[root@vm Temp]$ wget http://mirror.bit.edu.cn/apache/zookeeper/zookeeper-3.4.6/zookeeper-3.4.6.tar.gz[root@vm Temp]$ tar zxvf zookeeper-3.4.6.tar.gz[root@vm Temp]$ cd zookeeper-3.4.6[root@vm zookeeper-3.4.6]$ cp conf/zoo_sample.cfg conf/zoo.cfg[root@vm zookeeper-3.4.6]$ export ZOOKEEPER_HOME=/usr/local/src/zookeeper-3.4.5[root@vm zookeeper-3.4.6]$ export PATH=$ZOOKEEPER_HOME/bin:$PATH[root@vm zookeeper-3.4.6]$ bin/zkServer.sh start[root@vm zookeeper-3.4.6]$ bin/zkCli.sh -server 127.0.0.1:2181
2.客戶端常用操作
用zkCli.sh連接上Zookeeper服務后，用help能列出所有命令：
[html] view plain copy print?[root@BC-VM-edce4ac67d304079868c0bb265337bd4 zookeeper-3.4.6]# bin/zkCli.sh -127.0.0.1:2181  Connecting to localhost:2181  2015-06-11 10:55:14,387 [myid:] - INFO  [main:Environment@100] - Client environment:zookeeper.version=3.4.6-1569965, built on 02/20/2014 09:09 GMT      ...    [zk: localhost:2181(CONNECTED) 5] help  ZooKeeper -server host:port cmd args          connect host:port          get path [watch]          ls path [watch]          set path data [version]          rmr path          delquota [-n|-b] path          quit           printwatches on|off          create [-s] [-e] path data acl          stat path [watch]          close           ls2 path [watch]          history           listquota path          setAcl path acl          getAcl path          sync path          redo cmdno          addauth scheme auth          delete path [version]          setquota -n|-b val path  
[root@BC-VM-edce4ac67d304079868c0bb265337bd4 zookeeper-3.4.6]# bin/zkCli.sh -127.0.0.1:2181Connecting to localhost:21812015-06-11 10:55:14,387 [myid:] - INFO  [main:Environment@100] - Client environment:zookeeper.version=3.4.6-1569965, built on 02/20/2014 09:09 GMT    ...[zk: localhost:2181(CONNECTED) 5] helpZooKeeper -server host:port cmd args        connect host:port        get path [watch]        ls path [watch]        set path data [version]        rmr path        delquota [-n|-b] path        quit         printwatches on|off        create [-s] [-e] path data acl        stat path [watch]        close         ls2 path [watch]        history         listquota path        setAcl path acl        getAcl path        sync path        redo cmdno        addauth scheme auth        delete path [version]        setquota -n|-b val path
下面就試驗一下常用的命令：
create：創(chuàng)建路徑結(jié)點。ls：查看路徑下的所有結(jié)點。get：獲得結(jié)點上的值。set：修改結(jié)點上的值。delete：刪除結(jié)點。[html] view plain copy print?[zk: localhost:2181(CONNECTED) 6] create /zktest mydata  Created /zktest  [zk: localhost:2181(CONNECTED) 12] ls /  [zktest, zookeeper]  [zk: localhost:2181(CONNECTED) 7] ls /zktest  []  [zk: localhost:2181(CONNECTED) 13] get /zktest  mydata  cZxid = 0x1c  ctime = Thu Jun 11 10:58:06 CST 2015  mZxid = 0x1c  mtime = Thu Jun 11 10:58:06 CST 2015  pZxid = 0x1c  cversion = 0  dataVersion = 0  aclVersion = 0  ephemeralOwner = 0x0  dataLength = 6  numChildren = 0  [zk: localhost:2181(CONNECTED) 14] set /zktest junk  cZxid = 0x1c  ctime = Thu Jun 11 10:58:06 CST 2015  mZxid = 0x1f  mtime = Thu Jun 11 10:59:08 CST 2015  pZxid = 0x1c  cversion = 0  dataVersion = 1  aclVersion = 0  ephemeralOwner = 0x0  dataLength = 4  numChildren = 0  [zk: localhost:2181(CONNECTED) 15] delete /zktest  [zk: localhost:2181(CONNECTED) 16] ls /  [zookeeper]  
[zk: localhost:2181(CONNECTED) 6] create /zktest mydataCreated /zktest[zk: localhost:2181(CONNECTED) 12] ls /[zktest, zookeeper][zk: localhost:2181(CONNECTED) 7] ls /zktest[][zk: localhost:2181(CONNECTED) 13] get /zktestmydatacZxid = 0x1cctime = Thu Jun 11 10:58:06 CST 2015mZxid = 0x1cmtime = Thu Jun 11 10:58:06 CST 2015pZxid = 0x1ccversion = 0dataVersion = 0aclVersion = 0ephemeralOwner = 0x0dataLength = 6numChildren = 0[zk: localhost:2181(CONNECTED) 14] set /zktest junkcZxid = 0x1cctime = Thu Jun 11 10:58:06 CST 2015mZxid = 0x1fmtime = Thu Jun 11 10:59:08 CST 2015pZxid = 0x1ccversion = 0dataVersion = 1aclVersion = 0ephemeralOwner = 0x0dataLength = 4numChildren = 0[zk: localhost:2181(CONNECTED) 15] delete /zktest[zk: localhost:2181(CONNECTED) 16] ls /[zookeeper]
