Zookeeper：实现“分布式锁”的 Demo

朴灿烈づ我的快乐病毒、 2023-09-23 16:10 46阅读 0赞

> Zookeeper 能保证数据的强一致性，用户任何时候都可以相信集群中每个节点的数据都是相同的。一个用户创建一个节点作为锁，另一个用户检测该节点，如果存在，代表别的用户已经锁住，如果不存在，则可以创建一个节点，代表拥有一个锁。
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> 本篇内容包括：Demo 概述、代码实现、测试结果

--------------------

### 文章目录 ###

*   *  一、Demo 概述
     *   *   *  1、关于 zookeeper “命名服务协调”
             *  2、Demo 设计
             *  3、Demo 前提
     *  二、代码实现
     *   *   *  1、引用 Maven 依赖
             *  2、ConnectionWatcher 类创建 Zookeeper 连接
             *  3、ActiveKeyValueStore 类读写 Zookeeper 数据
             *  4、ZkLock 类实现分布式锁
     *  三、测试结果

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## 一、Demo 概述 ##

#### 1、关于 zookeeper “命名服务协调” ####

Zookeeper 能保证数据的强一致性，用户任何时候都可以相信集群中每个节点的数据都是相同的。一个用户创建一个节点作为锁，另一个用户检测该节点，如果存在，代表别的用户已经锁住，如果不存在，则可以创建一个节点，代表拥有一个锁。

#### 2、Demo 设计 ####

分布式锁本质，就是多个资源竞争者对一份资源的排他占有

*  我们设置多个线程，分别在同一 path 下创建节点
 *  没个线程获取当前 path 下子节点，看最小子节点是否为自身，是则加锁成功（更好的方式是用 Watcher 对前一个地址监控，这里图方便用子节点排序取最小的方式 ）
 *  线程加锁成功后，执行任务，执行完毕后解锁

#### 3、Demo 前提 ####

参考：[Mac通过Docker安装Zookeeper集群][Mac_Docker_Zookeeper]

--------------------

## 二、代码实现 ##

#### 1、引用 Maven 依赖 ####

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

#### 2、ConnectionWatcher 类创建 Zookeeper 连接 ####

import java.io.IOException;
    import java.util.concurrent.CountDownLatch;
    
    import org.apache.zookeeper.WatchedEvent;
    import org.apache.zookeeper.Watcher;
    import org.apache.zookeeper.ZooKeeper;
    
    public class ConnectionWatcher implements Watcher {
        
    
        private final CountDownLatch connectedSignal = new CountDownLatch(1);
        private static final int SESSION_TIMEOUT = 5000;
        protected ZooKeeper zk;
    
    
        public void connect(String hosts) throws IOException, InterruptedException {
        
            zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
            connectedSignal.await();
        }
    
        @Override
        public void process(WatchedEvent event) {
        
            if (event.getState() == Event.KeeperState.SyncConnected) {
        
                connectedSignal.countDown();
            }
        }
    
        public void close() throws InterruptedException {
        
            zk.close();
        }
    
    }

#### 3、ActiveKeyValueStore 类读写 Zookeeper 数据 ####

import java.nio.charset.Charset;
    import java.nio.charset.StandardCharsets;
    import java.util.List;
    
    import org.apache.zookeeper.CreateMode;
    import org.apache.zookeeper.KeeperException;
    import org.apache.zookeeper.Watcher;
    import org.apache.zookeeper.ZooDefs;
    import org.apache.zookeeper.data.Stat;
    
    public class ActiveKeyValueStore extends ConnectionWatcher {
        
    
        private static final Charset CHARSET = StandardCharsets.UTF_8;
    
        int state = 0;
    
        /**
         * 写入节点数据
         *
         * @param path  节点地址
         * @param value 数据值
         * @throws InterruptedException 中断异常
         * @throws KeeperException      ZooKeeper异常
         */
        public void write(String path, String value) throws InterruptedException, KeeperException {
        
            Stat stat = zk.exists(path, false);
            if (stat == null) {
        
                if (value == null) {
        
                    zk.create(path, null,
                            ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
                } else {
        
                    zk.create(path, value.getBytes(CHARSET),
                            ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
                }
    
