redis分布式锁RedissonLock的实现细节解析

redis分布式锁RedissonLock

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 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 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

 RFuture tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand 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.