ZooKeeper Java 示例
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监视客户端。这个 ZooKeeper 客户端监视 znode 的变化，并通过启动或停止程序来响应。
要求
客户有四个要求：
- 它作为参数：
- ZooKeeper 服务的地址
- znode 的名称 - 要监视的节点
- 要将输出写入到的文件的名称
- 带参数的可执行文件。
- 它获取与 znode 关联的数据并启动可执行文件。
- 如果 znode 发生变化，客户端会重新获取内容并重新启动可执行文件。
- 如果 znode 消失，客户端将终止可执行文件。
程序设计
按照惯例，ZooKeeper 应用程序分为两个单元，一个负责维护连接，另一个负责监控数据。在这个应用程序中，名为Executor的类维护 ZooKeeper 连接，名为DataMonitor的类监控 ZooKeeper 树中的数据。此外，Executor 包含主线程并包含执行逻辑。根据 znode 的状态，它负责少量的用户交互，以及与作为参数传入的可执行程序的交互以及示例（根据要求）关闭和重新启动的交互。
执行者类
Executor 对象是示例应用程序的主要容器。它包含ZooKeeper对象DataMonitor，如上面程序设计中所述。
// 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您在命令行中传入其名称的可执行文件。它这样做是为了响应 ZooKeeper 对象触发的事件。正如您在上面的代码中看到的那样，Executor 将一个对自身的引用作为 ZooKeeper 构造函数中的 Watcher 参数传递。它还将对自身的引用作为 DataMonitorListener 参数传递给 DataMonitor 构造函数。根据 Executor 的定义，它实现了这两个接口：
public class Executor implements Watcher, Runnable, DataMonitor.DataMonitorListener {
...
Watcher接口由ZooKeeper Java API 定义。ZooKeeper 使用它与它的容器进行通信。它只支持一种方法，process()ZooKeeper 使用它来传达主线程可能感兴趣的通用事件，例如 ZooKeeper 连接的状态或 ZooKeeper 会话。本例中的 Executor 只是将这些事件转发给 DataMonitor 以决定如何处理它们。它这样做只是为了说明这一点，按照惯例，Executor 或一些类似 Executor 的对象“拥有”ZooKeeper 连接，但可以自由地将事件委托给其他事件给其他对象。它还使用它作为触发监视事件的默认通道。（稍后会详细介绍。）
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界面，专为此示例应用程序而设计。DataMonitor 对象使用它与它的容器进行通信，该容器也是 Executor 对象。DataMonitorListener 接口如下所示：
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* The ZooKeeper session is no longer valid.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);
}
该接口在 DataMonitor 类中定义，在 Executor 类中实现。当Executor.exists()被调用时，Executor 根据需求决定是启动还是关闭。回想一下，当 znode 不再存在时，需要杀死可执行文件。
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。
您可能已经猜到，DataMonitor 是调用这些方法的对象，以响应 ZooKeeper 状态的变化。
以下是 Executor 对DataMonitorListener.exists()and的实现DataMonitorListener.closing：
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();
}
}
数据监视器类
DataMonitor 类具有 ZooKeeper 逻辑的核心。它主要是异步和事件驱动的。DataMonitor 在构造函数中启动：
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
检查 znode 是否存在的调用ZooKeeper.exists()，设置监视，并将对自身 ( this) 的引用作为完成回调对象传递。从这个意义上说，它开始了事情，因为真正的处理发生在手表被触发时。
笔记
不要将完成回调与监视回调混淆。
ZooKeeper.exists()完成回调，恰好是 DataMonitor 对象中实现的方法，当监视操作 (by ) 的异步设置在服务器上完成StatCallback.processResult()时被调用。ZooKeeper.exists()另一方面，watch 的触发会向Executor对象发送一个事件，因为 Executor 注册为 ZooKeeper 对象的 Watcher。
顺便说一句，您可能会注意到 DataMonitor 还可以将自己注册为此特定监视事件的监视程序。这是 ZooKeeper 3.0.0 的新功能（支持多个观察者）。但是，在此示例中，DataMonitor 并未注册为 Watcher。
当ZooKeeper.exists()服务器上的操作完成时，ZooKeeper API 在客户端调用这个完成回调：
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);</emphasis>
prevData = b;
}
}
该代码首先检查 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，则从znode获取数据，如果状态发生变化则调用Executor的exists()回调。注意，它不必为 getData 调用做任何异常处理，因为它有任何可能导致错误的等待处理：如果节点在调用之前被删除ZooKeeper.getData()，则由设置的监视事件ZooKeeper.exists()触发回调；如果存在通信错误，则在连接恢复时触发连接监视事件。
最后，注意 DataMonitor 如何处理监视事件：
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:
// 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);
}
}
如果客户端 ZooKeeper 库可以在会话到期（Expired 事件）之前重新建立到 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动物园管理员 3.0.0)。有关这方面的更多信息，请参阅程序员指南中的ZooKeeper Watches。在这个函数中稍微低一点，当 DataMonitor 得到一个 znode 的事件时，它会调用ZooKeeper.exists()以找出发生了什么变化。
完整的源清单
执行器.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();
}
}
}
}
数据监视器.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;
}
}
}