上一篇文章已经完成了ZooKeeper的基本搭建和使用的介绍，现在开始用代码说话。参考 https://zookeeper.apache.org/doc/current/javaExample.html ，但对场景和代码都做了简化，只实现基本的Watcher功能。
1 场景设计
目的是体验ZooKeeper的Watcher功能。程序监控ZooKeeper的/watcher节点数据变化，当创建或修改数据时，控制台打印当前的数据内容和版本号；当/watcher被删除时，程序退出。
/watcher的创建、修改和删除操作，使用控制台或zkui操作。
2 搭建Maven项目
代码相对比较简单，就不用SpringBoot这个大杀器了，使用一个普通的Maven项目即可。
ZooKeeper客户端使用官方提供的Java库，org.apache.zookeeper: zookeeper: 3.5.5。日志框架使用习惯的slf4j+log4j2，ZooKeeper缺省使用的是log4j V1，因此在引入的时候需要excludes。另外，使用了lombok来简化一些代码。
以下是pom.xml的内容
| xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> |
3 log4j2.xml
在项目的 src/main/resources 下创建一个文件 log4j2.xml，内容为
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4 创建ZooKeeper连接
创建连接代码比较简单，只需要创建 ZooKeeper对象就行，
ZooKeeper构造函数的定义
| /** * 创建一个 ZooKeeper 客户端对象 * @param connectString 逗号分隔的 host:port 字符串， * 单点如 127.0.0.1:2181， * 集群如 192.168.5.128:2181,192.168.5.129:2181,192.168.5.130:2181， * 还可以指定根节点，如 127.0.0.1:2181/foo/bar * @param sessionTimeout 毫秒为单位的超时时间 * @param watcher Watcher对象，用于接收 MatcherEvent * @throws IOException 网络错误时抛出异常 * @throws IllegalArgumentException 如果Root路径设置错误 */ public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher) throws IOException; |
写一段测试代码，创建zk对象后判断某一个zNode是否存在。
| public class ZooKeeperWatcher implements Watcher { /** ZooKeeper的客户端连接 */ ZooKeeper zk;
public ZooKeeperWatcher(String hostPort, String zNode) throws KeeperException, IOException { zk = new ZooKeeper(hostPort, 3000, this); try { Stat exists = zk.exists(zNode, true); if(exists == null){ log.info(“{} 不存在”, zNode) } } catch (InterruptedException e) { log.error("InterruptedException", e); } } } |
运行这段代码，发现会抛异常
| java.net.SocketException: Socket is not connected at sun.nio.ch.Net.translateToSocketException(Net.java:162) ~[?:?] at sun.nio.ch.Net.translateException(Net.java:196) ~[?:?] at sun.nio.ch.Net.translateException(Net.java:202) ~[?:?] at sun.nio.ch.SocketAdaptor.shutdownInput(SocketAdaptor.java:400) ~[?:?] at org.apache.zookeeper.ClientCnxnSocketNIO.cleanup(ClientCnxnSocketNIO.java:198) [zookeeper-3.5.5.jar:3.5.5] at org.apache.zookeeper.ClientCnxn$SendThread.cleanup(ClientCnxn.java:1338) [zookeeper-3.5.5.jar:3.5.5] at org.apache.zookeeper.ClientCnxn$SendThread.cleanAndNotifyState(ClientCnxn.java:1276) [zookeeper-3.5.5.jar:3.5.5] at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1254) [zookeeper-3.5.5.jar:3.5.5] Caused by: java.nio.channels.NotYetConnectedException at sun.nio.ch.SocketChannelImpl.shutdownInput(SocketChannelImpl.java:917) ~[?:?] at sun.nio.ch.SocketAdaptor.shutdownInput(SocketAdaptor.java:398) ~[?:?] ... 4 more |
NotYetConnectedException的字面意思是连接还没有创建好，网络搜索了一下，建立连接需要一些时间，创建zk对象后马上调用exists命令，这时候连接还没有创建好，就会抛异常。ZooKeeper在连接建立成功之后，会发送一个WatchedEvent事件，我们可以利用这个事件完成建立连接的过程。修改后的代码如下，顺便添加了slf4j-ext中的Profiler，用于记录所消耗的时间。
| public class ZooKeeperWatcher implements Watcher { /** 等待连接建立成功的信号 */ private CountDownLatch connectedSemaphore = new CountDownLatch(1);
/** ZooKeeper的客户端连接 */ ZooKeeper zk;
