为了向您介绍ZooKeeper Java API，我们在这里开发了一个非常简单的观看客户端。该ZooKeeper客户端通过启动或停止程序来观察ZooKeeper节点的更改并进行响应。
要求
有四个要求：
1.它作为参数：
ZooKeeper服务的地址
那么znode的名字就是被观看的
具有参数的可执行文件
2.它获取与znode相关联的数据，并启动可执行文件。
3.如果znode更改，客户端将重新启动内容并重新启动可执行文件。
4.如果znode消失，客户端将杀死可执行文件。
程序设计
通常，ZooKeeper应用程序分为两个单元，一个维护连接，另一个用于监视数据。在此应用程序中，名为Executor的类维护ZooKeeper连接，并且名为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。它负责什么样的用户交互，以及与您作为参数传递的可执行程序的交互以及根据znode的状态关闭和重新启动示例。
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) {
}
}
//以响应ZooKeeper连接永久消失。
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数据至文件
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。 它是一个完全定制的界面，专为此示例应用程序而设计。
* DataMonitor对象使用它来回传给它的容器，它也是Executor对象。
*/
public class DataMonitor implements StatCallback {
//Executor或一些类似Executor的对象“拥有”ZooKeeper连接，但可以将事件委托给其他事件到其他对象。
ZooKeeper zk;
String znode;
Watcher chainedWatcher;
boolean dead;
//简单地将这些事件转发到DataMonitor来决定如何处理它们
DataMonitorListener listener;
byte prevData[];
// 它主要是异步和事件驱动
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);
}
/**
* 该接口在DataMonitor类中定义，并在Executor类中实现。 当调用Executor.exists（）时，执行器根据要求决定是启动还是关闭。
* 当znode不再存在时，需要说的是杀死可执行文件。
*/
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);
}
//响应ZooKeeper状态的更改
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 proce***esult(int rc, String path, Object ctx, Stat stat) {
/**
* 首先检查znode存在，致命错误和可恢复错误的错误代码。
* 如果文件（或znode）存在，它将从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调用执行异常处理，因为它具有挂起的任何可能导致错误的监视器：
* 如果节点在调用ZooKeeper.getData（）之前被删除，则由ZooKeeper设置的监视事件 .exists（）触发回调;
*如果发生通信错误，连接回显将触发连接监视事件。
*/
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;
}
}
}
调试:
参数列表：192.168.0.10:2181 /hellojd_node filename calc
192.168.0.10:2181：ZK地址
/hellojd_node :监视node
filename :备份数据文件
calc:命令
来自:http://zookeeper.apache.org/doc/current/javaExample.html