在 Zookeeper 中實現分布式鎖是一種常見的用例。Zookeeper 提供了強一致性、高可用性的分布式協調服務，使得它非常適合用來實現分布式鎖。以下是詳細的步驟和代碼示例，展示如何在 Zookeeper 中實現分布式鎖。

1. Zookeeper 分布式鎖的基本原理

Zookeeper 分布式鎖的實現主要依賴于以下幾點：

1. 臨時順序節點：每個客戶端在嘗試獲取鎖時，會在特定節點下創建一個臨時順序節點。
2. 節點排序：所有客戶端創建的節點會按順序排列，最小的節點獲得鎖。
3. 監視前一個節點：每個客戶端監視它前一個節點的變化，當前一個節點被刪除時，當前客戶端重新檢查自己是否是最小的節點，如果是，則獲得鎖。

2. 添加依賴

首先，需要在項目中添加 Zookeeper 的依賴。以下是 Maven 和 Gradle 的依賴配置：

Maven

在 pom.xml 中添加 Zookeeper 依賴：

<dependency><groupId>org.apache.zookeeper</groupId><artifactId>zookeeper</artifactId><version>3.7.0</version>
</dependency>

Gradle

在 build.gradle 中添加 Zookeeper 依賴：

dependencies {implementation 'org.apache.zookeeper:zookeeper:3.7.0'
}

3. 實現分布式鎖

以下是一個完整的實現分布式鎖的示例代碼。

ZookeeperClient.java

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;public class ZookeeperClient {private static final String ZK_ADDRESS = "localhost:2181";private static final int SESSION_TIMEOUT = 3000;private ZooKeeper zooKeeper;public void connect() throws Exception {zooKeeper = new ZooKeeper(ZK_ADDRESS, SESSION_TIMEOUT, new Watcher() {@Overridepublic void process(WatchedEvent event) {System.out.println("Received event: " + event);}});}public void close() throws InterruptedException {if (zooKeeper != null) {zooKeeper.close();}}public ZooKeeper getZooKeeper() {return zooKeeper;}public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();// 在這里可以測試分布式鎖client.close();} catch (Exception e) {e.printStackTrace();}}
}

DistributedLock.java

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;import java.util.Collections;
import java.util.List;public class DistributedLock {private static final String LOCK_ROOT = "/locks";private static final String LOCK_NODE = LOCK_ROOT + "/lock_";private ZooKeeper zooKeeper;private String lockPath;public DistributedLock(ZooKeeper zooKeeper) throws Exception {this.zooKeeper = zooKeeper;Stat stat = zooKeeper.exists(LOCK_ROOT, false);if (stat == null) {zooKeeper.create(LOCK_ROOT, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);}}public void acquireLock() throws Exception {lockPath = zooKeeper.create(LOCK_NODE, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);System.out.println("Lock path: " + lockPath);while (true) {List<String> children = zooKeeper.getChildren(LOCK_ROOT, false);Collections.sort(children);String smallestChild = LOCK_ROOT + "/" + children.get(0);if (lockPath.equals(smallestChild)) {System.out.println("Acquired lock: " + lockPath);return;}String watchNode = null;for (int i = children.size() - 1; i >= 0; i--) {String child = LOCK_ROOT + "/" + children.get(i);if (child.compareTo(lockPath) < 0) {watchNode = child;break;}}if (watchNode != null) {final Object lock = new Object();Watcher watcher = new Watcher() {@Overridepublic void process(WatchedEvent event) {synchronized (lock) {lock.notifyAll();}}};Stat stat = zooKeeper.exists(watchNode, watcher);if (stat != null) {synchronized (lock) {lock.wait();}}}}}public void releaseLock() throws Exception {if (lockPath != null) {zooKeeper.delete(lockPath, -1);System.out.println("Released lock: " + lockPath);lockPath = null;}}public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();ZooKeeper zooKeeper = client.getZooKeeper();DistributedLock lock = new DistributedLock(zooKeeper);lock.acquireLock();// 模擬業務邏輯Thread.sleep(5000);lock.releaseLock();client.close();} catch (Exception e) {e.printStackTrace();}}
}

4. 代碼說明

ZookeeperClient 類

ZookeeperClient 類負責連接和關閉 Zookeeper 客戶端，并提供獲取 ZooKeeper 實例的方法。

DistributedLock 類

DistributedLock 類負責實現分布式鎖的邏輯，包括獲取鎖和釋放鎖。

• acquireLock() 方法：嘗試獲取鎖，創建臨時順序節點，然后檢查自己是否是最小的節點。如果是，則獲得鎖；否則，監視前一個節點的變化。
• releaseLock() 方法：釋放鎖，刪除自己創建的臨時順序節點。

5. 測試分布式鎖

在 DistributedLock 類的 main 方法中，創建 ZookeeperClient 實例并連接 Zookeeper，然后創建 DistributedLock 實例并嘗試獲取和釋放鎖。可以通過運行多個實例來測試分布式鎖的功能。

public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();ZooKeeper zooKeeper = client.getZooKeeper();DistributedLock lock = new DistributedLock(zooKeeper);lock.acquireLock();// 模擬業務邏輯Thread.sleep(5000);lock.releaseLock();client.close();} catch (Exception e) {e.printStackTrace();}
}

總結

1. 添加依賴：在項目中添加 Zookeeper 的依賴。
2. 實現 ZookeeperClient 類：負責連接和關閉 Zookeeper 客戶端，并提供獲取 ZooKeeper 實例的方法。
3. 實現 DistributedLock 類：負責實現分布式鎖的邏輯，包括獲取鎖和釋放鎖。
4. 測試分布式鎖：通過運行多個實例來測試分布式鎖的功能。

通過以上方法，可以在 Zookeeper 中實現分布式鎖，確保其高效穩定地運行。根據實際情況和需求，選擇適合你的實現方法并進行實施。