Zookeeper系列三：Zookeeper客户端的使用（Zookeeper原生API如何进行调用、ZKClient、Curator）和Zookeeper会话

一、Zookeeper原生API如何进行调用

准备工作：

首先在新建一个maven项目ZK-Demo,然后在pom.xml里面引入zk的依赖

    <dependency>
      <groupId>org.apache.zookeeper</groupId>
      <artifactId>zookeeper</artifactId>
      <version>3.4.10</version>
    </dependency>

1. 连接zk并监听事件

package com.study.demo.zk;

import java.io.IOException;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.ZooKeeper;

//连接zk并监听事件
public class ZKDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);

    public static void main(String[] args) throws IOException {
        ZooKeeper zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKDemo());
        System.out.println(zk.getState());

        try {
            cdl.await();
        } catch (Exception e) {
            System.out.println("ZK Session established.");
        }
    }


    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        System.out.println("Receive watched event:" + event);
        if (KeeperState.SyncConnected == event.getState()) {
            cdl.countDown();
        }
    }
}

输出结果：

CONNECTING
Receive watched event:WatchedEvent state:SyncConnected type:None path:null

2. 创建znode并监听事件

package com.study.demo.zk;

import java.io.IOException;
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.KeeperState;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

//创建znode并监听事件
public class ZKOperateDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        ZooKeeper zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKOperateDemo());
        cdl.await();

        String path1 = zk.create("/zk-test-", "123".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
        System.out.println("Success create path: " + path1);
        String path2 = zk.create("/zk-test-", "456".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        System.out.println("Success create path: " + path2);
    }

    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        System.out.println("Receive watched event:" + event);
        if (KeeperState.SyncConnected == event.getState()) {
            cdl.countDown();
        }
    }
}

输出结果：

Receive watched event:WatchedEvent state:SyncConnected type:None path:null
Success create path: /zk-test-
Success create path: /zk-test-0000000011

3. 改变znode数据并监听事件

package com.study.demo.zk;

import java.io.IOException;
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;

//改变znode数据并监听事件
public class ZKDataDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);
    private static ZooKeeper zk = null;
    private static Stat stat = new Stat();

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKDataDemo());
        cdl.await();

        zk.create("/zk-test", "123".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
        System.out.println(new String(zk.getData("/zk-test", true, stat)));

        zk.getData("/zk-test", true, stat);
        System.out.println(stat.getCzxid() + ", " + stat.getMzxid() + ", " + stat.getVersion());
        zk.setData("/zk-test", "123".getBytes(), -1);

        Thread.sleep(Integer.MAX_VALUE);
    }

    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        if (KeeperState.SyncConnected == event.getState()) {
            if (EventType.None == event.getType() && null == event.getPath()) {
                cdl.countDown();
            } else if (event.getType() == EventType.NodeDataChanged) {
                try {
                    System.out.println(new String(zk.getData(event.getPath(), true, stat)));
                    System.out.println(stat.getCzxid() + ", " + stat.getMzxid() + ", " + stat.getVersion());
                } catch (Exception e) {
                }
            }
        }
    }
}

输出结果：

123
4294967354, 4294967354, 0
123
4294967354, 4294967355, 1

4. 改变子节点并监听事件

package com.study.demo.zk;

import java.io.IOException;
import java.util.List;
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;

//改变子节点并监听事件
public class ZKChildrenDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);
    private static ZooKeeper zk = null;

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKChildrenDemo());
        cdl.await();

        zk.create("/zk-test", "123".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

        zk.create("/zk-test/c1", "456".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

        List<String> list = zk.getChildren("/zk-test", true);
        for (String str : list)
            System.out.println(str);

        zk.create("/zk-test/c2", "789".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

        Thread.sleep(Integer.MAX_VALUE);
    }

    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        if (KeeperState.SyncConnected == event.getState())
            if (EventType.None == event.getType() && null == event.getPath()) {
                cdl.countDown();
            } else if (event.getType() == EventType.NodeChildrenChanged) {
                try {
                    System.out.println("Child: " + zk.getChildren(event.getPath(), true));
                } catch (Exception e) {
                }
            }
    }
}

输出结果：

c1
Child: [c1, c2]

5. 异步调用并完成回调

package com.study.demo.zk;

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

import org.apache.zookeeper.AsyncCallback;
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;

//异步调用并完成回调
class ChildrenCallback implements AsyncCallback.Children2Callback {
    
    public void processResult(int rc, String path, Object ctx, List<String> children, Stat stat) {
        System.out.println(
                "Child: " + rc + ", path: " + path + ", ctx: " + ctx + ", children: " + children + ", stat: " + stat);
    }
}

public class ZKChildrenAsyncDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);
    private static ZooKeeper zk = null;

