Zk学习笔记——检测节点是否存在

参考：从Paxos到Zookeeper分布式一致性原理和实践

使用的zk依赖是cdh5.16.2的3.4.5

<!-- zookeeper -->
<dependency>
    <groupId>org.apache.zookeeper</groupId>
    <artifactId>zookeeper</artifactId>
    <version>3.4.5-cdh5.16.2</version>
</dependency>

 代码，exist函数来检查节点是否存在，同时会注册一个watch

package com.bigdata.zookeeper;


import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;

import java.util.List;
import java.util.concurrent.CountDownLatch;

public class ZkExample implements Watcher {

    public static CountDownLatch connectedSemaphore = new CountDownLatch(1);
    //    private static Stat stat = new Stat();
    private static ZooKeeper zk;

    public static void main(String[] args) throws Exception {
        zk = new ZooKeeper("master:2181", 5000, new ZkExample());
        System.out.println(zk.getState());
        try {
            connectedSemaphore.await();
            // 创建一个节点
            String path = "/app6";
            // 注册watch
            Stat stat = zk.exists(path, true);

            zk.create(path, "123".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

            // version=-1，修改节点
            zk.setData(path, "555".getBytes(), -1);
            // 注册watch
            zk.exists(path, true);
            System.out.println(stat.getVersion());

            // version=1，修改节点
            Stat stat2 = zk.setData(path, "55555".getBytes(), stat.getVersion());
            // 注册watch
            zk.exists(path, true);
            System.out.println(stat2.getVersion());
            
            // version=-1，删除节点，正常
            zk.delete(path, stat2.getVersion());


            Thread.sleep(10000); // 10秒延时

        } catch (InterruptedException e) {
            System.out.println("Zk session established" + e);
        }
    }

    @Override
    public void process(WatchedEvent watchedEvent) {
        System.out.println(watchedEvent);
        if (Event.KeeperState.SyncConnected == watchedEvent.getState()) {
            if (Event.EventType.None == watchedEvent.getType() && null == watchedEvent.getPath()) {
                connectedSemaphore.countDown();
            } else if (watchedEvent.getType() == Event.EventType.NodeChildrenChanged ||
                    watchedEvent.getType() == Event.EventType.NodeDeleted || watchedEvent.getType() == Event.EventType.NodeDataChanged) {
                try {
                    System.out.println(zk.getChildren(watchedEvent.getPath(), true));
//
                } catch (Exception e) {
                    System.out.println(e);
                }
            }

        }
    }
}

 输出