3.用Curator管理Zookeeper
Curator的Maven依賴如下，一般直接使用curator-recipes就行了，如果需要自己封裝一些底層些的功能的話，例如增加連接管理重試機制等，則可以引入curator-framework包。
[html] view plain copy print?<dependency>         <groupId>org.apache.curator</groupId>         <artifactId>curator-recipes</artifactId>         <version>2.7.0</version>     </dependency>  
 <dependency>        <groupId>org.apache.curator</groupId>        <artifactId>curator-recipes</artifactId>        <version>2.7.0</version>    </dependency>
3.1 Client操作
利用Curator提供的客戶端API，可以完全實現(xiàn)上面原生客戶端的功能。值得注意的是，Curator采用流式風格API。
[java] view plain copy print?package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;    import org.apache.curator.framework.CuratorFramework;  import org.apache.curator.framework.CuratorFrameworkFactory;  import org.apache.curator.retry.RetryNTimes;    /**  * Curator framework's client test.  * Output:  *  $ create /zktest hello   *  $ ls /   *  [zktest, zookeeper]  *  $ get /zktest   *  hello  *  $ set /zktest world   *  $ get /zktest   *  world  *  $ delete /zktest   *  $ ls /   *  [zookeeper]  */  public class CuratorClientTest {        /** Zookeeper info */      private static final String ZK_ADDRESS = "192.168.1.100:2181";      private static final String ZK_PATH = "/zktest";        public static void main(String[] args) throws Exception {          // 1.Connect to zk          CuratorFramework client = CuratorFrameworkFactory.newClient(                  ZK_ADDRESS,                  new RetryNTimes(10, 5000)          );          client.start();          System.out.println("zk client start successfully!");            // 2.Client API test          // 2.1 Create node          String data1 = "hello";          print("create", ZK_PATH, data1);          client.create().                  creatingParentsIfNeeded().                  forPath(ZK_PATH, data1.getBytes());            // 2.2 Get node and data          print("ls", "/");          print(client.getChildren().forPath("/"));          print("get", ZK_PATH);          print(client.getData().forPath(ZK_PATH));            // 2.3 Modify data          String data2 = "world";          print("set", ZK_PATH, data2);          client.setData().forPath(ZK_PATH, data2.getBytes());          print("get", ZK_PATH);          print(client.getData().forPath(ZK_PATH));            // 2.4 Remove node          print("delete", ZK_PATH);          client.delete().forPath(ZK_PATH);          print("ls", "/");          print(client.getChildren().forPath("/"));      }        private static void print(String... cmds) {          StringBuilder text = new StringBuilder("$ ");          for (String cmd : cmds) {              text.append(cmd).append(" ");          }          System.out.println(text.toString());      }        private static void print(Object result) {          System.out.println(                  result instanceof byte[]                      ? new String((byte[]) result)                          : result);      }    }  
package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.RetryNTimes;/** * Curator framework's client test. * Output: *  $ create /zktest hello  *  $ ls /  *  [zktest, zookeeper] *  $ get /zktest  *  hello *  $ set /zktest world  *  $ get /zktest  *  world *  $ delete /zktest  *  $ ls /  *  [zookeeper] */public class CuratorClientTest {    /** Zookeeper info */    private static final String ZK_ADDRESS = "192.168.1.100:2181";    private static final String ZK_PATH = "/zktest";    public static void main(String[] args) throws Exception {        // 1.Connect to zk        CuratorFramework client = CuratorFrameworkFactory.newClient(                ZK_ADDRESS,                new RetryNTimes(10, 5000)        );        client.start();        System.out.println("zk client start successfully!");        // 2.Client API test        // 2.1 Create node        String data1 = "hello";        print("create", ZK_PATH, data1);        client.create().                