使用ZooKeeper实现分布式锁

引言

[1. ZooKeeper简介](#1. ZooKeeper简介)

[2. 分布式锁实现原理](#2. 分布式锁实现原理)

[3. 分布式锁实现步骤](#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](#4. 总结)

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。

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。
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。
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。
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。