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKChildrenAsyncDemo());
        cdl.await();

        zk.create("/zk-test", "123".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

        zk.create("/zk-test/c1", "456".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

        zk.getChildren("/zk-test", true, new ChildrenCallback(), "ok");

        Thread.sleep(Integer.MAX_VALUE);
    }

    
    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        if (KeeperState.SyncConnected == event.getState())
            if (EventType.None == event.getType() && null == event.getPath()) {
                cdl.countDown();
            } else if (event.getType() == EventType.NodeChildrenChanged) {
                try {
                    System.out.println("Child: " + zk.getChildren(event.getPath(), true));
                } catch (Exception e) {
                }
            }
    }
}

输出结果：

Child: 0, path: /zk-test, ctx: ok, children: [c1], stat: 4294967369,4294967369,1535536716381,1535536716381,0,1,0,0,3,1,4294967370

6. 连接后创建回调

package com.study.demo.zk;

import java.io.IOException;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.AsyncCallback;
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.KeeperState;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

//连接后创建回调
class IStringCallback implements AsyncCallback.StringCallback {
    public void processResult(int rc, String path, Object ctx, String name) {
        System.out.println("create path result: [" + rc + ", " + path + "," + ctx + ", real path name: " + name);
    }
}

public class ZKAsyncDemo implements Watcher {
    private static final CountDownLatch cdl = new CountDownLatch(1);

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        ZooKeeper zk = new ZooKeeper("192.168.152.130:2181", 5000, new ZKAsyncDemo());
        cdl.await();

        zk.create("/zk-test-", "123".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL, new IStringCallback(),
                new String("I am context"));

        zk.create("/zk-test-", "456".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL,
                new IStringCallback(), new String("I am context"));

        zk.create("/zk-test-", "789".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL,
                new IStringCallback(), new String("I am context"));

        Thread.sleep(Integer.MAX_VALUE);
    }

    //监听到事件时进行处理
    public void process(WatchedEvent event) {
        System.out.println("Receive watched event:" + event);
        if (KeeperState.SyncConnected == event.getState()) {
            cdl.countDown();
        }
    }
}

输出结果：

Receive watched event:WatchedEvent state:SyncConnected type:None path:null
create path result: [0, /zk-test-,I am context, real path name: /zk-test-
create path result: [-110, /zk-test-,I am context, real path name: null
create path result: [0, /zk-test-,I am context, real path name: /zk-test-0000000016

Chroot命名空间：
主要为了对业务进行隔离性

示例：
Zookeeper client=new Zookeeper(“192.168.56.101:2181/zk-client”, ........)
/zk-client就是Chroot命名空间。Chroot命名空间可以多级
后续的操作都只能在/zk-client及它的子节点下进行，由此进行了业务隔离

二、ZKClient

ZKClient的优点：

1）可以递归创建。在zookeeper命令行和zookeeper的原生API里面得先创建父节点才能创建子节点
2）可以递归删除。在zookeeper命令行和zookeeper的原生API里面得先删除子节点才能删除父节点
3）避免不存在的异常

准备工作：

首先在新建一个maven项目ZK-Demo,然后在pom.xml里面引入ZKClient的依赖

        <dependency>
            <groupId>com.101tec</groupId>
            <artifactId>zkclient</artifactId>
            <version>0.10</version>
        </dependency>

1. ZkClient递归创建顺序节点

package com.study.demo.client;

import org.I0Itec.zkclient.ZkClient;

/**
 * 
* @Description: ZkClient递归创建顺序节点
* @author leeSmall
* @date 2018年9月2日
*
 */
public class CreateNodeDemo {
    public static void main(String[] args) {
        ZkClient client = new ZkClient("192.168.152.130:2181", 5000);
        String path = "/zk-client/c1";
        // 递归创建顺序节点 true：先创建父节点/zk-client
        client.createPersistent(path, true);
    }
}

 创建成功：

2. ZkClient获取数据并监听事件

package com.study.demo.client;

import org.I0Itec.zkclient.IZkDataListener;
import org.I0Itec.zkclient.ZkClient;

/**
* 
* @Description: ZkClient获取数据
* @author leeSmall
* @date 2018年9月2日
*
*/
public class GetDataDemo {
    public static void main(String[] args) throws InterruptedException {
        String path = "/zk-client";
        ZkClient client = new ZkClient("192.168.152.130:2181", 5000);
        //创建临时节点
        client.createEphemeral(path, "123");

