基于Redisson的联锁(MultiLock)

基于Redis的分布式MultiLock对象允许对Lock对象进行分组并将它们作为单个锁进行处理。每个RLock对象可能属于不同的Redisson实例。

如果获取的Redisson实例MultiLock崩溃，那么它可能永远挂在获取状态。为了避免这种情况，Redisson维护了一个锁看门狗，它会在持有者Redisson实例处于活动状态时延长锁过期时间。默认情况下，锁定看门狗超时为30s，可以通过Config.lockWatchdogTimeout设置进行更改。作者的另外一篇文章有对看门狗机制有解析：基于Redisson的可重入分布式锁

leaseTime：在指定的时间间隔后锁将自动释放

MultiLock对象的行为符合java锁规范。这意味着只有锁的拥有者线程才能解锁它，否则会抛出IllegalMonitorStateException异常。否则考虑使用RSemaphore对象。

使用示例

普通使用示例：

RLock lock1 = redisson1.getLock("lock1");
RLock lock2 = redisson2.getLock("lock2");
RLock lock3 = redisson3.getLock("lock3");RLock multiLock = anyRedisson.getMultiLock(lock1, lock2, lock3);// traditional lock method
multiLock.lock();// or acquire lock and automatically unlock it after 10 seconds
multiLock.lock(10, TimeUnit.SECONDS);// or wait for lock aquisition up to 100 seconds 
// and automatically unlock it after 10 seconds
boolean res = multiLock.tryLock(100, 10, TimeUnit.SECONDS);
if (res) {
try {
...
} finally {
multiLock.unlock();
}
}

Async接口使用的代码示例：

RLock lock1 = redisson1.getLock("lock1");
RLock lock2 = redisson2.getLock("lock2");
RLock lock3 = redisson3.getLock("lock3");RLock multiLock = anyRedisson.getMultiLock(lock1, lock2, lock3);RFuture<Void> lockFuture = multiLock.lockAsync();// or acquire lock and automatically unlock it after 10 seconds
RFuture<Void> lockFuture = multiLock.lockAsync(10, TimeUnit.SECONDS);// or wait for lock aquisition up to 100 seconds 
// and automatically unlock it after 10 seconds
RFuture<Boolean> lockFuture = multiLock.tryLockAsync(100, 10, TimeUnit.SECONDS);lockFuture.whenComplete((res, exception) -> {// ...multiLock.unlockAsync();
});

Reactive接口使用的代码示例:

RedissonReactiveClient anyRedisson = redissonClient.reactive();RLockReactive lock1 = redisson1.getLock("lock1");
RLockReactive lock2 = redisson2.getLock("lock2");
RLockReactive lock3 = redisson3.getLock("lock3");RLockReactive multiLock = anyRedisson.getMultiLock(lock1, lock2, lock3);Mono<Void> lockMono = multiLock.lock();// or acquire lock and automatically unlock it after 10 seconds
Mono<Void> lockMono = multiLock.lock(10, TimeUnit.SECONDS);// or wait for lock aquisition up to 100 seconds 
// and automatically unlock it after 10 seconds
Mono<Boolean> lockMono = multiLock.tryLock(100, 10, TimeUnit.SECONDS);lockMono.doOnNext(res -> {// ...
})
.doFinally(multiLock.unlock())
.subscribe();

RxJava3接口使用的代码示例:

RedissonRxClient anyRedisson = redissonClient.rxJava();RLockRx lock1 = redisson1.getLock("lock1");
RLockRx lock2 = redisson2.getLock("lock2");
RLockRx lock3 = redisson3.getLock("lock3");RLockRx multiLock = anyRedisson.getMultiLock(lock1, lock2, lock3);Completable lockRes = multiLock.lock();// or acquire lock and automatically unlock it after 10 seconds
Completable lockRes = multiLock.lock(10, TimeUnit.SECONDS);// or wait for lock aquisition up to 100 seconds 
// and automatically unlock it after 10 seconds
Single<Boolean> lockRes = multiLock.tryLock(100, 10, TimeUnit.SECONDS);lockRes.doOnSuccess(res -> {// ...
})
.doFinally(multiLock.unlock())
.subscribe();

源码解析(RedissonMultiLock)

• Redisson获取联锁

// 这里相对简单，就是创建了一个RLock集合，为了后续分别去获取锁
final List<RLock> locks = new ArrayList<>();
@Override
public RLock getMultiLock(RLock... locks) {return new RedissonMultiLock(locks);
}
public RedissonMultiLock(RLock... locks) {if (locks.length == 0) {throw new IllegalArgumentException("Lock objects are not defined");}this.locks.addAll(Arrays.asList(locks));
}

