermvnResponsityorgspringframeworkspring-expression4.3.24.RELEASEspring-expression-4.3.24.RELEASE.jar;E:developermvnResponsityorgspringframeworkootspring-boot-autoconfigure1.5.21.RELEASEspring-boot-autoconfigure-1.5.21.RELEASE.jar;E:developermvnResponsityorgspringframeworkootspring-boot-starter-logging1.5.21.RELEASEspring-boot-starter-logging-1.5.21.RELEASE.jar;E:developermvnResponsitychqoslogbacklogback-classic1.1.11logback-classic-1.1.11.jar;E:developermvnResponsitychqoslogbacklogback-core1.1.11logback-core-1.1.11.jar;E:developermvnResponsityorgslf4jjcl-over-slf4j1.7.26jcl-over-slf4j-1.7.26.jar;E:developermvnResponsityorgslf4jjul-to-slf4j1.7.26jul-to-slf4j-1.7.26.jar;E:developermvnResponsityorgslf4jlog4j-over-slf4j1.7.26log4j-over-slf4j-1.7.26.jar;E:developermvnResponsityorgspringframeworkspring-core4.3.24.RELEASEspring-core-4.3.24.RELEASE.jar;E:developermvnResponsityorgyamlsnakeyaml1.17snakeyaml-1.17.jar;E:developermvnResponsityorgapachezookeeperzookeeper3.4.14zookeeper-3.4.14.jar;E:developermvnResponsityorgslf4jslf4j-api1.7.26slf4j-api-1.7.26.jar;E:developermvnResponsityorgslf4jslf4j-log4j121.7.26slf4j-log4j12-1.7.26.jar;E:developermvnResponsitycomgithubspotbugsspotbugs-annotations3.1.9spotbugs-annotations-3.1.9.jar;E:developermvnResponsitycomgooglecodefindbugsjsr3053.0.2jsr305-3.0.2.jar;E:developermvnResponsitylog4jlog4j1.2.17log4j-1.2.17.jar;E:developermvnResponsityjlinejline.9.94jline-0.9.94.jar;E:developermvnResponsityorgapacheyetusaudience-annotations.5.0audience-annotations-0.5.0.jar;E:developermvnResponsityio etty etty3.10.6.Final etty-3.10.6.Final.jar 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.library.path=E:Javajdk1.8.0_172in;C:WindowsSunJavain;C:Windowssystem32;C:Windows;E:/Java/jdk1.8.0_172/jre/bin/server;E:/Java/jdk1.8.0_172/jre/bin;E:/Java/jdk1.8.0_172/jre/lib/amd64;E:xftp;C:Windowssystem32;C:Windows;C:WindowsSystem32Wbem;C:WindowsSystem32WindowsPowerShellv1.0;E:Javajdk1.8.0_172lib;E:Javajdk1.8.0_172jrein;E:mavenapache-maven-3.5.3in;E:softwaremysqlmysql-8.0.15-winx64;C:UsersAdministratorAppDataLocalMicrosoftWindowsApps;;E:stsspring-tool-suite-3.9.4.RELEASE-e4.7.3a-win32-x86_64sts-bundlests-3.9.4.RELEASE;;. 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.io.tmpdir=C:UsersADMINI~1AppDataLocalTemp 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:java.compiler=<NA> 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:os.name=Windows 10 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:os.arch=amd64 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:os.version=10.0 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:user.name=Administrator 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:user.home=C:UsersAdministrator 14:11:26.434 [main] INFO org.apache.zookeeper.ZooKeeper - Client environment:user.dir=E:codeservletjava-zk 14:11:26.435 [main] INFO org.apache.zookeeper.ZooKeeper - Initiating client connection, connectString=127.0.0.1:2181 sessionTimeout=2000 watcher=com.zk.watcher.ZookeeperWatcher@2d38eb89 14:11:26.437 [main] DEBUG org.apache.zookeeper.ClientCnxn - zookeeper.disableAutoWatchReset is false LOGzk 开始启动连接服务器.... 14:11:26.550 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.SaslServerPrincipal - Canonicalized address to 127.0.0.1 14:11:26.552 [main-SendThread(127.0.0.1:2181)] INFO org.apache.zookeeper.ClientCnxn - Opening socket connection to server 127.0.0.1/127.0.0.1:2181. Will not attempt to authenticate using SASL (unknown error) 14:11:26.553 [main-SendThread(127.0.0.1:2181)] INFO org.apache.zookeeper.ClientCnxn - Socket connection established to 127.0.0.1/127.0.0.1:2181, initiating session 14:11:26.555 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Session establishment request sent on 127.0.0.1/127.0.0.1:2181 14:11:26.559 [main-SendThread(127.0.0.1:2181)] WARN org.apache.zookeeper.ClientCnxnSocket - Connected to an old server; r-o mode will be unavailable 14:11:26.559 [main-SendThread(127.0.0.1:2181)] INFO org.apache.zookeeper.ClientCnxn - Session establishment complete on server 127.0.0.1/127.0.0.1:2181, sessionid = 0x16b263e2f4d0003, negotiated timeout = 4000 LOGzk 建立连接成功! 14:11:26.564 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x16b263e2f4d0003, packet:: clientPath:null serverPath:null finished:false header:: 1,3 replyHeader:: 1,188,-101 request:: '/createNode,T response:: 14:11:26.571 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Got notification sessionid:0x16b263e2f4d0003 14:11:26.573 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Got WatchedEvent state:SyncConnected type:NodeCreated path:/createNode for sessionid 0x16b263e2f4d0003 LOG事件通知,新增node节点/createNode 14:11:26.574 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x16b263e2f4d0003, packet:: clientPath:null serverPath:null finished:false header:: 2,1 replyHeader:: 2,189,0 request:: '/createNode,#70612d363434303634,v{s{31,s{'world,'anyone}}},0 response:: '/createNode LOG节点创建成功, Path:/createNode,data:pa-644064 14:11:26.577 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x16b263e2f4d0003, packet:: clientPath:null serverPath:null finished:false header:: 3,3 replyHeader:: 3,189,0 request:: '/createNode,T response:: s{189,189,1559715086567,1559715086567,0,0,0,0,9,0,189} 14:11:26.581 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Got notification sessionid:0x16b263e2f4d0003 14:11:26.581 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Got WatchedEvent state:SyncConnected type:NodeDataChanged path:/createNode for sessionid 0x16b263e2f4d0003 LOG事件通知,当前node节点/createNode被修改.... 14:11:26.582 [main-SendThread(127.0.0.1:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x16b263e2f4d0003, packet:: clientPath:null serverPath:null finished:false header:: 4,5 replyHeader:: 4,190,0 request:: '/createNode,#37383934353631,-1 response:: s{189,190,1559715086567,1559715086579,1,0,0,0,7,0,189}
