分布式锁用 Redis 好，还是 ZooKeeper 好？

    privatestaticfinal String LOCK_SUCCESS = "OK";
    privatestaticfinal Long RELEASE_SUCCESS = 1L;
    privatestaticfinal String SET_IF_NOT_EXIST = "NX";
    privatestaticfinal String SET_WITH_EXPIRE_TIME = "PX";
    
    @Override
    public String acquire() {
    try {
        // 获取锁的超时时间，超过这个时间则放弃获取锁
        long end = System.currentTimeMillis() + acquireTimeout;
        // 随机生成一个 value
        String requireToken = UUID.randomUUID().toString();
        while (System.currentTimeMillis() < end) {
            String result = jedis
                .set(lockKey, requireToken, SET_IF_NOT_EXIST, SET_WITH_EXPIRE_TIME, expireTime);
            if (LOCK_SUCCESS.equals(result)) {
                return requireToken;
            }
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    } catch (Exception e) {
        log.error("acquire lock due to error", e);
    }
    
    returnnull;
    }
    
    @Override
    public boolean release(String identify) {
    if (identify == null) {
        returnfalse;
    }
    //通过 lua 脚本进行比对删除操作，保证原子性
    String script = "if redis.call('get', KEYS[1]) == ARGV[1] then return redis.call('del', KEYS[1]) else return 0 end";
    Object result = new Object();
    try {
        result = jedis.eval(script, Collections.singletonList(lockKey),
            Collections.singletonList(identify));
        if (RELEASE_SUCCESS.equals(result)) {
            log.info("release lock success, requestToken:{}", identify);
            returntrue;
        }
    } catch (Exception e) {
        log.error("release lock due to error", e);
    } finally {
        if (jedis != null) {
            jedis.close();
        }
    }
    
    log.info("release lock failed, requestToken:{}, result:{}", identify, result);
    returnfalse;}

    思考：加锁和释放锁的原子性可以用 lua 脚本来保证，那锁的自动续期改如何实现呢？

    <dependency>
    <groupId>org.redisson</groupId>
    <artifactId>redisson</artifactId>
    <version>3.13.4</version>
    </dependency>

    private void test() {
    //分布式锁名  锁的粒度越细，性能越好
    RLock lock = redissonClient.getLock("test_lock");
    lock.lock();
    try {
        //具体业务......
    } finally {
        lock.unlock();
    }}

    // 最常见的使用方法

      lock.lock();
    // 加锁以后10秒钟自动解锁
    // 无需调用unlock方法手动解锁
    lock.lock(10, TimeUnit.SECONDS);

    public void test() {
    RLock lock = redissonClient.getLock("test_lock");
    lock.lock(30, TimeUnit.SECONDS);
    try {
        //.......具体业务
    } finally {
        //手动释放锁
        lock.unlock();
    }}

    * 当第一个线程进来时会去父节点上创建一个临时的顺序节点。
    * 第二个线程进来发现锁已经被持有了，就会为当前持有锁的节点注册一个 watcher 监听器。
    * 第三个线程进来发现锁已经被持有了，因为是顺序节点的缘故，就会为上一个节点去创建一个 watcher 监听器。
    * 当第一个线程释放锁后，删除节点，由它的下一个节点去占有锁。

    public class ZooKeeperDistributedLock implements Watcher {
    
    private ZooKeeper zk;
    private String locksRoot = "/locks";
    private String productId;
    private String waitNode;
    private String lockNode;
    private CountDownLatch latch;
    private CountDownLatch connectedLatch = new CountDownLatch(1);
    private int sessionTimeout = 30000;
    
    public ZooKeeperDistributedLock(String productId) {
        this.productId = productId;
        try {
            String address = "192.168.189.131:2181,192.168.189.132:2181";
            zk = new ZooKeeper(address, sessionTimeout, this);
            connectedLatch.await();
        } catch (IOException e) {
            throw new LockException(e);
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        }
    }
    
    public void process(WatchedEvent event) {
        if (event.getState() == KeeperState.SyncConnected) {
            connectedLatch.countDown();
            return;
        }
    
        if (this.latch != null) {
            this.latch.countDown();
        }
    }
    
    public void acquireDistributedLock() {
        try {
            if (this.tryLock()) {
                return;
            } else {
                waitForLock(waitNode, sessionTimeout);
            }
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        }
    }
    //获取锁
    public boolean tryLock() {
        try {
        // 传入进去的 locksRoot + “/” + productId
        // 假设 productId 代表了一个商品 id，比如说 1
        // locksRoot = locks
        // /locks/10000000000，/locks/10000000001，/locks/10000000002
        lockNode = zk.create(locksRoot + "/" + productId, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
    
        // 看看刚创建的节点是不是最小的节点
        // locks：10000000000，10000000001，10000000002
        List<String> locks = zk.getChildren(locksRoot, false);
        Collections.sort(locks);
    
        if(lockNode.equals(locksRoot+"/"+ locks.get(0))){
            //如果是最小的节点,则表示取得锁
            return true;
        }
    
        //如果不是最小的节点，找到比自己小 1 的节点
      int previousLockIndex = -1;
            for(int i = 0; i < locks.size(); i++) {
        if(lockNode.equals(locksRoot + “/” + locks.get(i))) {
                    previousLockIndex = i - 1;
            break;
        }
       }
    
       this.waitNode = locks.get(previousLockIndex);
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        }
        return false;
    }
    
    private boolean waitForLock(String waitNode, long waitTime) throws InterruptedException, KeeperException {
        Stat stat = zk.exists(locksRoot + "/" + waitNode, true);
        if (stat != null) {
            this.latch = new CountDownLatch(1);
            this.latch.await(waitTime, TimeUnit.MILLISECONDS);
            this.latch = null;
        }
        return true;
    }
    
    //释放锁
    public void unlock() {
        try {
            System.out.println("unlock " + lockNode);
            zk.delete(lockNode, -1);
            lockNode = null;
            zk.close();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }
    //异常
    public class LockException extends RuntimeException {
        private static final long serialVersionUID = 1L;
    
        public LockException(String e) {
            super(e);
        }
    
        public LockException(Exception e) {
            super(e);
        }
    }
    }