為了向您介紹ZooKeeper Java API，我們在這裏開發了一個非常簡單的觀看客戶端。該ZooKeeper客戶端通過啟動或停止程序來觀察ZooKeeper節點的更改並進行響應。
要求
有四個要求：
1.它作為參數：
ZooKeeper服務的地址
那麽znode的名字就是被觀看的
具有參數的可執行文件

2.它獲取與znode相關聯的數據，並啟動可執行文件。
3.如果znode更改，客戶端將重新啟動內容並重新啟動可執行文件。
4.如果znode消失，客戶端將殺死可執行文件。

程序設計

通常，ZooKeeper應用程序分為兩個單元，一個維護連接，另一個用於監視數據。

此外，Executor包含主線程並包含執行邏輯。它負責什麽樣的用戶交互，以及與您作為參數傳遞的可執行程序的交互以及根據znode的狀態關閉和重新啟動示例。

1. Executor.java

package com.hellojd.cloud;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
/**
 * A simple example program to use DataMonitor to start and
 * stop executables based on a znode. The program watches the
 * specified znode and saves the data that corresponds to the
 * znode in the filesystem. It also starts the specified program
 * with the specified arguments when the znode exists and kills
 * the program if the znode goes away.
 */
public class Executor
        implements Watcher, Runnable, DataMonitor.DataMonitorListener
{

    DataMonitor dm;

    ZooKeeper zk;

    String filename;

    String exec[];

    Process child;

    public Executor(String hostPort, String znode, String filename,
                    String exec[]) throws KeeperException, IOException {
        this.filename = filename;
        this.exec = exec;
        zk = new ZooKeeper(hostPort, 3000, this);
        dm = new DataMonitor(zk, znode, null, this);
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        if (args.length < 4) {
            System.err
                    .println("USAGE: Executor hostPort znode filename program [args ...]");
            System.exit(2);
        }
        String hostPort = args[0];
        String znode = args[1];
        String filename = args[2];
        String exec[] = new String[args.length - 3];
        System.arraycopy(args, 3, exec, 0, exec.length);
        try {
            new Executor(hostPort, znode, filename, exec).run();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /***************************************************************************
     * We do process any events ourselves, we just need to forward them on.
     *
     * @see org.apache.zookeeper.Watcher#process(org.apache.zookeeper.proto.WatcherEvent)
     */
    public void process(WatchedEvent event) {
        System.out.println("Watcher process");
        dm.process(event);
    }

    public void run() {
        try {
            synchronized (this) {
                while (!dm.dead) {
                    wait();
                }
            }
        } catch (InterruptedException e) {
        }
    }
    //以響應ZooKeeper連接永久消失。
    public void closing(int rc) {
        synchronized (this) {
            notifyAll();
        }
    }

    static class StreamWriter extends Thread {
        OutputStream os;

        InputStream is;

        StreamWriter(InputStream is, OutputStream os) {
            this.is = is;
            this.os = os;
            start();
        }

        public void run() {
            byte b[] = new byte[80];
            int rc;
            try {
                while ((rc = is.read(b)) > 0) {
                    os.write(b, 0, rc);
                }
            } catch (IOException e) {
            }

        }
    }

    public void exists(byte[] data) {
        if (data == null) {
            if (child != null) {
                System.out.println("Killing process");
                child.destroy();
                try {
                    child.waitFor();
                } catch (InterruptedException e) {
                }
            }
            child = null;
        } else {
            if (child != null) {
                System.out.println("Stopping child");
                child.destroy();
                try {
                    child.waitFor();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //保存znode數據至文件
            try {
                FileOutputStream fos = new FileOutputStream(filename);
                fos.write(data);
                fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                System.out.println("Starting child");

                child = Runtime.getRuntime().exec(exec);
                new StreamWriter(child.getInputStream(), System.out);
                new StreamWriter(child.getErrorStream(), System.err);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

2. DataMonitor.java

/**
 * A simple class that monitors the data and existence of a ZooKeeper
 * node. It uses asynchronous ZooKeeper APIs.
 */
package com.hellojd.cloud;
import java.util.Arrays;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.AsyncCallback.StatCallback;
import org.apache.zookeeper.KeeperException.Code;
import org.apache.zookeeper.data.Stat;