        //注册父节点数据改变的事件
        client.subscribeDataChanges(path, new IZkDataListener() {
            
            //父节点数据改变事件
            public void handleDataChange(String dataPath, Object data) throws Exception {
                System.out.println(dataPath + " changed: " + data);
            }

            //父节点数据删除事件
            public void handleDataDeleted(String dataPath) throws Exception {
                System.out.println(dataPath + " deleted");
            }
        });

        System.out.println(client.readData(path).toString());
        client.writeData(path, "456");
        Thread.sleep(1000);
        client.delete(path);
        //sleep的目的是为了更好的观察事件变化
        Thread.sleep(Integer.MAX_VALUE);
    }
}

 输出结果：

123
/zk-client changed: 456
/zk-client deleted

3. ZkClient获取子节点数据并监听事件

package com.study.demo.client;

import java.util.List;

import org.I0Itec.zkclient.IZkChildListener;
import org.I0Itec.zkclient.ZkClient;

/**
* 
* @Description: ZkClient获取子节点数据
* @author leeSmall
* @date 2018年9月2日
*
*/
public class GetChildrenDemo {
    public static void main(String[] args) throws InterruptedException {
        String path = "/zk-client";
        ZkClient client = new ZkClient("192.168.152.130:2181", 5000);
        //注册子节点数据改变的事件
        client.subscribeChildChanges(path, new IZkChildListener() {
            
            //子节点数据改变事件
            public void handleChildChange(String parentPath, List<String> currentChilds) throws Exception {
                System.out.println(parentPath + "的子发生变化: " + currentChilds);
            }
        });

        //创建顺序节点
        client.createPersistent(path);
        Thread.sleep(1000);
        //获取子节点数据 此时还没有创建获取不到
        System.out.println(client.getChildren(path));
        //在前面的父节点 /zk-client下创建子节点c1
        client.createPersistent(path + "/c1");
        Thread.sleep(1000);
        //删除子节点
        client.delete(path + "/c1");
        Thread.sleep(1000);
        //删除父节点
        client.delete(path);
        Thread.sleep(Integer.MAX_VALUE);
    }
}

 输出结果：

/zk-client的子发生变化: []
[]
/zk-client的子发生变化: [c1]
/zk-client的子发生变化: []
/zk-client的子发生变化: null

三、Curator

curator是连接ZK应用最广泛的工具

原因如下：

1）zk应用场景（分布式锁，Master选举等等），curator包含了这些场景。
2）应用场景出现极端的情况下，curator考虑到处理了。

准备工作：

首先在新建一个maven项目ZK-Demo,然后在pom.xml里面引入curator的依赖

        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-framework</artifactId>
            <version>4.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-recipes</artifactId>
            <version>4.0.0</version>
        </dependency>

1. curator创建连接session

package com.study.demo.curator;

import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;

/**
* 
* @Description: curator创建连接session
* @author leeSmall
* @date 2018年9月2日
*
*/
public class CreateSessionDemo {
    public static void main(String[] args) throws InterruptedException {
        RetryPolicy policy = new ExponentialBackoffRetry(1000, 3);
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(policy).build();
        client.start();
        Thread.sleep(Integer.MAX_VALUE);
    }
}

这里介绍一种算法：Backoff退避算法

有这样一种场景，有多个请求，如果网络出现阻塞,每1分钟重试一次。
20：25 request1（block）
20：26 request2（block）
20：27 request3（block）
当网络通顺的时候，请求都累在一起来发送
20：28 request4（通顺）request2、3、4
那么前面的请求就没有意义了，所以就有了退避算法，按照指数间隔重试，比如第一次1分钟，第二次2分钟......随着时间的推移，重试间隔越长。

2. curator递归创建顺序节点

package com.study.demo.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.zookeeper.CreateMode;

/**
* 
* @Description: curator递归创建顺序节点
* @author leeSmall
* @date 2018年9月2日
*
*/
public class CreateNodeDemo {
    public static void main(String[] args) throws Exception {
        String path = "/zk-curator/c1";
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        client.start();
        client.create().creatingParentsIfNeeded().withMode(CreateMode.PERSISTENT).forPath(path, "test".getBytes());
    }
}

创建成功：

3. curator异步创建临时节点

package com.study.demo.curator;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.api.BackgroundCallback;
import org.apache.curator.framework.api.CuratorEvent;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.zookeeper.CreateMode;

/**
* 
* @Description: curator异步创建临时节点
* @author leeSmall
* @date 2018年9月2日
*
*/
public class CreateNodeAsyncDemo {
    static CountDownLatch cdl = new CountDownLatch(2);
    static ExecutorService es = Executors.newFixedThreadPool(2);

    public static void main(String[] args) throws Exception {
        String path = "/zk-curator";
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        client.start();
        
