ZooKeeper Java Example
原链接http://zookeeper.apache.org/doc/trunk/javaExample.html
一个简单的监控客户端
为了向你介绍zookeeper的Java API,在这里我们开发了一个简单的监控客户端。zookeeper客户端通过监控一个zookeeper节点的状态改变来使一个程序的启动和关闭。必要条件
客户端需要满足以下四点
接受4个参数
zookeeper服务的地址
znode的名字-需要监控的那个节点
写输出的文件名
一个带参数的可执行程序
Program Design
一般的，zookeeper被分成两个模块，一个维持连接，另一个监控数据。在这个程序中，维持连接的类叫Executor，DataMonitor监控zookeeper树上的数据。同时，Executor包含主线程和执行逻辑。他事无巨细的负责用户交互。也负责你使用参数传入的可执行程序，还有根据znode的状态来关闭和重启样例程序。
Executor类
Executor对象是样例程序的原始容器。他包含zookeeper对象，DataMonitor，就像在程序设计中所描述的那样。
// 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) {
}
}回顾一下，Executor的工作是用来启动和关闭那个可执行程序（你使用命令行参数传入名称的那个）。它通过响应zookeeper对象激发的事件来完成这些工作。正像以上你看到的代码，Executor向zookeeper构造器传递了一个自身的引用作为监视器的参数。它同时也向DataMonitor构造器传递了一个自身的引用作为数据监视器的参数。每一个Executor的定义，都实现了所有这些接口。
Watcher接口定义在zookeeper Java API里。zookeeper使用它和它的拥有者(原文是container，作者本意应该就是持有Watcher接口的容器，也即实现Watcher接口的类)通讯。它只支持一个方法，process()，ZooKeeper 使用它来交互一些主线程感兴趣的一般事件，例如zookeeper连接状态或zookeeper会话。在这个例子中Executor只是简单将这些事件转发到DataMonitor并由DataMonitor决定怎么处理他们。它做这些只是为了演示一点，按惯例，Executor或一些类似Executor的对象拥有zookeeper连接，但它可以将事件委托给其他对象。
public void process(WatchedEvent event) {
dm.process(event);
}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* The ZooKeeper session is no longer valid.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);
}这个接口在DataMonitor类中定义，实现在Executor类中。当Executor.exists()方法被调用的时候，Executor根据需要决定是否启动或关闭。回想必要条件中所述当znode不存在时杀死那个可执行程序。
当Executor.closing()被调用的时候，Executor决定是否关闭自己来响应zookeeper连接的永久消失。
正如你所猜的，DataMonitor是一个调用这些方法的对象，来响应zookeeper状态的改变。
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类拥有zookeeper逻辑的内容。它经常是异步的和事件驱动。DataMonitor kicks things off in the constructor with。
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);
}注意
不要把完成回调和监控器回调弄混。ZooKeeper.exists()完成回调，它发生在实现在DataMonitor的方法StatCallback.processResult()，被调用当异步的设置监视器操作（由ZooKeeper.exists()）由server完成时。
监视器的触发，在另一方面，向Executor发送一个事件，只要Executor向zookeeper对象注册一个监视器对象。
作为一个旁白，你可能注意到DataMonitor也可以将自己注册为监视器为了一个特别的监视事件。这是zooke3.0.0（支持多个触发器）的新特性；在这个例子中，然而，DataMonitor没有注册为一个监视器。
当ZooKeeper.exists()操作在服务器上完成的时候，ZooKeeper API调用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);
prevData = b;
}
}最后，注意DataMonitor怎么处理监控事件。
如果在会话失效以前客户端zookeeper库能够和ZooKeeper建立连接通道。则所有的会话监控器会与服务自动重新建立（自动重置监控器是zookeeper3.0.0里的一个新特性）。欲了解更多请在开发者向导中查看zookeeper 监控器一节。在这个函数的底部,当DataMonitor想要获取一个节点的事件，它调用ZooKeeper.exists()来查找发生了什么。
源代码列表
Executor.java
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);
}
}
完整代码列表
/**
* 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;
}
}
}