public ZooKeeperWatcher(String hostPort, String zNode) throws KeeperException, IOException { Profiler profiler = new Profiler("连接到ZooKeeper"); profiler.start("开始链接"); zk = new ZooKeeper(hostPort, 3000, this); try { profiler.start("等待连接成功的Event"); connectedSemaphore.await(); Stat exists = zk.exists(zNode, true); if(exists == null){ log.info(“{} 不存在”, zNode) } } catch (InterruptedException e) { log.error("InterruptedException", e); } profiler.stop(); profiler.setLogger(log); profiler.log(); }
/** 收到ZooKeeper的WatchedEvent */ @Override public void process(WatchedEvent event) { log.info("event = {}", event); if (Event.EventType.None.equals(event.getType())) { // 连接状态发生变化 if (Event.KeeperState.SyncConnected.equals(event.getState())) { // 连接建立成功 connectedSemaphore.countDown(); } } } } |
修改代码之后的执行记录日志如下，可以看到等待连接成功的Event耗时9秒多。网络上有文章说关闭防火墙可以秒连，但我测试过，没发现有什么变化，使用systemctl stop firewalld之后重新执行程序，仍然需要9秒多。
| [INFO] - ZooKeeperWatcher.process(61) - event = WatchedEvent state:SyncConnected type:None path:null [DEBUG] - ZooKeeperWatcher.log(201) - + Profiler [连接到ZooKeeper] |-- elapsed time [开始链接] 78.912 milliseconds. |-- elapsed time [等待连接成功的Event] 9330.606 milliseconds. |-- Total [连接到ZooKeeper] 9409.926 milliseconds.
[INFO] - ZooKeeperWatcher.readNodeData(95) - /watcher 不存在 |
5 读取WatchedEvent
前面的代码，只是处理了建立连接成功时的Event，下面再来看看读取数据的过程。关键代码如下：
| if (Event.EventType.NodeDataChanged.equals(event.getType()) || Event.EventType.NodeCreated.equals(event.getType())) { String path = event.getPath(); if (path != null && path.equals(zNode)) { // 节点数据被修改 readNodeData(); } }
/** 读节点数据 */ private void readNodeData() { try { Stat stat = new Stat(); byte[] data = zk.getData(zNode, true, stat); if (data != null) { log.info("{}, value={}, version={}", zNode, new String(data), stat.getVersion()); } } catch (KeeperException e) { log.info("{} 不存在", zNode); } catch (InterruptedException e) { log.error("InterruptedException", e); } } |
当接收到创建节点和修改节点的WatchedEvent，都会将数据读出并打印在控制台。
6 调整后的完整程序清单
对前面的代码做了部分调整，同时添加了退出系统的机制：节点被删除。
| package tech.codestory.zookeeper.watcher;
import java.io.IOException; import java.util.concurrent.CountDownLatch; import org.apache.zookeeper.*; import org.apache.zookeeper.data.Stat; import org.slf4j.profiler.Profiler; import lombok.extern.slf4j.Slf4j;
/** * 用于测试 ZooKeeper的 WatchedEvent用法 * @author code story * @date 2019/8/13 */ @Slf4j public class ZooKeeperWatcher implements Watcher, Runnable { /** 等待连接建立成功的信号 */ private CountDownLatch connectedSemaphore = new CountDownLatch(1); /** 退出系统的信号 */ static Integer quitSemaphore = Integer.valueOf(-1);
String zNode; ZooKeeper zk;
public ZooKeeperWatcher(String hostPort, String zNode) throws KeeperException, IOException { this.zNode = zNode;
Profiler profiler = new Profiler("连接到ZooKeeper"); profiler.start("开始链接"); zk = new ZooKeeper(hostPort, 3000, this); try { profiler.start("等待连接成功的Event"); connectedSemaphore.await(); } catch (InterruptedException e) { log.error("InterruptedException", e); } profiler.stop(); profiler.setLogger(log); profiler.log();
// 先读当前的数据 readNodeData(); }
