redis分布式锁RedissonLock的实现细节

简单使用

String key = "key-lock";
RLock lock = redisson.getLock(key);
lock.lock();
try {
    // TODO
} catch (Exception e){
    log.error(e.getMessage(), e);
} finally {
    lock.unlock();
}

String key = "key-tryLock";
long maxWaitTime = 3_000;
RLock lock = redisson.getLock(key);
if (lock.tryLock(maxWaitTime, TimeUnit.MILLISECONDS)){
    try {
        // TODO
    } catch (Exception e){
        log.error(e.getMessage(), e);
    } finally {
        lock.unlock();
    }
} else {
    log.debug("redis锁竞争失败");
}

流程图

多个线程节点锁竞争的正常流程如下图：

多个线程节点锁竞争，并出现节点下线的异常流程如下图：

源码解析

RedissonLock是可重入锁，使用redis的hash结构作为锁的标识存储，锁的名称作为hash的key，UUID + 线程ID作为hash的field，锁被重入的次数作为hash的value。如图所示：

private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {
    long threadId = Thread.currentThread().getId();
    // 尝试获取锁,锁获取成功则ttl为null;获取失败则返回锁的剩余过期时间
    Long ttl = tryAcquire(leaseTime, unit, threadId);
    if (ttl == null) {
        return;
    }

    // 锁被其他线程占用而索取失败,使用线程通知而非自旋的方式等待锁
    // 使用redis的发布订阅pub/sub功能来等待锁的释放通知
    RFuture<RedissonLockEntry> future = subscribe(threadId);
    commandExecutor.syncSubscription(future);

    try {
        while (true) {
            ttl = tryAcquire(leaseTime, unit, threadId);
            // 尝试获取锁,锁获取成功则ttl为null;获取失败则返回锁的剩余过期时间
            if (ttl == null) {
                break;
            }

            if (ttl >= 0) {
                // 使用LockSupport.parkNanos方法线程休眠
                try {
                    getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                } catch (InterruptedException e) {
                    if (interruptibly) {
                        throw e;
                    }
                    getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                }
            } else {
                if (interruptibly) {
                    getEntry(threadId).getLatch().acquire();
                } else {
                    getEntry(threadId).getLatch().acquireUninterruptibly();
                }
            }
        }
    } finally {
        // 退出锁竞争(锁获取成功或者放弃获取锁),则取消锁的释放订阅
        unsubscribe(future, threadId);
    }
}

public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
    long time = unit.toMillis(waitTime);
    long current = System.currentTimeMillis();
    long threadId = Thread.currentThread().getId();
    Long ttl = tryAcquire(leaseTime, unit, threadId);
    if (ttl == null) {
        return true;
    }
    
    time -= System.currentTimeMillis() - current;
    if (time <= 0) {
        acquireFailed(threadId);
        return false;
    }
    
    current = System.currentTimeMillis();
    RFuture<RedissonLockEntry> subscribeFuture = subscribe(threadId);
    if (!await(subscribeFuture, time, TimeUnit.MILLISECONDS)) {
        if (!subscribeFuture.cancel(false)) {
            subscribeFuture.onComplete((res, e) -> {
                if (e == null) {
                    unsubscribe(subscribeFuture, threadId);
                }
            });
        }
        acquireFailed(threadId);
        return false;
    }

    try {
        time -= System.currentTimeMillis() - current;
        if (time <= 0) {
            acquireFailed(threadId);
            return false;
        }
    
        while (true) {
            long currentTime = System.currentTimeMillis();
            ttl = tryAcquire(leaseTime, unit, threadId);
            // lock acquired
            if (ttl == null) {
                return true;
            }

            time -= System.currentTimeMillis() - currentTime;
            if (time <= 0) {
                acquireFailed(threadId);
                return false;
            }

            currentTime = System.currentTimeMillis();
            if (ttl >= 0 && ttl < time) {
                getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
            } else {
                getEntry(threadId).getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);
            }

            time -= System.currentTimeMillis() - currentTime;
            if (time <= 0) {
                acquireFailed(threadId);
                return false;
            }
        }
    } finally {
        unsubscribe(subscribeFuture, threadId);
    }
}

RedissonLock实现的是可重入锁，通过redis的hash结构实现，而非加单的set nx ex。为了实现原子性的复杂的加锁逻辑，而通过lua脚本实现。获取锁会有如下三种状态：

