【分布式】Zookeeper使用--开源客户端

转载自：http://www.cnblogs.com/leesf456/p/6032716.html

一、前言

　　上一篇博客已经介绍了如何使用Zookeeper提供的原生态Java API进行操作，本篇博文主要讲解如何通过开源客户端来进行操作。

二、ZkClient

　　ZkClient是在Zookeeper原声API接口之上进行了包装，是一个更易用的Zookeeper客户端，其内部还实现了诸如Session超时重连、Watcher反复注册等功能。

　　2.1 添加依赖

　　在pom.xml文件中添加如下内容即可。　　

        <dependency>            <groupId>com.101tec</groupId>            <artifactId>zkclient</artifactId>            <version>0.2</version>        </dependency>

　　2.2 创建会话

　　使用ZkClient可以轻松的创建会话，连接到服务端。　　

package com.hust.grid.leesf.zkclient.examples;import java.io.IOException;import org.I0Itec.zkclient.ZkClient;public class Create_Session_Sample {    public static void main(String[] args) throws IOException, InterruptedException {        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 5000);        System.out.println("ZooKeeper session established.");    }}

　　运行结果：　　

ZooKeeper session established.

　　结果表明已经成功创建会话。

　　2.3 创建节点　

　　ZkClient提供了递归创建节点的接口，即其帮助开发者完成父节点的创建，再创建子节点。

package com.hust.grid.leesf.zkclient.examples;import org.I0Itec.zkclient.ZkClient;public class Create_Node_Sample {    public static void main(String[] args) throws Exception {        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 5000);        String path = "/zk-book/c1";        zkClient.createPersistent(path, true);        System.out.println("success create znode.");    }}

　　运行结果：　

success create znode.

　　结果表明已经成功创建了节点，值得注意的是，在原生态接口中是无法创建成功的（父节点不存在），但是通过ZkClient可以递归的先创建父节点，再创建子节点。

　　

　　可以看到确实成功创建了/zk-book和/zk-book/c1两个节点。

　　2.4 删除节点

　　ZkClient提供了递归删除节点的接口，即其帮助开发者先删除所有子节点（存在），再删除父节点。　　

package com.hust.grid.leesf.zkclient.examples;import org.I0Itec.zkclient.ZkClient;public class Del_Data_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 5000);        zkClient.createPersistent(path, "");        zkClient.createPersistent(path+"/c1", "");        zkClient.deleteRecursive(path);        System.out.println("success delete znode.");    }}

　　运行结果:　　

success delete znode.

　　结果表明ZkClient可直接删除带子节点的父节点，因为其底层先删除其所有子节点，然后再删除父节点。

　　2.5 获取子节点　　

package com.hust.grid.leesf.zkclient.examples;import java.util.List;import org.I0Itec.zkclient.IZkChildListener;import org.I0Itec.zkclient.ZkClient;public class Get_Children_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 5000);        zkClient.subscribeChildChanges(path, new IZkChildListener() {            public void handleChildChange(String parentPath, List<String> currentChilds) throws Exception {                System.out.println(parentPath + " 's child changed, currentChilds:" + currentChilds);            }        });        zkClient.createPersistent(path);        Thread.sleep(1000);        zkClient.createPersistent(path + "/c1");        Thread.sleep(1000);        zkClient.delete(path + "/c1");        Thread.sleep(1000);        zkClient.delete(path);        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：　　

/zk-book 's child changed, currentChilds:[]/zk-book 's child changed, currentChilds:[c1]/zk-book 's child changed, currentChilds:[]/zk-book 's child changed, currentChilds:null

