zk实现分布式锁

一、为什么需要分布式锁

    如果服务运行在多台服务器上，或者运行在多个JVM上，对于一些公共资源，就需要有锁保证资源的独享性。比如，网上银行存钱的时候，客户端发出存钱的指令，被分发到多台服务器上，同时执行了存钱的服务，就可能会导致数据库中有两条相同的记录。所以需要保证同一时刻，只有一个客户端在执行这条指令。

二、分布式锁方案

1、每一次获取锁的时候，都在根节点下创建临时序列化节点，释放锁的时候删除该节点；

2、获取锁时，调用根节点的getChildren()方法，查看所有的子节点，如果发现自己创建的节点是子节点中序列号最小，就定义客户端获取到了锁；否则，找到比自己小的前一个节点，注册监听事件；如果比自己小的节点不存在，再次判断自己所创建的节点是否最小。

zk分布式锁的原理不做多述，直接上代码：

I. 分布式锁的接口

public interface DistributedLock {

    /**
     * 获取分布式锁
     * @return
     */
    boolean dLock();

    /**
     * 在一定时间内获取分布式锁
     * @param time
     * @return
     */
    boolean dLock(long time);

    /**
     * 释放分布式锁
     */
    void unDLock();
}

II. 分布式锁的实现

public class DefaultDistributedLock implements DistributedLock, Watcher {

    private ZooKeeper zkClient;

    // 分布式锁持久化节点名称
    private static String LOCK_PERSIST= "/DIS_LOCK";

    // 临时节点前缀
    private static String LOCK_ELEPHANT_PREFIX = LOCK_PERSIST+"/dis_";

    // zk连接的ip
    private String ips;

    // session过期时间
    private static int sessionTimeout = 300000;

    // 主线程等待连接建立好后才启动
    private CountDownLatch connectedSemaphore = new CountDownLatch(1);

    // 当前线程创建临时节点后返回的路径
    private String selfPath;
    // 等锁路径
    private String waitPath;

    private String lockName;

    private CountDownLatch latch;

    public DefaultDistributedLock(String ips, String lockName) {
        this(ips,sessionTimeout, lockName);
    }

    public DefaultDistributedLock(String ips, int sessionTimeout, String lockName) {
        this.ips = ips;
        this.sessionTimeout = sessionTimeout;
        this.lockName = lockName;
        createRootNode(LOCK_PERSIST,"根节点");
    }



    public boolean dLock() {
        try {
            selfPath = zkClient.create(LOCK_ELEPHANT_PREFIX, null, ZooDefs.Ids.OPEN_ACL_UNSAFE,
                    CreateMode.EPHEMERAL_SEQUENTIAL);
            System.out.println(lockName+" 创建临时节点路径"+selfPath);
            return checkMinPath(selfPath);
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

    public boolean dLock(long time){
        try {
            if(dLock()){
                return true;
            }

            return waitForLock(time);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return false;
    }

    public void unDLock()  {
        System.out.println(lockName+"删除本节点：" + selfPath);
        try {
            zkClient.delete(selfPath, -1);
            selfPath = null;
            releaseConnection();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }

    public void releaseConnection() {
        if (this.zkClient != null) {
            try {
                this.zkClient.close();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(lockName+"释放连接");
    }

    // 判断比自己小的一个节点是否存在，如果不存在，直接返回，无需等待
    private boolean waitForLock(long t) throws KeeperException, InterruptedException {
        Stat  stat = zkClient.exists(waitPath, true);
        if(stat != null){
            this.latch = new CountDownLatch(1);
            // 如果超过等待时间会抛出异常
            latch.await(t, TimeUnit.MILLISECONDS);
            this.latch = null;
        }
        return true;
    }


    // 校验本线程创建的节点是否是所有节点中的最小节点
    private boolean checkMinPath(String selfPath) throws KeeperException, InterruptedException {
        List subNodes = zkClient.getChildren(LOCK_PERSIST,false);
        Collections.sort(subNodes);
        String str = selfPath.substring(LOCK_PERSIST.length()+1);
        int index = subNodes.indexOf(str);

        switch (index){
            case -1:{
                System.out.println(lockName+"--本节点已不在了..." + selfPath);
                return false;
            }
            case 0:{
                System.out.println(lockName+"--本节点是最小节点..." + selfPath);
                return true;
            }
            default:{
                waitPath = LOCK_PERSIST+"/"+subNodes.get(index-1);
                System.out.println(lockName+"--获取子节点中，排在我前面的"+ waitPath);
                // 对前一个节点注册监听事件
                try {
                    zkClient.getData(waitPath,true,new Stat());
                    return false;
                } catch (Exception e) {
                    // 跑出异常的时候，判断节点是否存在
                   if(zkClient.exists(waitPath, false) == null){
                       System.out.println(lockName+ "--子节点中，排在我前面的" + waitPath + "已失踪,重新检查");
                       return checkMinPath(selfPath);
                   }else{
                       throw new RuntimeException(waitPath+"node disappered");
                   }
                }
            }
        }
    }




    // 创建根节点,根节点不需要进行watch
    private boolean createRootNode(String path, String data) {
        try {
            createConnection();
            if(zkClient.exists(path, false) == null){
                String retPath = zkClient.create(path, data.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
                System.out.println("创建根节点:path" + retPath + "content" + data);
            }
            return true;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return false;
    }


    private void createConnection() throws IOException, InterruptedException {
        if(zkClient == null){
            zkClient = new ZooKeeper(ips, sessionTimeout, this);
            connectedSemaphore.await();
        }
    }


    public void process(WatchedEvent watchedEvent) {
        if(watchedEvent == null){
            return;
        }

        Event.EventType eventType = watchedEvent.getType();
        Event.KeeperState state = watchedEvent.getState();

        if(Event.KeeperState.SyncConnected == state){
            if(Event.EventType.None == eventType){
                System.out.println("正在启动连接服务器");
                connectedSemaphore.countDown();
            }else if (Event.KeeperState.Disconnected == state) {
                System.out.println("与ZK服务器断开连接");
            } else if (Event.KeeperState.Expired == state) {
                System.out.println("会话失效");
            }
        }
    }
}

3、测试用例

public class DisMainTest {

    private static final int THREAD_NUM = 100;
    private static final CountDownLatch threadSemaphore = new CountDownLatch(THREAD_NUM);

    public static void main(String[] args) {
        final String ips = "localhost:2181,localhost:2182";
        for(int i=0; i< THREAD_NUM;i++){
            final int threadId = i + 1;
            new Thread(){
                @Override
                public void run() {
                    try {
                        DistributedLock dc = new DefaultDistributedLock(ips, threadId + "");
                        if (dc.dLock()) {
                            System.out.println(threadId+"获取到锁，并执行了任务");
                        }
                        dc.unDLock();
                        threadSemaphore.countDown();
                    } catch (Exception e) {
                        System.out.println("第" + threadId + "个线程抛出的异常：");
                        e.printStackTrace();
                    }
                }
            }.start();

        }

        try {
            threadSemaphore.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}