Java实现ZooKeeper的zNode监控

当我们使用ZooKeeper作为分布式协调框架时，监视zNode的变化是很常见的任务，因为zNode的变化往往意味着某些与服务相关的状态变化。本文将详细讲解如何使用Java实现ZooKeeper的zNode监视。

步骤一：导入ZooKeeper依赖

首先，在项目的pom.xml文件中添加以下ZooKeeper依赖：

<dependency>
    <groupId>org.apache.zookeeper</groupId>
    <artifactId>zookeeper</artifactId>
    <version>3.5.3-beta</version>
</dependency>

步骤二：编写监控代码

接下来，我们来编写Java代码实现zNode的监控。以下是完整的Java实现代码：

import org.apache.zookeeper.*;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;

public class ZookeeperWatcher implements Watcher {

    private static final Logger LOGGER = LoggerFactory.getLogger(ZookeeperWatcher.class);
    private static final String ZOOKEEPER_HOST = "localhost:2181";
    private static final int SESSION_TIMEOUT = 5000;

    private ZooKeeper zooKeeper;
    private CountDownLatch connectedSignal = new CountDownLatch(1);

    @Override
    public void process(WatchedEvent event) {
        if (event.getState() == KeeperState.SyncConnected) {
            connectedSignal.countDown();
        } else if (event.getType() == EventType.NodeChildrenChanged) {
            LOGGER.info("Node '"+ event.getPath() +"' children changed.");
            try {
                List<String> children = zooKeeper.getChildren(event.getPath(), this);
                LOGGER.info("Node '"+ event.getPath() +"' has children: "+ children);
            } catch (KeeperException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private ZooKeeper connect() throws IOException, InterruptedException {
        zooKeeper = new ZooKeeper(ZOOKEEPER_HOST, SESSION_TIMEOUT, this);
        connectedSignal.await();
        return zooKeeper;
    }

    public void close() throws InterruptedException {
        zooKeeper.close();
    }

    public static void main(String[] args) throws InterruptedException, IOException {
        ZookeeperWatcher watcher = new ZookeeperWatcher();
        watcher.connect();
        List<String> children = watcher.zooKeeper.getChildren("/test", watcher);
        LOGGER.info("Node '/test' has children: "+ children);
        Thread.sleep(Integer.MAX_VALUE);
    }

}

代码解析：

• 我们定义了一个ZookeeperWatcher类来实现监控功能，继承了Watcher接口，实现了process()方法来处理事件。
• 在process()方法中，首先判断事件的状态是否为SyncConnected，如果是则通过connectedSignal来发出一个信号，表示客户端与ZooKeeper服务器已建立连接。
• 如果是节点变化事件类型且节点的子节点发生了变化，则通过节点路径获取其子节点列表，并打印出来。

示例一：监控zNode的子节点变化

我们在以下示例中，将/test路径下的zNode作为示例节点，监控其子节点的变化。如果有新的子节点添加到该节点下，我们将立即得到通知。

public static void main(String[] args) throws InterruptedException, IOException {
    ZookeeperWatcher watcher = new ZookeeperWatcher();
    watcher.connect();
    List<String> children = watcher.zooKeeper.getChildren("/test", watcher);
    LOGGER.info("Node '/test' has children: "+ children);
    Thread.sleep(Integer.MAX_VALUE);
}

运行代码后，我们可以按以下步骤来模拟节点变化：

1. 在ZooKeeper的console中用以下命令创建/test节点：

create /test mydata

1. 用以下命令创建/test/node1子节点：

create /test/node1 mydata

1. 然后你将在ZookeeperWatcher类中看到如下输出：

Node '/test' has children: [node1]
Node '/test' children changed.
Node '/test' has children: [node1, node2]

示例二：监控zNode的数据变化

除了监控zNode的子节点变化，我们还可以监视指定zNode的数据变化。以下示例代码演示了如何监视/test节点的数据变化：

public class DataWatcher implements Watcher {

    private static final Logger LOGGER = LoggerFactory.getLogger(DataWatcher.class);
    private static final String ZOOKEEPER_HOST = "localhost:2181";
    private static final int SESSION_TIMEOUT = 5000;

    private ZooKeeper zooKeeper;
    private CountDownLatch connectedSignal = new CountDownLatch(1);
    private String nodePath;

    public DataWatcher(String nodePath) {
        this.nodePath = nodePath;
    }

    @Override
    public void process(WatchedEvent event) {
        if(event.getType() == EventType.NodeDataChanged) {
            LOGGER.info("Node '"+ event.getPath() +"' data changed.");
            try {
                byte[] data = zooKeeper.getData(nodePath, this, null);
                LOGGER.info("Node '"+ nodePath +"' data is: "+ new String(data));
            } catch (KeeperException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private ZooKeeper connect() throws IOException, InterruptedException {
        zooKeeper = new ZooKeeper(ZOOKEEPER_HOST, SESSION_TIMEOUT, this);
        connectedSignal.await();
        return zooKeeper;
    }

    public void close() throws InterruptedException {
        zooKeeper.close();
    }

    public static void main(String[] args) throws InterruptedException, IOException, KeeperException {
        DataWatcher dataWatcher = new DataWatcher("/test");
        ZooKeeper zooKeeper = dataWatcher.connect();
        byte[] data = zooKeeper.getData("/test", dataWatcher, null);
        LOGGER.info("Node '/test' has data: "+ new String(data));
        Thread.sleep(Integer.MAX_VALUE);
    }

}

运行示例代码后，我们可以按以下步骤来模拟节点数据的变化：

1. 在ZooKeeper的console中用以下命令创建/test节点：

create /test mydata

1. 用以下命令修改/test节点的数据：

set /test mynewdata

1. 然后你将在DataWatcher类中看到如下输出：

Node '/test' has data: mydata
Node '/test' data changed.
Node '/test' data is: mynewdata

至此，Java实现ZooKeeper的zNode监控已经完成。

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