　　结果表明：

　　客户端可以对一个不存在的节点进行子节点变更的监听。

　　一旦客户端对一个节点注册了子节点列表变更监听之后，那么当该节点的子节点列表发生变更时，服务端都会通知客户端，并将最新的子节点列表发送给客户端

　　该节点本身的创建或删除也会通知到客户端。

　　2.6 获取数据

package com.hust.grid.leesf.zkclient.examples;import org.I0Itec.zkclient.IZkDataListener;import org.I0Itec.zkclient.ZkClient;public class Get_Data_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 5000);        zkClient.createEphemeral(path, "123");        zkClient.subscribeDataChanges(path, new IZkDataListener() {            public void handleDataDeleted(String dataPath) throws Exception {                System.out.println("Node " + dataPath + " deleted.");            }            public void handleDataChange(String dataPath, Object data) throws Exception {                System.out.println("Node " + dataPath + " changed, new data: " + data);            }        });        System.out.println(zkClient.readData(path));        zkClient.writeData(path, "456");        Thread.sleep(1000);        zkClient.delete(path);        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：　

123Node /zk-book changed, new data: 456Node /zk-book deleted.

　　结果表明可以成功监听节点数据变化或删除事件。

　　2.7 检测节点是否存在　　

package com.hust.grid.leesf.zkclient.examples;import org.I0Itec.zkclient.ZkClient;public class Exist_Node_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        ZkClient zkClient = new ZkClient("127.0.0.1:2181", 2000);        System.out.println("Node " + path + " exists " + zkClient.exists(path));    }}

　　运行结果：

Node /zk-book exists false

　　结果表明，可以通过ZkClient轻易检测节点是否存在，其相比于原生态的接口更易于理解。

三、Curator客户端

　　Curator解决了很多Zookeeper客户端非常底层的细节开发工作，包括连接重连，反复注册Watcher和NodeExistsException异常等，现已成为Apache的顶级项目。

　　3.1 添加依赖

　　在pom.xml文件中添加如下内容即可。　　

        <!-- https://mvnrepository.com/artifact/org.apache.curator/apache-curator -->        <dependency>            <groupId>org.apache.curator</groupId>            <artifactId>curator-framework</artifactId>            <version>2.4.2</version>        </dependency>

　　3.2 创建会话

　　Curator除了使用一般方法创建会话外，还可以使用fluent风格进行创建。

package com.hust.grid.leesf.curator.examples;import org.apache.curator.RetryPolicy;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.ExponentialBackoffRetry;public class Create_Session_Sample {    public static void main(String[] args) throws Exception {        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);        CuratorFramework client = CuratorFrameworkFactory.newClient("127.0.0.1:2181", 5000, 3000, retryPolicy);        client.start();        System.out.println("Zookeeper session1 established. ");        CuratorFramework client1 = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")                .sessionTimeoutMs(5000).retryPolicy(retryPolicy).namespace("base").build();        client1.start();        System.out.println("Zookeeper session2 established. ");            }}

　　运行结果：　

Zookeeper session1 established. Zookeeper session2 established. 

　　值得注意的是session2会话含有隔离命名空间，即客户端对Zookeeper上数据节点的任何操作都是相对/base目录进行的，这有利于实现不同的Zookeeper的业务之间的隔离。

　　3.3 创建节点

　　通过使用Fluent风格的接口，开发人员可以进行自由组合来完成各种类型节点的创建。　　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.zookeeper.CreateMode;public class Create_Node_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book/c1";        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();        client.start();        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "init".getBytes());        System.out.println("success create znode: " + path);    }}

　　运行结果：　

success create znode: /zk-book/c1

　　其中，也创建了/zk-book/c1的父节点/zk-book节点。

　　3.4 删除节点　　

package com.hust.grid.leesf.curator.examples;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;public class Del_Data_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book/c1";        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();        client.start();        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "init".getBytes());        Stat stat = new Stat();        System.out.println(new String(client.getData().storingStatIn(stat).forPath(path)));        client.delete().deletingChildrenIfNeeded().withVersion(stat.getVersion()).forPath(path);        System.out.println("success delete znode " + path);        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：　

initsuccess delete znode /zk-book/c1

　　结果表明成功删除/zk-book/c1节点。

　　3.5 获取数据　

package com.hust.grid.leesf.curator.examples;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;public class Get_Data_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();        client.start();        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "init".getBytes());        Stat stat = new Stat();        System.out.println(new String(client.getData().storingStatIn(stat).forPath(path)));    }}