creatingParentsIfNeeded().                forPath(ZK_PATH, data1.getBytes());        // 2.2 Get node and data        print("ls", "/");        print(client.getChildren().forPath("/"));        print("get", ZK_PATH);        print(client.getData().forPath(ZK_PATH));        // 2.3 Modify data        String data2 = "world";        print("set", ZK_PATH, data2);        client.setData().forPath(ZK_PATH, data2.getBytes());        print("get", ZK_PATH);        print(client.getData().forPath(ZK_PATH));        // 2.4 Remove node        print("delete", ZK_PATH);        client.delete().forPath(ZK_PATH);        print("ls", "/");        print(client.getChildren().forPath("/"));    }    private static void print(String... cmds) {        StringBuilder text = new StringBuilder("$ ");        for (String cmd : cmds) {            text.append(cmd).append(" ");        }        System.out.println(text.toString());    }    private static void print(Object result) {        System.out.println(                result instanceof byte[]                    ? new String((byte[]) result)                        : result);    }}
3.2 監(jiān)聽器
Curator提供了三種Watcher(Cache)來監(jiān)聽結(jié)點的變化：
Path Cache：監(jiān)視一個路徑下1）孩子結(jié)點的創(chuàng)建、2）刪除，3）以及結(jié)點數(shù)據(jù)的更新。產(chǎn)生的事件會傳遞給注冊的PathChildrenCacheListener。Node Cache：監(jiān)視一個結(jié)點的創(chuàng)建、更新、刪除，并將結(jié)點的數(shù)據(jù)緩存在本地。Tree Cache：Path Cache和Node Cache的“合體”，監(jiān)視路徑下的創(chuàng)建、更新、刪除事件，并緩存路徑下所有孩子結(jié)點的數(shù)據(jù)。
下面就測試一下最簡單的Path Watcher：
[java] view plain copy print?package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;    import org.apache.curator.framework.CuratorFramework;  import org.apache.curator.framework.CuratorFrameworkFactory;  import org.apache.curator.framework.recipes.cache.ChildData;  import org.apache.curator.framework.recipes.cache.PathChildrenCache;  import org.apache.curator.framework.recipes.cache.PathChildrenCache.StartMode;  import org.apache.curator.retry.RetryNTimes;    /**  * Curator framework watch test.  */  public class CuratorWatcherTest {        /** Zookeeper info */      private static final String ZK_ADDRESS = "192.168.1.100:2181";      private static final String ZK_PATH = "/zktest";        public static void main(String[] args) throws Exception {          // 1.Connect to zk          CuratorFramework client = CuratorFrameworkFactory.newClient(                  ZK_ADDRESS,                  new RetryNTimes(10, 5000)          );          client.start();          System.out.println("zk client start successfully!");            // 2.Register watcher          PathChildrenCache watcher = new PathChildrenCache(                  client,                  ZK_PATH,                  true    // if cache data          );          watcher.getListenable().addListener((client1, event) -> {              ChildData data = event.getData();              if (data == null) {                  System.out.println("No data in event[" + event + "]");              } else {                  System.out.println("Receive event: "                          + "type=[" + event.getType() + "]"                          + ", path=[" + data.getPath() + "]"                          + ", data=[" + new String(data.getData()) + "]"                          + ", stat=[" + data.getStat() + "]");              }          });          watcher.start(StartMode.BUILD_INITIAL_CACHE);          System.out.println("Register zk watcher successfully!");            Thread.sleep(Integer.MAX_VALUE);      }    }  
