前言

上一篇说过，zookeeper是一个类似文件系统的数据结构，每个节点都可以看做是一个文件目录，也就是说，我们所创建的节点是唯一的，那么分布式锁的原理就是基于这个来的。

代码仓库：https://gitee.com/LIRUIYI/test-zk.git

非公平锁

通过创建节点，并判断节点是否存在实现分布式锁。

1. 创建临时节点，如/lock，临时节点是当出现异常，没有删除导致永久加锁的情况发生
2. 当节点不存在，创建节点成功，也就意味着枷锁成功，如果创建失败，那么就是加锁失败
3. 其他需要加锁的线程，监听/lock节点（get -w /lock），当节点/lock删除后，zookeeper会通知到监听的节点

这种方式的加锁，不能保证需要加锁的线程能得到锁的概率一样，他们是随机的，有可能最先排队的加锁线程，到最后都不能得到锁，这就是非公平锁。

以原始客户端实现非公平锁：

这里zookeeper地址和上面不一样，因为之前是桥接模式，自动获取的，有时ip会变动，所以改NAT模式，设定了静态ip

package com.liry.zk;

import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import lombok.extern.slf4j.Slf4j;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;

/**
 * 分布式锁 - 非公平锁实现
 *
 * @author ALI
 * @since 2022/12/25
 */
@Slf4j
public class NonfairSyncLock {

    private static final String CONNECT_STR = "192.168.17.128:2181";

    private static final int TIME_OUT = 30000;

    private static final String LOCK_PATH = "/lock";

    private static ZooKeeper zookeeper = null;

    /**
     * 获取客户端
     */
    public static ZooKeeper getClient() throws IOException, InterruptedException {
        synchronized (LOCK_PATH) {
            final CountDownLatch latch = new CountDownLatch(1);
            if (zookeeper == null) {
                Watcher startWatcher = watchedEvent -> {
                    if (watchedEvent.getState() == Watcher.Event.KeeperState.SyncConnected) {
                        log.info("与zookeeper建立连接");
                        latch.countDown();
                    }
                };
                zookeeper = new ZooKeeper(CONNECT_STR, TIME_OUT, startWatcher);
                latch.await();
            } else if (zookeeper.getState() != ZooKeeper.States.CONNECTED) {
                latch.await();
            }
        }
        return zookeeper;
    }

    /**
     * 加锁
     */
    public void lock() {
        while (true) {
            if (tryLock()) {
                return;
            }
            // 加锁失败，增加监听，然后阻塞
            try {
                watch();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
    }

    /**
     * 解锁
     */
    public void unlock() {
        try {
            getClient().delete(LOCK_PATH, -1);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * 尝试加锁
     */
    private boolean tryLock() {
        try {
            getClient().create(LOCK_PATH, "lock".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE,
                               CreateMode.EPHEMERAL);
        } catch (Exception e) {
            log.error("加锁失败");
            return false;
        }
        return true;
    }

    /**
     * 监听锁节点
     * 当节点存在时，线程阻塞，当节点不存在，直接退出监听
     */
    private void watch() throws InterruptedException, KeeperException, IOException {
        CountDownLatch latch = new CountDownLatch(1);
        Watcher dataWatch = event -> {
            if (event.getType() == Watcher.Event.EventType.NodeDeleted) {
                latch.countDown();
            }
        };
        try {
            getClient().getData(LOCK_PATH, dataWatch, null);
        } catch (KeeperException.NoNodeException e) {
            // 当创建监听时，节点不存在，说明有线程解锁了，那么直接退出，监听步骤，去争抢锁
            return;
        }
        latch.await();
    }

}

    static int count = 0;

    public static void main(String[] args) throws InterruptedException {
        nonFairLock();
    }

    private static void nonFairLock() throws InterruptedException {
        NonfairSyncLock lock = new NonfairSyncLock();

        CountDownLatch latch = new CountDownLatch(1000);
        List<Thread> threadList = IntStream.range(0, 1000).mapToObj(d -> new Thread(() -> {
            lock.lock();
            count += 1;
            latch.countDown();
            lock.unlock();
        }, "线程-" + d)).collect(Collectors.toList());

        threadList.forEach(Thread::start);

        latch.await();
        System.out.println("最终结果应是1000：" + count);
    }

公平锁

相对于非公平锁的实现，这个方式较为复杂一点。

1. 先创建一个根节点/lock

2. 再在/lock下创建临时有序节点，有序节点是因为所有需要加锁的节点需要按先来后到的顺序才能公平

3. 然后每个有序节点都监听它的前一个节点，如图，当前一个节点被删除，表示解锁了，那么zookeeper会通知到监听的节点，也就是下一个需要加锁的线程

   

这个在curator中已经有实现了，分布式锁不局限于zookeeper，了解其原理就行

static int count = 0;

    public static void main(String[] args) throws InterruptedException {
//        nonFairLock();
        fairLock();
    }

    private static void fairLock() throws InterruptedException {
        // 使用curator客户端
        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
        CuratorFramework client = CuratorFrameworkFactory.builder()
                                                         .connectString("92.168.17.128:2181")
                                                         .sessionTimeoutMs(3000)
                                                         .connectionTimeoutMs(3000)
                                                         .retryPolicy(retryPolicy)
                                                         .build();
        client.start();
        
        InterProcessMutex lock = new InterProcessMutex(client, "/lock");

        CountDownLatch latch = new CountDownLatch(1000);
        List<Thread> threadList = IntStream.range(0, 1000).mapToObj(d -> new Thread(() -> {
            try {
                lock.acquire();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
            count += 1;
            latch.countDown();
            try {
                lock.release();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }, "线程-" + d)).collect(Collectors.toList());

        threadList.forEach(Thread::start);

        latch.await();
        System.out.println("最终结果应是1000：" + count);
    }