/** 收到ZooKeeper的WatchedEvent */ @Override public void process(WatchedEvent event) { log.info("event = {}", event); if (Event.EventType.None.equals(event.getType())) { // 连接状态发生变化 if (Event.KeeperState.SyncConnected.equals(event.getState())) { // 连接建立成功 connectedSemaphore.countDown(); } } else if (Event.EventType.NodeDataChanged.equals(event.getType()) || Event.EventType.NodeCreated.equals(event.getType())) { String path = event.getPath(); if (path != null && path.equals(zNode)) { // 节点数据被修改 readNodeData(); } } else if (Event.EventType.NodeDeleted.equals(event.getType())) { String path = event.getPath(); if (path != null && path.equals(zNode)) { synchronized (quitSemaphore) { // 节点被删除，通知退出线程 quitSemaphore.notify(); } } } }
/** 读节点数据 */ private void readNodeData() { try { Stat stat = new Stat(); byte[] data = zk.getData(zNode, true, stat); if (data != null) { log.info("{}, value={}, version={}", zNode, new String(data), stat.getVersion()); } } catch (KeeperException e) { log.info("{} 不存在", zNode); try { // 目的是添加Watcher zk.exists(zNode, true); } catch (KeeperException ex) { } catch (InterruptedException ex) { } } catch (InterruptedException e) { log.error("InterruptedException", e); } }
@Override public void run() { try { synchronized (quitSemaphore) { quitSemaphore.wait(); log.info("{} 被删除，退出", zNode); } } catch (InterruptedException e) { log.error("InterruptedException", e); } }
/** 主程序，代码中写死了Server地址和zNode名，也可以改成从args中读取 */ public static void main(String[] args) { String hostPort = "192.168.5.128:2181"; String zNode = "/watcher"; try { new ZooKeeperWatcher(hostPort, zNode).run(); } catch (Exception e) { log.error("new ZooKeeperExecutor()", e); } } } |
做一个测试，应用启动后创建节点，修改多次节点，最后删除节点，日志输出如下：
| 10:13:31:979 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:None path:null 10:13:31:982 [DEBUG] - ZooKeeperWatcher.log(201) - + Profiler [连接到ZooKeeper] |-- elapsed time [开始链接] 210.193 milliseconds. |-- elapsed time [等待连接成功的Event] 9385.467 milliseconds. |-- Total [连接到ZooKeeper] 9596.196 milliseconds.
10:13:31:996 [INFO] - ZooKeeperWatcher.readNodeData(84) - /watcher 不存在 10:15:43:451 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeCreated path:/watcher 10:15:43:463 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 00, version=0 10:15:50:906 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:15:50:910 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 01, version=1 10:15:56:004 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:15:56:007 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 02, version=2 10:16:00:246 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:16:00:249 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 03, version=3 10:16:06:399 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:16:06:402 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 10, version=4 10:16:10:217 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:16:10:220 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 11, version=5 10:16:14:444 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDataChanged path:/watcher 10:16:14:447 [INFO] - ZooKeeperWatcher.readNodeData(81) - /watcher, value=hello zk 12, version=6 10:16:20:118 [INFO] - ZooKeeperWatcher.process(50) - event = WatchedEvent state:SyncConnected type:NodeDeleted path:/watcher 10:16:20:118 [INFO] - ZooKeeperWatcher.run(101) - /watcher 被删除，退出 |