ZooKeeper----Java实例文档

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一个简单的手表客户端
要求
程序设计
执行者类
DataMonitor类
完成源代码列表

 

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一个简单的手表客户端
为了向您介绍ZooKeeper Java API,我们在这里开发了一个非常简单的手表客户端。这个ZooKeeper客户端监视一个ZooKeeper节点的变化,并通过启动或停止一个程序做出响应。

要求
客户有四个要求:

  • 它采用以下参数:
  • ZooKeeper服务的地址
  • 然后是znode的名称 - 要观看的节点
  • 一个带参数的可执行文件。

它获取与znode相关联的数据并启动可执行文件。
如果znode更改,客户端将重新获取内容并重新启动可执行文件。
如果znode消失,客户端会杀死可执行文件。
程序设计
通常,ZooKeeper应用程序分为两个单元,一个维护连接,另一个监视数据。在这个应用程序中,称为Executor的类维护ZooKeeper连接,而调用DataMonitor的类监视ZooKeeper树中的数据。此外,Executor包含主线程并包含执行逻辑。它负责什么小的用户交互,以及与作为参数传递的可执行程序的交互,以及示例(根据需求)关闭和重新启动,根据znode的状态。

执行者类

Executor对象是示例应用程序的主容器。 它包含ZooKeeper对象,DataMonitor,如上所述在程序设计。

// 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) {
        }
    }

 

回想一下,Executor的工作是启动和停止在命令行上传递其名称的可执行文件。 它响应由ZooKeeper对象发起的事件。 正如你可以在上面的代码中看到的,Executor传递一个引用自己作为ZooKeeper构造函数中的Watcher参数。 它还将对自身的引用传递给DataMonitor构造函数的DataMonitorListener参数。 根据执行者的定义,它实现这两个接口:

 

 

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

Watcher界面由ZooKeeper Java API定义。 ZooKeeper使用它来回传到它的容器。 它只支持一个方法,process()和ZooKeeper使用它来通信主线程将被插入的通用事件,例如ZooKeeper连接或ZooKeeper会话的状态。此示例中的执行者简单地将这些事件 到DataMonitor来决定如何处理它们。 它只是为了说明一点,按照惯例,执行器或一些类似执行器的对象“拥有”ZooKeeper连接,但它可以将事件委托给其他事件到其他对象。 它也使用此作为默认通道来触发观察事件。

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

另一方面,DataMonitorListener接口不是ZooKeeper API的一部分。 它是一个完全自定义的接口,为此示例应用程序设计。 DataMonitor对象使用它来回传到它的容器,这也是Executor对象。DataMonitorListener接口如下所示:

public interface DataMonitorListener {
    /**
    * 节点的存在状态已更改。
    */
    void exists(byte data[]);

    /**
    *ZooKeeper会话不再有效。
    * 
    * @param rc
    * the ZooKeeper reason code
    */
    void closing(int rc);
}

此接口在DataMonitor类中定义并在Executor类中实现。 当Executor.exists()被调用时,Executor决定是否根据需求启动或关闭。 回想一下,当znode停止存在时,需要说明杀死可执行文件。

当Executor.closing()被调用时,Executor决定是否关闭自己以响应ZooKeeper连接永久消失。

正如你可能已经猜到的,DataMonitor是调用这些方法的对象,以响应ZooKeeper的状态的更改。

这里是Executor的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
DataMonitor类具有ZooKeeper逻辑的类。 它主要是异步和事件驱动。 DataMonitor在构造函数中使用:

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ZooKeeper.exists()的调用检查znode的存在,设置一个watch,并传递一个引用本身(this)作为完成回调对象。 在这个意义上,它踢的东西,因为真正的处理发生在手表被触发。

 

注意
不要将完成回调与watch回调混淆。 当在服务器上完成watch操作的异步设置(通过ZooKeeper.exists())时,会调用ZooKeeper.exists()完成回调,
恰巧是在DataMonitor对象中实现的方法StatCallback.processResult()。 另一方面,触发手表会向Executor对象发送一个事件,因为Executor注册为ZooKeeper对象的Watcher。 另外,您可能会注意到,DataMonitor也可以将自己注册为此特定监视事件的监视器。 这是ZooKeeper 3.0.0的新功能(多个监视器的支持)。 但是,在本示例中,DataMonitor不会注册为观察器。

 当ZooKeeper.exists()操作在服务器上完成时,ZooKeeper API在客户端上调用此完成回调:

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;
    }
}

代码首先检查错误代码是否存在znode,致命错误和可恢复错误。 如果文件(或znode)存在,它从znode获取数据,然后调用Executor的exists()回调,如果状态已更改。 注意,它不必对getData调用进行任何异常处理,因为它已监视挂起任何可能导致错误的事件:如果节点在调用ZooKeeper.getData()之前被删除,ZooKeeper设置的watch事件 .exists()触发回调; 如果出现通信错误,则在连接恢复时触发连接监视事件。 最后,请注意DataMonitor如何处理观看事件:

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:
//在这个特定的例子中,我们不需要在这里做任何事情 - 手表会自动重新注册服务器和任何手表触发,而客户端断开连接将交付(按顺序)
                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);
        }
    }

如果客户端ZooKeeper库可以在会话到期(过期事件)之前重新建立与ZooKeeper的通信通道(SyncConnected事件),则所有会话的手表将自动与服务器重新建立(手表的自动重置是新的 ZooKeeper 3.0.0)。 有关更多信息,请参阅程序员指南中的ZooKeeper Watches。 在这个函数中位有点下降,当DataMonitor获取znode的事件时,它调用ZooKeeper.exists()来找出发生了什么变化。

完成源代码列表

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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http://zookeeper.apache.org/doc/r3.4.6/javaExample.html#sc_design

posted on 2016-11-17 13:29  Honey_Badger  阅读(1119)  评论(0编辑  收藏  举报

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