　　运行结果：　　

init

　　结果表明成功获取了节点的数据。

　　3.6 更新数据　　

package com.hust.grid.leesf.curator.examples;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;public class Set_Data_Sample {    public static void main(String[] args) throws Exception {        String path = "/zk-book";        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")                .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();        client.start();        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "init".getBytes());        Stat stat = new Stat();        client.getData().storingStatIn(stat).forPath(path);        System.out.println("Success set node for : " + path + ", new version: "                + client.setData().withVersion(stat.getVersion()).forPath(path).getVersion());        try {            client.setData().withVersion(stat.getVersion()).forPath(path);        } catch (Exception e) {            System.out.println("Fail set node due to " + e.getMessage());        }    }}

　　运行结果：　　

Success set node for : /zk-book, new version: 1Fail set node due to KeeperErrorCode = BadVersion for /zk-book

　　结果表明当携带数据版本不一致时，无法完成更新操作。

　　3.7 异步接口

　　如同Zookeeper原生API提供了异步接口，Curator也提供了异步接口。在Zookeeper中，所有的异步通知事件处理都是由EventThread这个线程来处理的，EventThread线程用于串行处理所有的事件通知，其可以保证对事件处理的顺序性，但是一旦碰上复杂的处理单元，会消耗过长的处理时间，从而影响其他事件的处理，Curator允许用户传入Executor实例，这样可以将比较复杂的事件处理放到一个专门的线程池中去。　

package com.hust.grid.leesf.curator.examples;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;public class Create_Node_Background_Sample {    static String path = "/zk-book";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    static CountDownLatch semaphore = new CountDownLatch(2);    static ExecutorService tp = Executors.newFixedThreadPool(2);    public static void main(String[] args) throws Exception {        client.start();        System.out.println("Main thread: " + Thread.currentThread().getName());        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() + "]" + ", Thread of processResult: " + Thread.currentThread().getName());                System.out.println();                semaphore.countDown();            }        }, tp).forPath(path, "init".getBytes());        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() + "]" + ", Thread of processResult: " + Thread.currentThread().getName());                semaphore.countDown();            }        }).forPath(path, "init".getBytes());        semaphore.await();        tp.shutdown();    }}

　　运行结果：

Main thread: mainevent[code: -110, type: CREATE], Thread of processResult: main-EventThreadevent[code: 0, type: CREATE], Thread of processResult: pool-3-thread-1

　　其中，创建节点的事件由线程池自己处理，而非默认线程处理。

　　Curator除了提供很便利的API，还提供了一些典型的应用场景，开发人员可以使用参考更好的理解如何使用Zookeeper客户端，所有的都在recipes包中，只需要在pom.xml中添加如下依赖即可

<dependency>   <groupId>org.apache.curator</groupId>   <artifactId>curator-recipes</artifactId>   <version>2.4.2</version>
</dependency>

　　3.8 节点监听　　

package com.hust.grid.leesf.curator.examples;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;public class NodeCache_Sample {    static String path = "/zk-book/nodecache";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(path, "init".getBytes());        final NodeCache cache = new NodeCache(client, path, false);        cache.start(true);        cache.getListenable().addListener(new NodeCacheListener() {            public void nodeChanged() throws Exception {                System.out.println("Node data update, new data: " + new String(cache.getCurrentData().getData()));            }        });        client.setData().forPath(path, "u".getBytes());        Thread.sleep(1000);        client.delete().deletingChildrenIfNeeded().forPath(path);        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：　　