package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.cache.ChildData;import org.apache.curator.framework.recipes.cache.PathChildrenCache;import org.apache.curator.framework.recipes.cache.PathChildrenCache.StartMode;import org.apache.curator.retry.RetryNTimes;/** * Curator framework watch test. */public class CuratorWatcherTest {    /** Zookeeper info */    private static final String ZK_ADDRESS = "192.168.1.100:2181";    private static final String ZK_PATH = "/zktest";    public static void main(String[] args) throws Exception {        // 1.Connect to zk        CuratorFramework client = CuratorFrameworkFactory.newClient(                ZK_ADDRESS,                new RetryNTimes(10, 5000)        );        client.start();        System.out.println("zk client start successfully!");        // 2.Register watcher        PathChildrenCache watcher = new PathChildrenCache(                client,                ZK_PATH,                true    // if cache data        );        watcher.getListenable().addListener((client1, event) -> {            ChildData data = event.getData();            if (data == null) {                System.out.println("No data in event[" + event + "]");            } else {                System.out.println("Receive event: "                        + "type=[" + event.getType() + "]"                        + ", path=[" + data.getPath() + "]"                        + ", data=[" + new String(data.getData()) + "]"                        + ", stat=[" + data.getStat() + "]");            }        });        watcher.start(StartMode.BUILD_INITIAL_CACHE);        System.out.println("Register zk watcher successfully!");        Thread.sleep(Integer.MAX_VALUE);    }}下面是在zkCli.sh中操作時Java程序的輸出：
[html] view plain copy print?Java: zk client start successfully!  Java: Register zk watcher successfully!    zkCli: [zk: localhost:2181(CONNECTED) 11] create /zktest/hello mydata  Java: Receive event: type=[CHILD_ADDED], path=[/zktest/hello], data=[mydata], stat=[121,121,1434001221097,1434001221097,0,0,0,0,6,0,121]    zkCli: [zk: localhost:2181(CONNECTED) 12] set /zktest/hello otherdata  Java: Receive event: type=[CHILD_UPDATED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]    zkCli: [zk: localhost:2181(CONNECTED) 13] delete /zktest/hello  Java: Receive event: type=[CHILD_REMOVED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]  
Java: zk client start successfully!Java: Register zk watcher successfully!zkCli: [zk: localhost:2181(CONNECTED) 11] create /zktest/hello mydataJava: Receive event: type=[CHILD_ADDED], path=[/zktest/hello], data=[mydata], stat=[121,121,1434001221097,1434001221097,0,0,0,0,6,0,121]zkCli: [zk: localhost:2181(CONNECTED) 12] set /zktest/hello otherdataJava: Receive event: type=[CHILD_UPDATED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]zkCli: [zk: localhost:2181(CONNECTED) 13] delete /zktest/helloJava: Receive event: type=[CHILD_REMOVED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]
4.Curator“菜譜”
既然Maven包叫做curator-recipes，那說明Curator有它獨特的“菜譜”：
鎖：包括共享鎖、共享可重入鎖、讀寫鎖等。選舉：Leader選舉算法。Barrier：阻止分布式計算直至某個條件被滿足的“柵欄”，可以看做JDK Concurrent包中Barrier的分布式實現(xiàn)。緩存：前面提到過的三種Cache及監(jiān)聽機制。持久化結(jié)點：連接或session終止后仍然在Zookeeper中存在的結(jié)點。隊列：分布式隊列、分布式優(yōu)先級隊列等。
4.1 分布式鎖
分布式編程時，比如最容易碰到的情況就是應用程序在線上多機部署，于是當多個應用同時訪問某一資源時，就需要某種機制去協(xié)調(diào)它們。例如，現(xiàn)在一臺應用正在rebuild緩存內(nèi)容，要臨時鎖住某個區(qū)域暫時不讓訪問；又比如調(diào)度程序每次只想一個任務被一臺應用執(zhí)行等等。
下面的程序會啟動兩個線程t1和t2去爭奪鎖，拿到鎖的線程會占用5秒。運行多次可以觀察到，有時是t1先拿到鎖而t2等待，有時又會反過來。Curator會用我們提供的lock路徑的結(jié)點作為全局鎖，這個結(jié)點的數(shù)據(jù)類似這種格式：[_c_64e0811f-9475-44ca-aa36-c1db65ae5350-lock-0000000005]，每次獲得鎖時會生成這種串，釋放鎖時清空數(shù)據(jù)。
[java] view plain copy print?package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;    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.RetryNTimes;    import java.util.concurrent.TimeUnit;    /**  * Curator framework's distributed lock test.  */  public class CuratorDistrLockTest {        /** Zookeeper info */      private static final String ZK_ADDRESS = "192.168.1.100:2181";      private static final String ZK_LOCK_PATH = "/zktest";        public static void main(String[] args) throws InterruptedException {          // 1.Connect to zk          CuratorFramework client = CuratorFrameworkFactory.newClient(                  ZK_ADDRESS,                  new RetryNTimes(10, 5000)          );          client.start();          System.out.println("zk client start successfully!");            Thread t1 = new Thread(() -> {              doWithLock(client);          }, "t1");          Thread t2 = new Thread(() -> {              doWithLock(client);          }, "t2");            t1.start();          t2.start();      }        private static void doWithLock(CuratorFramework client) {          InterProcessMutex lock = new InterProcessMutex(client, ZK_LOCK_PATH);          try {              if (lock.acquire(10 * 1000, TimeUnit.SECONDS)) {                  System.out.println(Thread.currentThread().getName() + " hold lock");                  Thread.sleep(5000L);                  System.out.println(Thread.currentThread().getName() + " release lock");              }          } catch (Exception e) {              e.printStackTrace();          } finally {              try {                  lock.release();              } catch (Exception e) {                  e.printStackTrace();              }          }        }    }  
package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;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.RetryNTimes;import java.util.concurrent.TimeUnit;/** * Curator framework's distributed lock test. */public class CuratorDistrLockTest {    /** Zookeeper info */    private static final String ZK_ADDRESS = "192.168.1.100:2181";    private static final String ZK_LOCK_PATH = "/zktest";    public static void main(String[] args) throws InterruptedException {        // 1.Connect to zk        CuratorFramework client = CuratorFrameworkFactory.newClient(                ZK_ADDRESS,                new RetryNTimes(10, 5000)        );        client.start();        System.out.println("zk client start successfully!");        Thread t1 = new Thread(() -> {            doWithLock(client);        }, "t1");        Thread t2 = new Thread(() -> {            doWithLock(client);        }, "t2");        t1.start();        t2.start();    }    private static void doWithLock(CuratorFramework client) {        InterProcessMutex lock = new InterProcessMutex(client, ZK_LOCK_PATH);        try {            if (lock.acquire(10 * 1000, TimeUnit.SECONDS)) {                System.out.println(Thread.currentThread().getName() + " hold lock");                Thread.sleep(5000L);                System.out.println(Thread.currentThread().getName() + " release lock");            }        } catch (Exception e) {            e.printStackTrace();        } finally {            try {                lock.release();            } catch (Exception e) {                e.printStackTrace();            }        }    }}
4.2 Leader選舉
當集群里的某個服務down機時，我們可能要從slave結(jié)點里選出一個作為新的master，這時就需要一套能在分布式環(huán)境中自動協(xié)調(diào)的Leader選舉方法。Curator提供了LeaderSelector監(jiān)聽器實現(xiàn)Leader選舉功能。同一時刻，只有一個Listener會進入takeLeadership()方法，說明它是當前的Leader。注意：當Listener從takeLeadership()退出時就說明它放棄了“Leader身份”，這時Curator會利用Zookeeper再從剩余的Listener中選出一個新的Leader。autoRequeue()方法使放棄Leadership的Listener有機會重新獲得Leadership，如果不設置的話放棄了的Listener是不會再變成Leader的。
[java] view plain copy print?package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;    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.LeaderSelectorListener;  import org.apache.curator.framework.state.ConnectionState;  import org.apache.curator.retry.RetryNTimes;  import org.apache.curator.utils.EnsurePath;    /**  * Curator framework's leader election test.  * Output:  *  LeaderSelector-2 take leadership!  *  LeaderSelector-2 relinquish leadership!  *  LeaderSelector-1 take leadership!  *  LeaderSelector-1 relinquish leadership!  *  LeaderSelector-0 take leadership!  *  LeaderSelector-0 relinquish leadership!   *      ...  */  public class CuratorLeaderTest {        /** Zookeeper info */      private static final String ZK_ADDRESS = "192.168.1.100:2181";      private static final String ZK_PATH = "/zktest";        public static void main(String[] args) throws InterruptedException {          LeaderSelectorListener listener = new LeaderSelectorListener() {              @Override              public void takeLeadership(CuratorFramework client) throws Exception {                  System.out.println(Thread.currentThread().getName() + " take leadership!");                    // takeLeadership() method should only return when leadership is being relinquished.                  