ZooKeeper Java示例
一个简单的监视客户端
为了向您介绍ZooKeeper Java API，我们在此开发了一个非常简单的监视客户端。这个ZooKeeper客户端会监视一个znode的变化，并通过启动或停止程序来响应变化。
需求
客户端有四个需求：
- 参数包括：
- ZooKeeper服务的地址
- 要监视的znode名称
- 输出写入的文件名
- 一个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启可执行文件。
- 如果znode消失，客户端将终止可执行程序。
程序设计
按照惯例，ZooKeeper应用程序被分为两个单元：一个负责维护连接，另一个负责监控数据。在本应用中，名为Executor的类负责维护ZooKeeper连接，而名为DataMonitor的类则监控ZooKeeper树中的数据。此外，Executor包含主线程和执行逻辑，负责处理少量的用户交互，以及根据znode的状态与你传入作为参数的可执行程序进行交互（根据需求示例会关闭并重启该程序）。
执行器类
Executor对象是示例应用程序的主要容器。它包含上述程序设计中描述的ZooKeeper对象和DataMonitor。
// 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的任务是启动和停止你在命令行中传入的可执行文件。它通过响应ZooKeeper对象触发的事件来完成这一操作。正如你在上面的代码中所看到的，Executor在ZooKeeper构造函数中将自身引用作为Watcher参数传入。同时它也将自身引用作为DataMonitorListener参数传递给DataMonitor构造函数。根据Executor的定义，它同时实现了这两个接口：
public class Executor implements Watcher, Runnable, DataMonitor.DataMonitorListener {
...
Watcher 接口由 ZooKeeper Java API 定义。ZooKeeper 通过该接口与容器进行通信。它仅支持一个方法 process()，ZooKeeper 使用该方法传递主线程可能感兴趣的通用事件，例如 ZooKeeper 连接状态或 ZooKeeper 会话状态。本示例中的 Executor 只是简单地将这些事件转发给 DataMonitor 来决定如何处理。这样做只是为了说明一个惯例：通常由 Executor 或类似 Executor 的对象"拥有" ZooKeeper 连接，但可以自由地将事件委托给其他对象。同时它也将其作为触发监视事件的默认通道。（后续会有更多说明。）
public void process(WatchedEvent event) {
dm.process(event);
}
另一方面，DataMonitorListener接口并非ZooKeeper API的组成部分。这是一个完全自定义的接口，专为此示例应用程序设计。DataMonitor对象通过该接口与其容器（同时也是Executor对象）进行通信。DataMonitorListener接口定义如下：
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* The ZooKeeper session is no longer valid.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);
}
该接口定义在DataMonitor类中，并在Executor类中实现。当调用Executor.exists()时，Executor会根据需求决定是启动还是关闭。回想一下需求说明：当znode节点不再存在时，需要终止可执行程序。
当调用Executor.closing()时，Executor会决定是否在ZooKeeper连接永久消失时自行关闭。
正如你可能猜到的，DataMonitor是响应ZooKeeper状态变化而调用这些方法的对象。
以下是Executor对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类包含了ZooKeeper逻辑的核心部分。它主要是异步和事件驱动的。DataMonitor在构造函数中通过以下方式启动流程：
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
调用ZooKeeper.exists()会检查znode是否存在，设置一个监视器，并将自身引用(this)作为完成回调对象传入。从这个意义上说，它启动了整个过程，因为真正的处理发生在监视器被触发时。
注意
不要将完成回调与监视回调混淆。
ZooKeeper.exists()的完成回调（恰好是DataMonitor对象中实现的StatCallback.processResult()方法）会在服务器上异步设置监视操作（通过ZooKeeper.exists()）完成时被调用。另一方面，监视器的触发会向Executor对象发送一个事件，因为Executor被注册为ZooKeeper对象的Watcher。
顺便提一下，您可能注意到DataMonitor也可以将自己注册为该特定监视事件的Watcher。这是ZooKeeper 3.0.0新增的功能（支持多个Watcher）。不过在本示例中，DataMonitor并未注册为Watcher。
当服务器上的ZooKeeper.exists()操作完成时，ZooKeeper API会在客户端调用此完成回调：
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);</emphasis>
prevData = b;
}
}
代码首先检查znode是否存在、致命错误和可恢复错误的错误代码。如果文件（或znode）存在，它会从znode获取数据，然后在状态发生变化时调用Executor的exists()回调。注意，它不需要为getData调用做任何异常处理，因为它已经为任何可能导致错误的情况设置了监视：如果在调用ZooKeeper.getData()之前节点被删除，由ZooKeeper.exists()设置的监视事件会触发回调；如果出现通信错误，当连接恢复时会触发连接监视事件。
最后，请注意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库能在会话过期(Expired事件)之前重新建立与ZooKeeper的通信通道(SyncConnected事件)，那么该会话的所有监视器都将自动在服务器端重新建立(监视器自动重置是ZooKeeper 3.0.0的新特性)。更多详情请参阅程序员指南中的ZooKeeper Watches。在本函数稍后部分，当DataMonitor接收到znode事件时，它会调用ZooKeeper.exists()来查明发生了什么变化。
完整源代码清单
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;
}
}
}