Node data update, new data: u

　　当节点数据变更后收到了通知。NodeCache不仅可以监听数据节点的内容变更，也能监听指定节点是否存在。

　　3.9 子节点监听　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.cache.PathChildrenCache;import org.apache.curator.framework.recipes.cache.PathChildrenCache.StartMode;import org.apache.curator.framework.recipes.cache.PathChildrenCacheEvent;import org.apache.curator.framework.recipes.cache.PathChildrenCacheListener;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.zookeeper.CreateMode;public class PathChildrenCache_Sample {    static String path = "/zk-book";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .retryPolicy(new ExponentialBackoffRetry(1000, 3)).sessionTimeoutMs(5000).build();    public static void main(String[] args) throws Exception {        client.start();        PathChildrenCache cache = new PathChildrenCache(client, path, true);        cache.start(StartMode.POST_INITIALIZED_EVENT);        cache.getListenable().addListener(new PathChildrenCacheListener() {            public void childEvent(CuratorFramework client, PathChildrenCacheEvent event) throws Exception {                switch (event.getType()) {                case CHILD_ADDED:                    System.out.println("CHILD_ADDED," + event.getData().getPath());                    break;                case CHILD_UPDATED:                    System.out.println("CHILD_UPDATED," + event.getData().getPath());                    break;                case CHILD_REMOVED:                    System.out.println("CHILD_REMOVED," + event.getData().getPath());                    break;                default:                    break;                }            }        });        client.create().withMode(CreateMode.PERSISTENT).forPath(path);        Thread.sleep(1000);        client.create().withMode(CreateMode.PERSISTENT).forPath(path + "/c1");        Thread.sleep(1000);        client.delete().forPath(path + "/c1");        Thread.sleep(1000);        client.delete().forPath(path);        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：

CHILD_ADDED,/zk-book/c1CHILD_REMOVED,/zk-book/c1

　　监听节点的子节点，包括新增、数据变化、删除三类事件。

　　3.10 Master选举

　　借助Zookeeper，开发者可以很方便地实现Master选举功能，其大体思路如下：选择一个根节点，如/master_select，多台机器同时向该节点创建一个子节点/master_select/lock，利用Zookeeper特性，最终只有一台机器能够成功创建，成功的那台机器就是Master。

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.leader.LeaderSelector;import org.apache.curator.framework.recipes.leader.LeaderSelectorListenerAdapter;import org.apache.curator.retry.ExponentialBackoffRetry;public class Recipes_MasterSelect {    static String master_path = "/curator_recipes_master_path";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        LeaderSelector selector = new LeaderSelector(client, master_path, new LeaderSelectorListenerAdapter() {            public void takeLeadership(CuratorFramework client) throws Exception {                System.out.println("成为Master角色");                Thread.sleep(3000);                System.out.println("完成Master操作，释放Master权利");            }        });        selector.autoRequeue();        selector.start();        Thread.sleep(Integer.MAX_VALUE);    }}

　　运行结果：

成为Master角色完成Master操作，释放Master权利成为Master角色

　　以上结果会反复循环，并且当一个应用程序完成Master逻辑后，另外一个应用程序的相应方法才会被调用，即当一个应用实例成为Master后，其他应用实例会进入等待，直到当前Master挂了或者推出后才会开始选举Master。

　　3.11 分布式锁

　　为了保证数据的一致性，经常在程序的某个运行点需要进行同步控制。以流水号生成场景为例，普通的后台应用通常采用时间戳方式来生成流水号，但是在用户量非常大的情况下，可能会出现并发问题。　

package com.hust.grid.leesf.curator.examples;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.CountDownLatch;public class Recipes_NoLock {    public static void main(String[] args) throws Exception {        final CountDownLatch down = new CountDownLatch(1);        for (int i = 0; i < 10; i++) {            new Thread(new Runnable() {                public void run() {                    try {                        down.await();                    } catch (Exception e) {                    }                    SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss|SSS");                    String orderNo = sdf.format(new Date());                    System.err.println("生成的订单号是 : " + orderNo);                }            }).start();        }        down.countDown();    }}

