Zookeeper-Watcher(事件通知)

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
（-1）

客户端与服务端成功建立连接

 

SyncConnected
（0）

NodeCreated
（1）

Watcher监听的对应数据节点被创建

 

 

NodeDeleted
（2）

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
（3）

Watcher监听的对应数据节点的数据内容发生变更

 

 

NodeChildChanged
（4）

Wather监听的对应数据节点的子节点列表发生变更

 

Disconnected
（0）

None
（-1）

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
（-112）

Node
（-1）

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
（4）

None
（-1）

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}

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原文地址：https://www.cnblogs.com/920913cheng/p/10969217.html