ZooKeeper官方文档之Java客户端开发案例翻译

目录

• 一个简单的监听客户端
• 需求
• 程序设计
• Executor类
• DataMonitor类
• 完整代码清单

官网原文标题《ZooKeeper Java Example》

官网原文地址：http://zookeeper.apache.org/doc/current/javaExample.html#sc_completeSourceCode

针对本篇翻译文章，我还有一篇对应的笔记《ZooKeeper官方Java例子解读》，如果对官网文档理解有困难，可以结合我的笔记理解。

一个简单的监听客户端

通过开发一个非常简单的监听客户端，为你介绍ZooKeeper的Java API。此ZooKeeper的客户端，监听ZooKeeper中node的变化并做出响应。

需求

这个客户端有如下四个需求：

1、它接收如下参数：

• ZooKeeper服务的地址
• 被监控的znode的名称
• 可执行命令参数

2、它会取得znode上关联的数据，然后执行命令

3、如果znode变化，客户端重新拉取数据，再次执行命令

4、如果znode消失了，客户端杀掉进行的执行命令。

程序设计

一般我们会这么做，把ZooKeeper的程序分成两个单元，一个维护连接，另外一个监控数据。本程序中Executor类维护ZooKeeper的连接，DataMonitor监控ZooKeeper的数据。同时，Executor维护主线程以及执行逻辑。它负责对用户的交互做出响应，这里的交互既指根据你传入参数做出响应，也指根据znode的状态，关闭和重启。

Executor类

// from the Executor class...
    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();
        }
    }
    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);
    }

    public void run() {
        try {
            synchronized (this) {
                while (!dm.dead) {
                    wait();
                }
            }
        } catch (InterruptedException e) {
        }
    }

回忆一下，Executor的工作是启停通过命令行传入的执行命令。他通过响应ZooKeeper对象触发的事件来实现。就像上面的代码，在ZooKeeper的构造器中，Executor传递自己的引用作为watcher参数。同时，他传递自己的引用作为DataMonitorLisrener参数给DataMonitor构造器。在Executor定义中，实现了这些接口。

public class Executor implements Watcher, Runnable, DataMonitor.DataMonitorListener {
...

ZooKeeper的Java API定义了Watcher接口。ZooKeeper用它来反馈给它的持有者。它仅支持一个方法process()，ZooKeeper用它来反馈主线程感兴趣的通用事件，例如ZooKeeper的连接状态，或者ZooKeeper session的状态。例子中的Executor只是简单的把事件传递给DataMonitor，由DataMonitor来决定怎么处理。为了方便，Executor或者其他的类似Executor的对象持有ZooKeeper连接，但是可以很自由的把事件委派给其他对象。它也用此作为触发watch事件的默认渠道。

public void process(WatchedEvent event) {
        dm.process(event);
    }

DataMonitorListener接口，并不是ZooKeeper提供的API。它是为这个示例程序设计的自定义接口。DataMonitor对象用它为它的持有者（也是Executor对象）反馈，

DataMonitorListener接口是下面这个样子：

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);
}

这个接口定义在DataMonitor类中，被Executor类实现。当调用Executor.exists()，Executor根据需求决定是否启动还是关闭。回忆一下，需求提到当znode不再存在时，杀掉进行中的执行命令。

当调用Executor.closing()，作为对ZooKeeper连接永久消失的响应，Executor决定是否关闭它自己。

就像你可能猜想的那样，，作为对ZooKeeper状态变化的响应，这些方法的调用者是DataMonitor。

下面是Exucutor中 DataMonitorListener.exists()和DataMonitorListener.closing()的实现

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();
            }
        }
        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();
        }
    }
}

public void closing(int rc) {
    synchronized (this) {
        notifyAll();
    }
}

DataMonitor类

ZooKeeper的逻辑都在DataMonitor类中。他是异步和事件驱动的。DataMonitor在构造函数中完成启动。

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);
    }

对zk.exists()的调用，会检查znode是否存在，设置watch，传递他自己的引用作为完成后的回调对象。这意味着，当watch被引发，真正的处理才开始。

Note

不要把完成回调和watch回调搞混。ZooKeeper.exists()完成时的回调，发生在DataMonitor对象实现的的StatCallback.processResult()方法中，调用发生在server上异步的watch设置操作（通过zk.exists()）完成时。

另一边，watch触发时，给Executor对象发送了一个事件，因为Executor注册成为ZooKeeper对象的一个watcher。

你可能注意到DataMonitor也可以注册它自己作为这个特定事件的watcher。这是ZooKeeper 3.0.0中加入的（多watcher的支持）。在这个例子中，DataMonitor并没有注册为watcher（译者：这里指zookeeper对象的watcher）。

当ZooKeeper.exists()在server上执行完成。ZooKeeper API将在客户端发起这个完成回调

public void processResult(int rc, String path, Object ctx, Stat stat) {
    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;
    }
    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;
    }
}

首先检查了znode存在返回的错误代码，致命的错误及可恢复的错误。如果znode存在，将从znode取得数据，如果状态发生改变，调用Executor的exists回调。不需要为getData做任何异常处理。因为它为任何可能引发错误的情况设置了监控：如果在调用ZooKeeper.getData()前，node被删除了，通过ZooKeeper.exists设置的监听事件被触发回调；如果发生了通信错误，当连接恢复时，连接的监听事件被触发。

最后，看一下DataMonitor是如何处理监听事件的：

public void process(WatchedEvent event) {
        String path = event.getPath();
        if (event.getType() == 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);
        }
    }

在session过期前，如果客户端zookeeper类库能重新发布和zookeeper的连接通道（SyncConnected event），session的所有watch将会重新发布。（zookeeper 3.0.0开始）。学习开发手册中的ZooKeeper Watches。继续往下讲，当DataMonitor从znode收到事件，他将会调用zookeeper.exists()，来找出发生了什么变化。

完整代码清单

Executor.java

/**
 * 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.
 */
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
public class Executor
    implements Watcher, Runnable, DataMonitor.DataMonitorListener
{
    String znode;
    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) {
        dm.process(event);
    }
     public void run() {
        try {
            synchronized (this) {
                while (!dm.dead) {
                    wait();
                }
            }
        } catch (InterruptedException e) {
        }
    }

    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();
                }
            }
            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();
            }
        }
    }
}

DataMonitor.java

/**
 * A simple class that monitors the data and existence of a ZooKeeper
 * node. It uses asynchronous ZooKeeper APIs.
 */
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;
public class DataMonitor implements Watcher, StatCallback {
    ZooKeeper zk;
    String znode;
    Watcher chainedWatcher;
    boolean dead;
    DataMonitorListener listener;
    byte prevData[];

    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);
    }

    /**
     * Other classes use the DataMonitor by implementing this method
     */
    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);
    }
    public void process(WatchedEvent event) {
        String path = event.getPath();
        if (event.getType() == 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) {
        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;
        }
        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;
        }
    }
}

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