Zookeeper（58）如何在Zookeeper中实现分布式锁？

在 Zookeeper 中实现分布式锁是一种常见的用例。Zookeeper 提供了强一致性、高可用性的分布式协调服务，使得它非常适合用来实现分布式锁。以下是详细的步骤和代码示例，展示如何在 Zookeeper 中实现分布式锁。

1. Zookeeper 分布式锁的基本原理

Zookeeper 分布式锁的实现主要依赖于以下几点：

临时顺序节点：每个客户端在尝试获取锁时，会在特定节点下创建一个临时顺序节点。节点排序：所有客户端创建的节点会按顺序排列，最小的节点获得锁。监视前一个节点：每个客户端监视它前一个节点的变化，当前一个节点被删除时，当前客户端重新检查自己是否是最小的节点，如果是，则获得锁。 2. 添加依赖

首先，需要在项目中添加 Zookeeper 的依赖。以下是 Maven 和 Gradle 的依赖配置：

Maven

在 pom.xml 中添加 Zookeeper 依赖：

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

在 build.gradle 中添加 Zookeeper 依赖：

dependencies { implementation 'org.apache.zookeeper:zookeeper:3.7.0' } 3. 实现分布式锁

以下是一个完整的实现分布式锁的示例代码。

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() { @Override public 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(); // 在这里可以测试分布式锁 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 pareTo(lockPath) < 0) { watchNode = child; break; } } if (watchNode != null) { final Object lock = new Object(); Watcher watcher = new Watcher() { @Override public 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(); // 模拟业务逻辑 Thread.sleep(5000); lock.releaseLock(); client.close(); } catch (Exception e) { e.printStackTrace(); } } } 4. 代码说明 ZookeeperClient 类

ZookeeperClient 类负责连接和关闭 Zookeeper 客户端，并提供获取 ZooKeeper 实例的方法。

DistributedLock 类

DistributedLock 类负责实现分布式锁的逻辑，包括获取锁和释放锁。

acquireLock() 方法：尝试获取锁，创建临时顺序节点，然后检查自己是否是最小的节点。如果是，则获得锁；否则，监视前一个节点的变化。releaseLock() 方法：释放锁，删除自己创建的临时顺序节点。 5. 测试分布式锁

在 DistributedLock 类的 main 方法中，创建 ZookeeperClient 实例并连接 Zookeeper，然后创建 DistributedLock 实例并尝试获取和释放锁。可以通过运行多个实例来测试分布式锁的功能。

public static void main(String[] args) { ZookeeperClient client = new ZookeeperClient(); try { client.connect(); ZooKeeper zooKeeper = client.getZooKeeper(); DistributedLock lock = new DistributedLock(zooKeeper); lock.acquireLock(); // 模拟业务逻辑 Thread.sleep(5000); lock.releaseLock(); client.close(); } catch (Exception e) { e.printStackTrace(); } } 总结 添加依赖：在项目中添加 Zookeeper 的依赖。实现 ZookeeperClient 类：负责连接和关闭 Zookeeper 客户端，并提供获取 ZooKeeper 实例的方法。实现 DistributedLock 类：负责实现分布式锁的逻辑，包括获取锁和释放锁。测试分布式锁：通过运行多个实例来测试分布式锁的功能。

通过以上方法，可以在 Zookeeper 中实现分布式锁，确保其高效稳定地运行。根据实际情况和需求，选择适合你的实现方法并进行实施。