1. 锁未被任何线程占用，则锁获取成功，返回null
2. 锁被当前线程占用，则锁获取成功并进行锁的重入，对锁的重入计数+1，返回null
3. 锁被其他线程占用，则锁获取失败，返回该锁的自动过期时间ttl

<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));
}

当锁因为被其他线程占用而 使用redis的发布订阅pub/sub功能，通过监听锁的释放通知（在其他线程通过RedissonLock释放锁时，会通过发布订阅pub/sub功能发起通知），等待锁被其他线程释放。通过如此的线程唤醒而非自旋的操作，提高了锁的效率。

public RFuture<E> subscribe(String entryName, String channelName) {
    AtomicReference<Runnable> listenerHolder = new AtomicReference<Runnable>();
    AsyncSemaphore semaphore = service.getSemaphore(new ChannelName(channelName));
    RPromise<E> newPromise = new RedissonPromise<E>() {
        @Override
        public boolean cancel(boolean mayInterruptIfRunning) {
            return semaphore.remove(listenerHolder.get());
        }
    };

    Runnable listener = new Runnable() {

        @Override
        public void run() {
            E entry = entries.get(entryName);
            if (entry != null) {
                entry.aquire();
                semaphore.release();
                entry.getPromise().onComplete(new TransferListener<E>(newPromise));
                return;
            }
            
            E value = createEntry(newPromise);
            value.aquire();
            
            E oldValue = entries.putIfAbsent(entryName, value);
            if (oldValue != null) {
                oldValue.aquire();
                semaphore.release();
                oldValue.getPromise().onComplete(new TransferListener<E>(newPromise));
                return;
            }
            
            RedisPubSubListener<Object> listener = createListener(channelName, value);
            service.subscribe(LongCodec.INSTANCE, channelName, semaphore, listener);
        }
    };
    semaphore.acquire(listener);
    listenerHolder.set(listener);
    
    return newPromise;
}

由于是可重入锁则需要在释放锁的时候做订阅通知，因此释放锁的操作同样是lua脚本实现。锁的释放会有如下三个状态：

1. 等待释放的锁不存在或者不是当前线程持有，返回null
2. 等待释放的锁被当前线程持有，且该锁当前被重入多次，则锁的重入计数-1，返回0
3. 等待释放的锁被当前线程持有，且该锁当前未被重入，则锁的删除并发布该锁释放的订阅通知，返回1

protected RFuture<Boolean> unlockInnerAsync(long threadId) {
    return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
            "if (redis.call('hexists', KEYS[1], ARGV[3]) == 0) then " +
                "return nil;" +
            "end; " +
            "local counter = redis.call('hincrby', KEYS[1], ARGV[3], -1); " +
            "if (counter > 0) then " +
                "redis.call('pexpire', KEYS[1], ARGV[2]); " +
                "return 0; " +
            "else " +
                "redis.call('del', KEYS[1]); " +
                "redis.call('publish', KEYS[2], ARGV[1]); " +
                "return 1; "+
            "end; " +
            "return nil;",
            Arrays.<Object>asList(getName(), getChannelName()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName(threadId));

}

Watchdog

RedissonLock为了避免应用获取锁后宕机，因为没人来释放锁而导致死锁情况的出现，默认每次锁的占用只有30秒的时间（org.redisson.config.Config#lockWatchdogTimeout = 30 * 1000）。于是便有了Watchdog设计，由独立的线程定时给未释放的锁续期，默认锁有效期的三分之一的时长即每10秒给锁自动续期。

private void renewExpiration() {
    ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());
    if (ee == null) {
        return;
    }
    
    // 默认10秒钟后执行锁续期任务
    Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
        @Override
        public void run(Timeout timeout) throws Exception {
            ExpirationEntry ent = EXPIRATION_RENEWAL_MAP.get(getEntryName());
            if (ent == null) {
                return;
            }
            Long threadId = ent.getFirstThreadId();
            if (threadId == null) {
                return;
            }
            
            RFuture<Boolean> future = renewExpirationAsync(threadId);
            future.onComplete((res, e) -> {
                if (e != null) {
                    log.error("Can't update lock " + getName() + " expiration", e);
                    return;
                }
                // 如果锁续期成功,则10秒钟后再次续期
                if (res) {
                    renewExpiration();
                }
            });
        }
    }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
    
    ee.setTimeout(task);
}

protected RFuture<Boolean> renewExpirationAsync(long threadId) {
    return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
            "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                "return 1; " +
            "end; " +
            "return 0;",
        Collections.<Object>singletonList(getName()), 
        internalLockLeaseTime, getLockName(threadId));
}