Redis(3) 分布式锁

1. Redisson 分布式锁

可重入：利用hash结构记录线程id和重入次数

可重试：利用信号量和PubSub功能实现等待、唤醒，获取锁失败的重试机制

超时续约：利用watchDog，开启一个定时任务，每隔一段时间（releaseTime / 3），重置超时时间

主从一致性问题：利用MultiLock锁，Redis不再区分主从节点，只有所有的节点都写入成功，才是加锁成功，假设现在某个节点挂了，那么他去获取锁的时候，只要有一个节点拿不到，都不能算是加锁成功，就保证了加锁的可靠性

MultiLock原理

构造函数

RLock lock1 = redissonClient.getLock("lock");
RLock lock2 = redissonClient2.getLock("lock");
RLock lock3 = redissonClient3.getLock("lock");
lock = redissonClient.getMultiLock(lock1, lock2, lock3);

public RedissonMultiLock(RLock... locks) {
    if (locks.length == 0) {
        throw new IllegalArgumentException("Lock objects are not defined");
    } else {
        this.locks.addAll(Arrays.asList(locks));
    }
}

联锁的tryLock

public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
    long newLeaseTime = -1L;
    //如果传入了释放时间
    if (leaseTime != -1L) {
        //再判断一下是否有等待时间
        if (waitTime == -1L) {
            //如果没传等待时间，不重试，则只获得一次
            newLeaseTime = unit.toMillis(leaseTime);
        } else {
            //想要重试，耗时较久，万一释放时间小于等待时间，则会有问题，所以这里将等待时间乘以二
            newLeaseTime = unit.toMillis(waitTime) * 2L;
        }
    }
    //获取当前时间
    long time = System.currentTimeMillis();
    //剩余等待时间
    long remainTime = -1L;
    if (waitTime != -1L) {
        remainTime = unit.toMillis(waitTime);
    }
    //锁等待时间，与剩余等待时间一样    
    long lockWaitTime = this.calcLockWaitTime(remainTime);
    //锁失败的限制，源码返回是的0
    int failedLocksLimit = this.failedLocksLimit();
    //已经获取成功的锁
    List<RLock> acquiredLocks = new ArrayList(this.locks.size());
    //迭代器，用于遍历
    ListIterator<RLock> iterator = this.locks.listIterator();

    while(iterator.hasNext()) {
        RLock lock = (RLock)iterator.next();

        boolean lockAcquired;
        try {
            //没有等待时间和释放时间，调用空参的tryLock
            if (waitTime == -1L && leaseTime == -1L) {
                lockAcquired = lock.tryLock();
            } else {
                //否则调用带参的tryLock
                long awaitTime = Math.min(lockWaitTime, remainTime);
                lockAcquired = lock.tryLock(awaitTime, newLeaseTime, TimeUnit.MILLISECONDS);
            }
        } catch (RedisResponseTimeoutException var21) {
            this.unlockInner(Arrays.asList(lock));
            lockAcquired = false;
        } catch (Exception var22) {
            lockAcquired = false;
        }
        //判断获取锁是否成功
        if (lockAcquired) {
            //成功则将锁放入成功锁的集合
            acquiredLocks.add(lock);
        } else {
            //如果获取锁失败
            //判断当前锁的数量，减去成功获取锁的数量，如果为0，则所有锁都成功获取，跳出循环
            if (this.locks.size() - acquiredLocks.size() == this.failedLocksLimit()) {
                break;
            }
            //否则将拿到的锁都释放掉
            if (failedLocksLimit == 0) {
                this.unlockInner(acquiredLocks);
                //如果等待时间为-1，则不想重试，直接返回false
                if (waitTime == -1L) {
                    return false;
                }

                failedLocksLimit = this.failedLocksLimit();
                //将已经拿到的锁都清空
                acquiredLocks.clear();
                //将迭代器往前迭代，相当于重置指针，放到第一个然后重试获取锁
                while(iterator.hasPrevious()) {
                    iterator.previous();
                }
            } else {
                --failedLocksLimit;
            }
        }
        //如果剩余时间不为-1，很充足
        if (remainTime != -1L) {
            //计算现在剩余时间
            remainTime -= System.currentTimeMillis() - time;
            time = System.currentTimeMillis();
            //如果剩余时间为负数，则获取锁超时了
            if (remainTime <= 0L) {
                //将之前已经获取到的锁释放掉，并返回false
                this.unlockInner(acquiredLocks);
                //联锁成功的条件是：每一把锁都必须成功获取，一把锁失败，则都失败
                return false;
            }
        }
    }
    //如果设置了锁的有效期
    if (leaseTime != -1L) {
        List<RFuture<Boolean>> futures = new ArrayList(acquiredLocks.size());
        //迭代器用于遍历已经获取成功的锁
        Iterator var24 = acquiredLocks.iterator();

        while(var24.hasNext()) {
            RLock rLock = (RLock)var24.next();
            //设置每一把锁的有效期
            RFuture<Boolean> future = ((RedissonLock)rLock).expireAsync(unit.toMillis(leaseTime), TimeUnit.MILLISECONDS);
            futures.add(future);
        }

        var24 = futures.iterator();

        while(var24.hasNext()) {
            RFuture<Boolean> rFuture = (RFuture)var24.next();
            rFuture.syncUninterruptibly();
        }
    }
    //但如果没设置有效期，则会触发WatchDog机制，自动帮我们设置有效期，所以大多数情况下，我们不需要自己设置有效期
    return true;
}