Zookeeper：实现“分布式锁”的 Demo

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”

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

二、代码实现

1、引用 Maven 依赖

<!--    选择对应的Zookeeper版本    --><dependency><groupId>org.apache.zookeeper</groupId><artifactId>zookeeper</artifactId><version>3.7.0</version></dependency>

2、ConnectionWatcher 类创建 Zookeeper 连接

importjava.io.IOException;importjava.util.concurrent.CountDownLatch;importorg.apache.zookeeper.WatchedEvent;importorg.apache.zookeeper.Watcher;importorg.apache.zookeeper.ZooKeeper;publicclassConnectionWatcherimplementsWatcher{privatefinalCountDownLatch connectedSignal =newCountDownLatch(1);privatestaticfinalintSESSION_TIMEOUT=5000;protectedZooKeeper zk;publicvoidconnect(String hosts)throwsIOException,InterruptedException{
        zk =newZooKeeper(hosts,SESSION_TIMEOUT,this);
        connectedSignal.await();}@Overridepublicvoidprocess(WatchedEvent event){if(event.getState()==Event.KeeperState.SyncConnected){
            connectedSignal.countDown();}}publicvoidclose()throwsInterruptedException{
        zk.close();}}

3、ActiveKeyValueStore 类读写 Zookeeper 数据

importjava.nio.charset.Charset;importjava.nio.charset.StandardCharsets;importjava.util.List;importorg.apache.zookeeper.CreateMode;importorg.apache.zookeeper.KeeperException;importorg.apache.zookeeper.Watcher;importorg.apache.zookeeper.ZooDefs;importorg.apache.zookeeper.data.Stat;publicclassActiveKeyValueStoreextendsConnectionWatcher{privatestaticfinalCharsetCHARSET=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
     */publicvoidwrite(String path,String value)throwsInterruptedException,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);}}}publicbooleanlock(String path,String name)throwsInterruptedException,KeeperException{boolean flag =tryLock(path, name);if(flag){
            state++;}return flag;}publicbooleantryLock(String path,String name)throwsInterruptedException,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)){returntrue;}CountDownLatch latch =newCountDownLatch(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,newWatcher(){@Overridepublicvoidprocess(WatchedEvent event){
                    latch.countDown();}});break;}
        latch.await();returntrue;}publicbooleanunlock(String path,String name){if(state >1){
            state--;returntrue;}String lockPath = path +"/"+ name;try{Stat stat = zk.exists(lockPath,false);int version = stat.getVersion();
            zk.delete(lockPath, version);
            state--;returntrue;}catch(Exception e){System.out.println("unlock:"+ lockPath +" ,exception,");}returnfalse;}/**
     * 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
     */publicList<String>readChildren(String path,Watcher watcher)throwsInterruptedException,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

importlombok.SneakyThrows;importorg.apache.zookeeper.KeeperException;importjava.io.IOException;importjava.util.concurrent.CountDownLatch;importjava.util.concurrent.TimeUnit;publicclassZkLock{/**
     * 开启的线程数，模拟多客户端操作
     */privatestaticfinalintCLIENTS_NUM=3;privatefinalActiveKeyValueStore store;publicZkLock(String hosts)throwsIOException,InterruptedException{//定义一个类
        store =newActiveKeyValueStore();//连接Zookeeper
        store.connect(hosts);}publicstaticvoidtestLock(){//线程计数器控制业务的执行finalCountDownLatch countDownLatch =newCountDownLatch(CLIENTS_NUM);for(int i =0; i <CLIENTS_NUM; i++){newThread(){@Overridepublicvoidrun(){}}.start();}try{// 堵塞线程，任务执行完后释放
            countDownLatch.await();}catch(InterruptedException e){
            e.printStackTrace();}}publicstaticvoidmain(String[] args)throwsIOException,InterruptedException,KeeperException{String hosts ="localhost:2181";ZkLock zkLock =newZkLock(hosts);// 创建父节点
        zkLock.store.write("/lock4","父亲节点");//CountDownLatch latch =newCountDownLatch(CLIENTS_NUM);for(int i =0; i <CLIENTS_NUM; i++){int finalI = i;newThread(){@SneakyThrows@Overridepublicvoidrun(){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