　　运行结果：　

生成的订单号是 : 16:29:10|590生成的订单号是 : 16:29:10|590生成的订单号是 : 16:29:10|591生成的订单号是 : 16:29:10|591生成的订单号是 : 16:29:10|590生成的订单号是 : 16:29:10|590生成的订单号是 : 16:29:10|591生成的订单号是 : 16:29:10|590生成的订单号是 : 16:29:10|592生成的订单号是 : 16:29:10|591

　　结果表示订单号出现了重复，即普通的方法无法满足业务需要，因为其未进行正确的同步。可以使用Curator来实现分布式锁功能。

package com.hust.grid.leesf.curator.examples;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.CountDownLatch;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.locks.InterProcessMutex;import org.apache.curator.retry.ExponentialBackoffRetry;public class Recipes_Lock {    static String lock_path = "/curator_recipes_lock_path";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        final InterProcessMutex lock = new InterProcessMutex(client, lock_path);        final CountDownLatch down = new CountDownLatch(1);        for (int i = 0; i < 30; i++) {            new Thread(new Runnable() {                public void run() {                    try {                        down.await();                        lock.acquire();                    } catch (Exception e) {                    }                    SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss|SSS");                    String orderNo = sdf.format(new Date());                    System.out.println("生成的订单号是 : " + orderNo);                    try {                        lock.release();                    } catch (Exception e) {                    }                }            }).start();        }        down.countDown();    }}

　　运行结果：

生成的订单号是 : 16:31:50|293生成的订单号是 : 16:31:50|319生成的订单号是 : 16:31:51|278生成的订单号是 : 16:31:51|326生成的订单号是 : 16:31:51|402生成的订单号是 : 16:31:51|420生成的订单号是 : 16:31:51|546生成的订单号是 : 16:31:51|602生成的订单号是 : 16:31:51|626生成的订单号是 : 16:31:51|656生成的订单号是 : 16:31:51|675生成的订单号是 : 16:31:51|701生成的订单号是 : 16:31:51|708生成的订单号是 : 16:31:51|732生成的订单号是 : 16:31:51|763生成的订单号是 : 16:31:51|785生成的订单号是 : 16:31:51|805生成的订单号是 : 16:31:51|823生成的订单号是 : 16:31:51|839生成的订单号是 : 16:31:51|853生成的订单号是 : 16:31:51|868生成的订单号是 : 16:31:51|884生成的订单号是 : 16:31:51|897生成的订单号是 : 16:31:51|910生成的订单号是 : 16:31:51|926生成的订单号是 : 16:31:51|939生成的订单号是 : 16:31:51|951生成的订单号是 : 16:31:51|965生成的订单号是 : 16:31:51|972生成的订单号是 : 16:31:51|983

　　结果表明此时已经不存在重复的流水号。

　　3.12 分布式计数器

　　分布式计数器的典型应用是统计系统的在线人数，借助Zookeeper也可以很方便实现分布式计数器功能：指定一个Zookeeper数据节点作为计数器，多个应用实例在分布式锁的控制下，通过更新节点的内容来实现计数功能。　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.atomic.AtomicValue;import org.apache.curator.framework.recipes.atomic.DistributedAtomicInteger;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.retry.RetryNTimes;public class Recipes_DistAtomicInt {    static String distatomicint_path = "/curator_recipes_distatomicint_path";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        DistributedAtomicInteger atomicInteger = new DistributedAtomicInteger(client, distatomicint_path,                new RetryNTimes(3, 1000));        AtomicValue<Integer> rc = atomicInteger.add(8);        System.out.println("Result: " + rc.succeeded());    }}

