目录

引言

1. ZooKeeper简介

2. 分布式锁实现原理

3. 分布式锁实现步骤

步骤一：创建ZooKeeper客户端

步骤二：创建分布式锁类

步骤三：使用分布式锁

4. 总结

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引言

在分布式系统中，实现分布式锁是一项常见的任务，可以用于保证同一时间只有一个客户端可以访问共享资源，从而避免竞争条件。ZooKeeper是一个开源的分布式协调服务，可以用来实现分布式锁。本文将介绍如何使用ZooKeeper实现分布式锁，并给出相应的代码示例。

1. ZooKeeper简介

ZooKeeper是一个高性能的分布式协调服务，提供了诸如配置管理、命名服务、分布式锁等功能。ZooKeeper通过维护一个具有层次结构的数据结构（类似于文件系统），来管理分布式应用程序的状态。

2. 分布式锁实现原理

在ZooKeeper中实现分布式锁的基本原理是利用ZooKeeper的顺序节点（Sequential Node）和临时节点（Ephemeral Node）特性。

1. 客户端尝试在ZooKeeper中创建一个带有指定路径的临时顺序节点，例如/locks/lock-000000001。
2. 客户端获取/locks节点下的所有子节点，并按节点名称的顺序排序。
3. 客户端判断自己创建的节点是否为最小节点，如果是，则认为获取锁成功；否则，监听自己前一个节点的删除事件，并进入等待状态。
4. 当前最小节点的客户端完成操作后，删除自己创建的节点，触发监听的客户端继续判断是否为最小节点，直到获取锁成功。

3. 分布式锁实现步骤

步骤一：创建ZooKeeper客户端

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;import java.io.IOException;
import java.util.concurrent.CountDownLatch;public class ZooKeeperClient {private static final String CONNECT_STRING = "localhost:2181";private static final int SESSION_TIMEOUT = 5000;private static ZooKeeper zooKeeper;public static ZooKeeper getZooKeeper() throws IOException, InterruptedException {final CountDownLatch connectedSignal = new CountDownLatch(1);zooKeeper = new ZooKeeper(CONNECT_STRING, SESSION_TIMEOUT, new Watcher() {public void process(WatchedEvent event) {if (event.getState() == Watcher.Event.KeeperState.SyncConnected) {connectedSignal.countDown();}}});connectedSignal.await();return zooKeeper;}public static void close() throws InterruptedException {if (zooKeeper != null) {zooKeeper.close();}}
}

步骤二：创建分布式锁类

import org.apache.zookeeper.*;import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;public class DistributedLock {private final ZooKeeper zooKeeper;private final String lockPath;private String currentLockPath;public DistributedLock(String lockPath) throws IOException, InterruptedException, KeeperException {this.zooKeeper = ZooKeeperClient.getZooKeeper();this.lockPath = lockPath;ensurePathExists(lockPath);}private void ensurePathExists(String path) throws KeeperException, InterruptedException {if (zooKeeper.exists(path, false) == null) {zooKeeper.create(path, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);}}public void lock() throws KeeperException, InterruptedException {currentLockPath = zooKeeper.create(lockPath + "/lock-", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);while (true) {List<String> children = zooKeeper.getChildren(lockPath, false);String minChild = getMinNode(children);if (currentLockPath.equals(lockPath + "/" + minChild)) {return;}waitForLock(minChild);}}private String getMinNode(List<String> children) {String minChild = children.get(0);for (String child : children) {if (child.compareTo(minChild) < 0) {minChild = child;}}return minChild;}private void waitForLock(String minChild) throws KeeperException, InterruptedException {final CountDownLatch latch = new CountDownLatch(1);Watcher watcher = new Watcher() {public void process(WatchedEvent event) {if (event.getType() == Event.EventType.NodeDeleted) {latch.countDown();}}};String prevNode = getPrevNode(minChild);zooKeeper.exists(lockPath + "/" + prevNode, watcher);latch.await();}private String getPrevNode(String minChild) throws KeeperException, InterruptedException {List<String> children = zooKeeper.getChildren(lockPath, false);String prevNode = null;for (String child : children) {if (child.equals(minChild)) {break;}prevNode = child;}return prevNode;}public void unlock() throws KeeperException, InterruptedException {zooKeeper.delete(currentLockPath, -1);currentLockPath = null;}
}

步骤三：使用分布式锁

public class Main {private static final String LOCK_PATH = "/locks";public static void main(String[] args) {try {DistributedLock lock = new DistributedLock(LOCK_PATH);lock.lock();// TODO: 处理业务逻辑lock.unlock();} catch (IOException | InterruptedException | KeeperException e) {e.printStackTrace();}}
}

4. 总结

本文介绍了使用ZooKeeper实现分布式锁的基本原理和步骤，并给出了相应的Java代码示例。在实际应用中，可以根据具体的需求和系统架构选择合适的分布式锁实现方式，从而保证系统的并发访问安全性。