Zk学习笔记——读取节点

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，在初始化zk的时候，会触发一个watchEvent，将CountDownLatch-1=0，从而开始读取/app1的value，此时为12345，

然后将其修改成123，会再次触发一个watchEvent，此时watchEvent的type是NodeDataChanged，之后10秒延时过后，输出/app1的value，此时为123

读取节点的getData放中的true表示是否注册一个watch，注意这里的watch是一次性的，触发通过之后就失效，需要反复注册watch

package com.bigdata.zookeeper;


import org.apache.zookeeper.AsyncCallback;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;

import java.util.concurrent.CountDownLatch;

public class ZkExample implements Watcher {

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

    public static void main(String[] args) throws Exception {
        ZooKeeper zk = new ZooKeeper("master:2181", 5000, new ZkExample());
        System.out.println(zk.getState());
        try {
            connectedSemaphore.await();
            System.out.println(new String(zk.getData("/app1", true, stat)));
            Thread.sleep(10000); // 10秒延时
            System.out.println(new String(zk.getData("/app1", true, stat)));
        } 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()) {
            connectedSemaphore.countDown();
        }
    }
}

输出

异步代码

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 = null;

    public static void main(String[] args) throws Exception {
        zk = new ZooKeeper("master:2181", 5000, new ZkExample());
        System.out.println(zk.getState());
        try {
            connectedSemaphore.await();
            // 注册一个watch，将能异步获取到为空的子节点
            zk.getChildren("/app1", true, new IChildren2Callback(), null);
            // 创建一个子节点
            zk.create("/app1/app12", "".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

            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) {
                try {
                    System.out.println(zk.getChildren(watchedEvent.getPath(), true));
                } catch (Exception e) {

                }
            }

        }
    }
}

class IChildren2Callback implements AsyncCallback.Children2Callback {

    @Override
    public void processResult(int i, String s, Object o, List<String> list, Stat stat) {
        System.out.println("Get children znode : [response code: " + i + ", path: " + s +
                ", children list" + list + ", stat : " + stat);
    }
}

输出