        //创建临时节点
        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).inBackground(new BackgroundCallback() {
            //回调事件处理
            public void processResult(CuratorFramework client, CuratorEvent event) throws Exception {
                System.out.println("event code: " + event.getResultCode() + ", type: " + event.getType());
                cdl.countDown();
            }
        }, es).forPath(path, "test".getBytes());

        //创建临时节点
        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).inBackground(new BackgroundCallback() {
            
            public void processResult(CuratorFramework client, CuratorEvent event) throws Exception {
                System.out.println("event code: " + event.getResultCode() + ", type: " + event.getType());
                cdl.countDown();
            }
        }).forPath(path, "test".getBytes());

        cdl.await();
        es.shutdown();
    }
}

 输出结果：

event code: 0, type: CREATE
event code: -110, type: CREATE

4. curator更新节点数据

package com.study.demo.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.data.Stat;

/**
* 
* @Description: curator更新节点数据
* @author leeSmall
* @date 2018年9月2日
*
*/
public class UpdateDataDemo {
    public static void main(String[] args) throws Exception {
        String path = "/zk-curator/c1";
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        client.start();
        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "test".getBytes());
        Stat stat = new Stat();
        client.getData().storingStatIn(stat).forPath(path);
        System.out.println("Current data: " + stat.getVersion());
        System.out.println("Update data: "
                + client.setData().withVersion(stat.getVersion()).forPath(path, "some".getBytes()).getVersion());
    }
}

 输出结果：

Current data: 0
Update data: 1

5. curator删除节点数据

package com.study.demo.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.data.Stat;

/**
* 
* @Description: curator删除节点数据
* @author leeSmall
* @date 2018年9月2日
*
*/
public class DeleteNodeDemo {
    public static void main(String[] args) throws Exception {
        String path = "/zk-curator/c1";
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        client.start();
        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "test".getBytes());
        Stat stat = new Stat();
        client.getData().storingStatIn(stat).forPath(path);
        client.delete().deletingChildrenIfNeeded().withVersion(stat.getVersion()).forPath(path);
    }
}

6. curator事件监听

package com.study.demo.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.cache.NodeCache;
import org.apache.curator.framework.recipes.cache.NodeCacheListener;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.zookeeper.CreateMode;

/**
* 
* @Description: curator事件监听
* @author leeSmall
* @date 2018年9月2日
*
*/
public class NodeCacheDemo {
    public static void main(String[] args) throws Exception {
        String path = "/zk-curator/nodecache";
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("192.168.152.130:2181")
                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        client.start();
        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "test".getBytes());

        final NodeCache nc = new NodeCache(client, path, false);
        nc.start();
        //通过回调函数监听事件
        nc.getListenable().addListener(new NodeCacheListener() {
            
            public void nodeChanged() throws Exception {
                System.out.println("update--current data: " + new String(nc.getCurrentData().getData()));
            }
        });
        
        client.setData().forPath(path, "test123".getBytes());
        Thread.sleep(1000);
        client.delete().deletingChildrenIfNeeded().forPath(path);
        Thread.sleep(5000);
        nc.close();
    }
}

 输出结果：

update--current data: test123

Curator事件监听：

NodeCache：节点处理监听（会使用缓存）。回调接口NodeCacheListener

PathChildrenCache：子节点缓存，处理子节点变化。回调接口PathChildrenCacheListener

TreeCache：NodeCache和PathChildrenCache的结合体。回调接口TreeCacheListener

四、zookeeper会话

1. zookeeper连接的几种状态

CONNECTING 正在连接
CONNECTED 已经连接
RECONNECTING 正在重新连接
RECONNECTED 重新连接上
CLOSE 会话关闭

2. session

2.1 session主要由几个类控制：

SessionTracker, LearnerSessionTracker, SessionTrackerImpl

session初始化的方法：

org.apache.zookeeper.server.SessionTrackerImpl.initializeNextSession(long)

public static long initializeNextSession(long id) {
        long nextSid = 0;
        nextSid = (System.currentTimeMillis() << 24) >>> 8;
        nextSid =  nextSid | (id <<56);
        return nextSid;
    }

 说明：

SessionID的分配（初始化）函数，策略如下：
1）取时间，并且左移24位得到的结果再右移8位（高8位，低16位都是0）
2）sid拿出来进行左移56位
3）和第一步的结果做或运算

2.2 Session分桶（zookeeper的一个特性）

按照Session会话过期时间进行分区块保存。
这样设计的好处：可以快速清理过期的session

2.3 session激活过程：

1）检测会话是否过期
2）计算会话下一次超时时间
3）定位会话的所在区块
4）迁移会话