分布式锁用 Redis 还是 Zookeeper？

// 获取锁
// NX是指如果key不存在就成功，key存在返回false，PX可以指定过期时间
SET anyLock unique_value NX PX 30000


// 释放锁：通过执行一段lua脚本
// 释放锁涉及到两条指令，这两条指令不是原子性的
// 需要用到redis的lua脚本支持特性，redis执行lua脚本是原子性的
if redis.call("get",KEYS[1]) == ARGV[1] then
return redis.call("del",KEYS[1])
else
return 0
end

Config config = new Config();
config.useClusterServers()
.addNodeAddress("redis://192.168.31.101:7001")
.addNodeAddress("redis://192.168.31.101:7002")
.addNodeAddress("redis://192.168.31.101:7003")
.addNodeAddress("redis://192.168.31.102:7001")
.addNodeAddress("redis://192.168.31.102:7002")
.addNodeAddress("redis://192.168.31.102:7003");

RedissonClient redisson = Redisson.create(config);


RLock lock = redisson.getLock("anyLock");
lock.lock();
lock.unlock();

// 加锁逻辑
private <T> RFuture<Long> tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId) {
    if (leaseTime != -1) {
        return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
    }
    // 调用一段lua脚本，设置一些key、过期时间
    RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
    ttlRemainingFuture.addListener(new FutureListener<Long>() {
        @Override
        public void operationComplete(Future<Long> future) throws Exception {
            if (!future.isSuccess()) {
                return;
            }

            Long ttlRemaining = future.getNow();
            // lock acquired
            if (ttlRemaining == null) {
                // 看门狗逻辑
                scheduleExpirationRenewal(threadId);
            }
        }
    });
    return ttlRemainingFuture;
}


<T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
    internalLockLeaseTime = unit.toMillis(leaseTime);

    return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
              "if (redis.call('exists', KEYS[1]) == 0) then " +
                  "redis.call('hset', KEYS[1], ARGV[2], 1); " +
                  "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                  "return nil; " +
              "end; " +
              "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                  "redis.call('hincrby', KEYS[1], ARGV[2], 1); " +
                  "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                  "return nil; " +
              "end; " +
              "return redis.call('pttl', KEYS[1]);",
                Collections.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));
}



// 看门狗最终会调用了这里
private void scheduleExpirationRenewal(final long threadId) {
    if (expirationRenewalMap.containsKey(getEntryName())) {
        return;
    }

    // 这个任务会延迟10s执行
    Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
        @Override
        public void run(Timeout timeout) throws Exception {

            // 这个操作会将key的过期时间重新设置为30s
            RFuture<Boolean> future = renewExpirationAsync(threadId);

            future.addListener(new FutureListener<Boolean>() {
                @Override
                public void operationComplete(Future<Boolean> future) throws Exception {
                    expirationRenewalMap.remove(getEntryName());
                    if (!future.isSuccess()) {
                        log.error("Can't update lock " + getName() + " expiration", future.cause());
                        return;
                    }

                    if (future.getNow()) {
                        // reschedule itself
                        // 通过递归调用本方法，无限循环延长过期时间
                        scheduleExpirationRenewal(threadId);
                    }
                }
            });
        }

    }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);

    if (expirationRenewalMap.putIfAbsent(getEntryName(), new ExpirationEntry(threadId, task)) != null) {
        task.cancel();
    }
}

另外，redisson还提供了对redlock算法的支持,它的用法也很简单：RedissonClient redisson = Redisson.create(config);
RLock lock1 = redisson.getFairLock("lock1");
RLock lock2 = redisson.getFairLock("lock2");
RLock lock3 = redisson.getFairLock("lock3");
RedissonRedLock multiLock = new RedissonRedLock(lock1, lock2, lock3);
multiLock.lock();
multiLock.unlock();
小结：
本节分析了使用redis作为分布式锁的具体落地方案以及其一些局限性然后介绍了一个redis的客户端框架redisson，这也是我推荐大家使用的，比自己写代码实现会少care很多细节。

InterProcessMutex interProcessMutex = new InterProcessMutex(client,"/anyLock");
interProcessMutex.acquire();
interProcessMutex.release();

private boolean internalLockLoop(long startMillis, Long millisToWait, String ourPath) throws Exception
{
    boolean  haveTheLock = false;
    boolean  doDelete = false;
    try {
        if ( revocable.get() != null ) {
            client.getData().usingWatcher(revocableWatcher).forPath(ourPath);
        }

        while ( (client.getState() == CuratorFrameworkState.STARTED) && !haveTheLock ) {
            // 获取当前所有节点排序后的集合
            List<String>        children = getSortedChildren();
            // 获取当前节点的名称
            String              sequenceNodeName = ourPath.substring(basePath.length() + 1); // +1 to include the slash
            // 判断当前节点是否是最小的节点
            PredicateResults    predicateResults = driver.getsTheLock(client, children, sequenceNodeName, maxLeases);
            if ( predicateResults.getsTheLock() ) {
                // 获取到锁
                haveTheLock = true;
            } else {
                // 没获取到锁，对当前节点的上一个节点注册一个监听器
                String  previousSequencePath = basePath + "/" + predicateResults.getPathToWatch();
                synchronized(this){
                    Stat stat = client.checkExists().usingWatcher(watcher).forPath(previousSequencePath);
                    if ( stat != null ){
                        if ( millisToWait != null ){
                            millisToWait -= (System.currentTimeMillis() - startMillis);
                            startMillis = System.currentTimeMillis();
                            if ( millisToWait <= 0 ){
                                doDelete = true;    // timed out - delete our node
                                break;
                            }
                            wait(millisToWait);
                        }else{
                            wait();
                        }
                    }
                }
                // else it may have been deleted (i.e. lock released). Try to acquire again
            }
        }
    }
    catch ( Exception e ) {
        doDelete = true;
        throw e;
    } finally{
        if ( doDelete ){
            deleteOurPath(ourPath);
        }
    }
    return haveTheLock;
}