/**
 * 另一方面，DataMonitorListener接口不是ZooKeeper API的一部分。 它是一個完全定制的界面，專為此示例應用程序而設計。
 * DataMonitor對象使用它來回傳給它的容器，它也是Executor對象。
 */
public class DataMonitor implements  StatCallback {
    //Executor或一些類似Executor的對象“擁有”ZooKeeper連接，但可以將事件委托給其他事件到其他對象。
    ZooKeeper zk;

    String znode;

    Watcher chainedWatcher;

    boolean dead;
    //簡單地將這些事件轉發到DataMonitor來決定如何處理它們
    DataMonitorListener listener;

    byte prevData[];
    // 它主要是異步和事件驅動
    public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
                       DataMonitorListener listener) {
        this.zk = zk;
        this.znode = znode;
        this.chainedWatcher = chainedWatcher;
        this.listener = listener;
        // Get things started by checking if the node exists. We are going
        // to be completely event driven
        zk.exists(znode, true, this, null);
    }

    /**
     * 該接口在DataMonitor類中定義，並在Executor類中實現。 當調用Executor.exists（）時，執行器根據要求決定是啟動還是關閉。
     * 當znode不再存在時，需要說的是殺死可執行文件。
     */
    public interface DataMonitorListener {
        /**
         * The existence status of the node has changed.
         */
        void exists(byte data[]);

        /**
         * The ZooKeeper session is no longer valid.
         *
         * @param rc
         *                the ZooKeeper reason code
         */
        void closing(int rc);
    }
    //響應ZooKeeper狀態的更改
    public void process(WatchedEvent event) {
        String path = event.getPath();
        if (event.getType() == Watcher.Event.EventType.None) {
            // We are are being told that the state of the
            // connection has changed
            switch (event.getState()) {
                case SyncConnected:
                    // In this particular example we don‘t need to do anything
                    // here - watches are automatically re-registered with
                    // server and any watches triggered while the client was
                    // disconnected will be delivered (in order of course)
                    break;
                case Expired:
                    // It‘s all over
                    dead = true;
                    listener.closing(KeeperException.Code.SessionExpired);
                    break;
            }
        } else {
            if (path != null && path.equals(znode)) {
                // Something has changed on the node, let‘s find out
                zk.exists(znode, true, this, null);
            }
        }
        if (chainedWatcher != null) {
            chainedWatcher.process(event);
        }
    }

    public void processResult(int rc, String path, Object ctx, Stat stat) {
/**
 * 首先檢查znode存在，致命錯誤和可恢復錯誤的錯誤代碼。
 * 如果文件（或znode）存在，它將從znode獲取數據，然後調用Executor的exists（）回調，
 * 如果狀態已更改。 註意，它不必對getData調用執行異常處理，因為它具有掛起的任何可能導致錯誤的監視器：
 * 如果節點在調用ZooKeeper.getData（）之前被刪除，則由ZooKeeper設置的監視事件 .exists（）觸發回調;
 *如果發生通信錯誤，連接回顯將觸發連接監視事件。
 */
        boolean exists;
        switch (rc) {
            case Code.Ok:
                exists = true;
                break;
            case Code.NoNode:
                exists = false;
                break;
            case Code.SessionExpired:
            case Code.NoAuth:
                dead = true;
                listener.closing(rc);
                return;
            default:
                // Retry errors
                zk.exists(znode, true, this, null);
                return;
        }
        //文件（或znode）存在
        byte b[] = null;
        if (exists) {
            try {
                b = zk.getData(znode, false, null);
            } catch (KeeperException e) {
                // We don‘t need to worry about recovering now. The watch
                // callbacks will kick off any exception handling
                e.printStackTrace();
            } catch (InterruptedException e) {
                return;
            }
        }
        if ((b == null && b != prevData)
                || (b != null && !Arrays.equals(prevData, b))) {
            listener.exists(b);
            prevData = b;
        }
    }
}

調試:

參數列表：192.168.0.10:2181 /hellojd_node filename calc

192.168.0.10:2181：ZK地址

/hellojd_node :監視node

filename :備份數據文件

calc:命令

來自:http://zookeeper.apache.org/doc/current/javaExample.html

本文出自 “簡單” 博客，請務必保留此出處http://dba10g.blog.51cto.com/764602/1975090

ZooKeeper Java Example(官方例子)