            } else {
        
                if (value == null) {
        
                    zk.setData(path, null, -1);
                } else {
        
                    zk.setData(path, value.getBytes(CHARSET), -1);
                }
    
            }
        }
      
       public boolean lock(String path, String name) throws InterruptedException, KeeperException {
        
            boolean flag = tryLock(path, name);
            if (flag) {
        
                state++;
            }
            return flag;
        }
    
        public boolean tryLock(String path, String name) throws InterruptedException, KeeperException {
        
    
            String lockPath = path + "/" + name;
            zk.create(lockPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
    
            List<String> waits = readChildren(path, null);
            Collections.sort(waits);
            if (waits.get(0).equals(name)) {
        
                return true;
            }
            CountDownLatch latch = new CountDownLatch(1);
            for (int i = 0; i < waits.size(); i++) {
        
                String cur = waits.get(i);
                if (!cur.equalsIgnoreCase(name)) {
        
                    continue;
                }
                String prePath = path + "/" + waits.get(i - 1);
                zk.exists(prePath, new Watcher() {
        
                    @Override
                    public void process(WatchedEvent event) {
        
                        latch.countDown();
                    }
                });
                break;
            }
            latch.await();
            return true;
        }
    
        public boolean unlock(String path, String name) {
        
            if (state > 1) {
        
                state--;
                return true;
            }
    
            String lockPath = path + "/" + name;
    
            try {
        
                Stat stat = zk.exists(lockPath, false);
                int version = stat.getVersion();
                zk.delete(lockPath, version);
                state--;
                return true;
            } catch (Exception e) {
        
                System.out.println("unlock:" + lockPath + " ,exception,");
            }
            return false;
        }
    
        /**
         * 获取所有子节点
         *
         * @param path    节点地址
         * @param watcher watcher
         * @return 所有子节点
         * @throws InterruptedException 中断异常
         * @throws KeeperException      ZooKeeper异常
         */
        public List<String> readChildren(String path, Watcher watcher) throws InterruptedException, KeeperException {
        
            List<String> childrens = null;
            if (watcher == null) {
        
                childrens = zk.getChildren(path, false);
            } else {
        
                childrens = zk.getChildren(path, watcher, null);
            }
            return childrens;
        }
    }

#### 4、ZkLock 类实现分布式锁 ####

import lombok.SneakyThrows;
    import org.apache.zookeeper.KeeperException;
    
    import java.io.IOException;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.TimeUnit;
    
    public class ZkLock {
        
    
    
        /**
         * 开启的线程数，模拟多客户端操作
         */
        private static final int CLIENTS_NUM = 3;
    
        private final ActiveKeyValueStore store;
    
        public ZkLock(String hosts) throws IOException, InterruptedException {
        
            //定义一个类
            store = new ActiveKeyValueStore();
            //连接Zookeeper
            store.connect(hosts);
        }
    
        public static void testLock() {
        
            //线程计数器控制业务的执行
            final CountDownLatch countDownLatch = new CountDownLatch(CLIENTS_NUM);
            for (int i = 0; i < CLIENTS_NUM; i++) {
        
                new Thread() {
        
                    @Override
                    public void run() {
        
    
                    }
                }.start();
            }
            try {
        
                // 堵塞线程，任务执行完后释放
                countDownLatch.await();
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    
    
        public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        
    
            String hosts = "localhost:2181";
            ZkLock zkLock = new ZkLock(hosts);
    
            // 创建父节点
            zkLock.store.write("/lock4", "父亲节点");
            //
            CountDownLatch latch = new CountDownLatch(CLIENTS_NUM);
            for (int i = 0; i < CLIENTS_NUM; i++) {
        
    
                int finalI = i;
                new Thread() {
        
                    @SneakyThrows
                    @Override
                    public void run() {
        
    
                        String name = "Thread-" + String.valueOf(finalI);
                        zkLock.store.lock("/lock4", name);
                        TimeUnit.SECONDS.sleep(2);
                        System.out.println("线程-" + name + "执行完毕");
                        latch.countDown();
                        zkLock.store.unlock("/lock4", name);
    
    
                    }
                }.start();
            }
            latch.await();
            System.out.println("end ...");
        }
    
    
    }

--------------------

## 三、测试结果 ##

ZkLock 代码测试结果如下：

线程-Thread-0执行完毕
    线程-Thread-1执行完毕
    线程-Thread-2执行完毕
    end ...

通过 ZkLock 打印的信息可以看出，已经成功模拟实现分布式锁

[Mac_Docker_Zookeeper]: https://blog.csdn.net/weixin_45187434/article/details/127958888