Thread.sleep(5000L);                    System.out.println(Thread.currentThread().getName() + " relinquish leadership!");              }                @Override              public void stateChanged(CuratorFramework client, ConnectionState state) {              }          };            new Thread(() -> {              registerListener(listener);          }).start();            new Thread(() -> {              registerListener(listener);          }).start();            new Thread(() -> {              registerListener(listener);          }).start();            Thread.sleep(Integer.MAX_VALUE);      }        private static void registerListener(LeaderSelectorListener listener) {          // 1.Connect to zk          CuratorFramework client = CuratorFrameworkFactory.newClient(                  ZK_ADDRESS,                  new RetryNTimes(10, 5000)          );          client.start();            // 2.Ensure path          try {              new EnsurePath(ZK_PATH).ensure(client.getZookeeperClient());          } catch (Exception e) {              e.printStackTrace();          }            // 3.Register listener          LeaderSelector selector = new LeaderSelector(client, ZK_PATH, listener);          selector.autoRequeue();          selector.start();      }    }  
package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;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.LeaderSelectorListener;import org.apache.curator.framework.state.ConnectionState;import org.apache.curator.retry.RetryNTimes;import org.apache.curator.utils.EnsurePath;/** * Curator framework's leader election test. * Output: *  LeaderSelector-2 take leadership! *  LeaderSelector-2 relinquish leadership! *  LeaderSelector-1 take leadership! *  LeaderSelector-1 relinquish leadership! *  LeaderSelector-0 take leadership! *  LeaderSelector-0 relinquish leadership!  *      ... */public class CuratorLeaderTest {    /** Zookeeper info */    private static final String ZK_ADDRESS = "192.168.1.100:2181";    private static final String ZK_PATH = "/zktest";    public static void main(String[] args) throws InterruptedException {        LeaderSelectorListener listener = new LeaderSelectorListener() {            @Override            public void takeLeadership(CuratorFramework client) throws Exception {                System.out.println(Thread.currentThread().getName() + " take leadership!");                // takeLeadership() method should only return when leadership is being relinquished.                Thread.sleep(5000L);                System.out.println(Thread.currentThread().getName() + " relinquish leadership!");            }            @Override            public void stateChanged(CuratorFramework client, ConnectionState state) {            }        };        new Thread(() -> {            registerListener(listener);        }).start();        new Thread(() -> {            registerListener(listener);        }).start();        new Thread(() -> {            registerListener(listener);        }).start();        Thread.sleep(Integer.MAX_VALUE);    }    private static void registerListener(LeaderSelectorListener listener) {        // 1.Connect to zk        CuratorFramework client = CuratorFrameworkFactory.newClient(                ZK_ADDRESS,                new RetryNTimes(10, 5000)        );        client.start();        // 2.Ensure path        try {            new EnsurePath(ZK_PATH).ensure(client.getZookeeperClient());        } catch (Exception e) {            e.printStackTrace();        }        // 3.Register listener        LeaderSelector selector = new LeaderSelector(client, ZK_PATH, listener);        selector.autoRequeue();        selector.start();    }}