　　运行结果：

Result: true

　　结果表明已经将数据成功写入数据节点中。

　　3.13 分布式Barrier

　　如同JDK的CyclicBarrier，Curator提供了DistributedBarrier来实现分布式Barrier。　　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.barriers.DistributedBarrier;import org.apache.curator.retry.ExponentialBackoffRetry;public class Recipes_Barrier {    static String barrier_path = "/curator_recipes_barrier_path";    static DistributedBarrier barrier;    public static void main(String[] args) throws Exception {        for (int i = 0; i < 5; i++) {            new Thread(new Runnable() {                public void run() {                    try {                        CuratorFramework client = CuratorFrameworkFactory.builder()                                .connectString("127.0.0.1:2181")                                .retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();                        client.start();                        barrier = new DistributedBarrier(client, barrier_path);                        System.out.println(Thread.currentThread().getName() + "号barrier设置");                        barrier.setBarrier();                        barrier.waitOnBarrier();                        System.err.println("启动...");                    } catch (Exception e) {                    }                }            }).start();        }        Thread.sleep(2000);        barrier.removeBarrier();    }}

　　运行结果：

Thread-1号barrier设置Thread-2号barrier设置Thread-4号barrier设置Thread-3号barrier设置Thread-0号barrier设置启动...启动...启动...启动...启动...

　　结果表明通过DistributedBarrier可以实现类似于CyclicBarrier的分布式Barrier功能。

四、Curator工具类

　　4.1 ZKPaths

　　其提供了简单的API来构建znode路径、递归创建、删除节点等。 　　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.utils.ZKPaths;import org.apache.curator.utils.ZKPaths.PathAndNode;import org.apache.zookeeper.ZooKeeper;public class ZKPaths_Sample {    static String path = "/curator_zkpath_sample";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        ZooKeeper zookeeper = client.getZookeeperClient().getZooKeeper();        System.out.println(ZKPaths.fixForNamespace(path, "sub"));        System.out.println(ZKPaths.makePath(path, "sub"));        System.out.println(ZKPaths.getNodeFromPath("/curator_zkpath_sample/sub1"));        PathAndNode pn = ZKPaths.getPathAndNode("/curator_zkpath_sample/sub1");        System.out.println(pn.getPath());        System.out.println(pn.getNode());        String dir1 = path + "/child1";        String dir2 = path + "/child2";        ZKPaths.mkdirs(zookeeper, dir1);        ZKPaths.mkdirs(zookeeper, dir2);        System.out.println(ZKPaths.getSortedChildren(zookeeper, path));        ZKPaths.deleteChildren(client.getZookeeperClient().getZooKeeper(), path, true);    }}

　　运行结果：　

/curator_zkpath_sample/sub/curator_zkpath_sample/subsub1/curator_zkpath_samplesub1[child1, child2]

　　借助ZKPaths可快速方便的完成节点的创建等操作。

　　4.2 EnsurePath

　　其提供了一种能够确保数据节点存在的机制，当上层业务希望对一个数据节点进行操作时，操作前需要确保该节点存在。　

package com.hust.grid.leesf.curator.examples;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.utils.EnsurePath;public class EnsurePathDemo {    static String path = "/zk-book/c1";    static CuratorFramework client = CuratorFrameworkFactory.builder().connectString("127.0.0.1:2181")            .sessionTimeoutMs(5000).retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();    public static void main(String[] args) throws Exception {        client.start();        client.usingNamespace("zk-book");        EnsurePath ensurePath = new EnsurePath(path);        ensurePath.ensure(client.getZookeeperClient());        ensurePath.ensure(client.getZookeeperClient());        EnsurePath ensurePath2 = client.newNamespaceAwareEnsurePath("/c1");        ensurePath2.ensure(client.getZookeeperClient());    }}

　　EnsurePath采取了如下节点创建方式，试图创建指定节点，如果节点已经存在，那么就不进行任何操作，也不对外抛出异常，否则正常创建数据节点。

五、总结

　　本篇介绍了使用Zookeeper的开源客户端如何操作Zookeeper的方法，相应的源码也已经上传至github，谢谢各位园友的观看～