• 加锁

leaseTime：指定加锁的时间。超过这个时间后锁便自动解开了。

为了方便我们的源码分析，假设我们的locks的size为6。leaseTime为2s

@Override
public void lock(long leaseTime, TimeUnit unit) {try {lockInterruptibly(leaseTime, unit);} catch (InterruptedException e) {Thread.currentThread().interrupt();}
}@Override
public void lockInterruptibly(long leaseTime, TimeUnit unit) throws InterruptedException {// 基础等待时间设置为连锁数量*1500，单位是毫秒 // 6*1500=9000ms 也就是9slong baseWaitTime = locks.size() * 1500;// 设置等待时间为-1long waitTime = -1;// 如果锁释放的时间为-1，就让等待时间等于基础等待时间9s// lock的无参方法默认leaseTime=-1if (leaseTime == -1) {waitTime = baseWaitTime;} else {// 如果锁的释放时间不为-1，把leaseTime转为毫秒leaseTime = unit.toMillis(leaseTime);// 把锁的释放时间传给等待时间，如果leaseTime=2s那么waitTime也等于2swaitTime = leaseTime;if (waitTime <= 2000) {// 也就是说leaseTime即使小于2s，waitTime也会被重置为2swaitTime = 2000;} else if (waitTime <= baseWaitTime) {// 如果leaseTime大于2s，并且小于9s，将重新设置等待时间，我们暂且还不知道这个等待时间做什么用。// 如果leaseTime等于6，那么waitTime=6，此时waitTime小于9s，重新设置waitTime// 将waitTime设置为大于等于3小于6的整数。(此处不明白看下面的解释)waitTime = ThreadLocalRandom.current().nextLong(waitTime/2, waitTime);} else {// 如果leaseTime大于2s而且大于9s(baseWaitTime)，同样重新设置waitTime的值// 如果传入的leaseTime=10s，那么waitTime一开始也是10s，并且大于baseWaitTime的9s// 将waitTime设置为大于等于9s，小于10s的整数。waitTime = ThreadLocalRandom.current().nextLong(baseWaitTime, waitTime);}}while (true) {// 传入waitTime开始尝试获取锁了if (tryLock(waitTime, leaseTime, TimeUnit.MILLISECONDS)) {return;}}
}

ThreadLocalRandom.current().nextLong(origin, bound)是用于生成一个指定范围内的随机长整数。

具体解释如下：

ThreadLocalRandom.current() 返回当前线程的 ThreadLocalRandom 实例，用于生成随机数。

nextLong(origin, bound) 生成一个介于 origin（包含）和 bound（不包含）之间的随机长整型数。这意味着生成的随机数大于等于 origin，并且小于 bound。

此处为什么需要去修改waitTime的值，为什么还得整个随机数，使用baseWaitTime调整waitTime的作用是什么？

• 尝试获取锁

waitTime：表示尝试获取锁的等待时间。它指定了在尝试获取锁时最长的等待时间。

leaseTime： 指定加锁的时间。超过这个时间后锁便自动解开了。

TimeUnit：时间单位

// 假设传入的waitTime=2s leaseTime=2s
public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {// 定义了一个新的释放时间newLeaseTime=-1long newLeaseTime = -1;// 如果传入了时间的tryLock，leaseTime就不等于-1，不传默认值为-1if (leaseTime != -1) {// 将新的锁释放时间设置为waitTime的2倍，单位是毫秒，也就是4000msnewLeaseTime = unit.toMillis(waitTime)*2;}// 获取当前时间（毫秒）long time = System.currentTimeMillis();// remain==保持，先翻译为保持时间，定义为-1long remainTime = -1;if (waitTime != -1) {// 保持时间设置为waitTime，2000msremainTime = unit.toMillis(waitTime);}// calcLockWaitTime(remainTime);-->return Math.max(remainTime / locks.size(), 1);// 300ms=lockWaitTimelong lockWaitTime = calcLockWaitTime(remainTime);// return 0int failedLocksLimit = failedLocksLimit();List<RLock> acquiredLocks = new ArrayList<>(locks.size());// 循环获取锁for (ListIterator<RLock> iterator = locks.listIterator(); iterator.hasNext();) {// 获取到的redisson实例生成的锁RLock lock = iterator.next();// 锁获取标识boolean lockAcquired;try {if (waitTime == -1 && leaseTime == -1) {lockAcquired = lock.tryLock();} else {// awaitTime=300mslong awaitTime = Math.min(lockWaitTime, remainTime);// 直接去获取锁，返回true or falselockAcquired = lock.tryLock(awaitTime, newLeaseTime, TimeUnit.MILLISECONDS);}} catch (RedisResponseTimeoutException e) {// 如果发生了RedisResponseTimeoutException，会先解锁。因为这个时候不确定是否加锁成功了，所以解锁设置标识为失败。unlockInner(Arrays.asList(lock));lockAcquired = false;} catch (Exception e) {// 其他异常设置标识为falselockAcquired = false;}if (lockAcquired) {// 如果加锁成功 放入集合中acquiredLocks.add(lock);} else {// 6-当前成功的数量=0，直接退出循环，也就是说超过了最大的失败限制// 这里RedissonRedLock有重写，红锁有自己的规则if (locks.size() - acquiredLocks.size() == failedLocksLimit()) {break;}// failedLocksLimit==0，那么只要失败就进入这个逻辑if (failedLocksLimit == 0) {// 会把获取到锁的一次性解锁unlockInner(acquiredLocks);if (waitTime == -1 && leaseTime == -1) {return false;}// 重置failedLocksLimit=0failedLocksLimit = failedLocksLimit();// 清空获取到锁的集合acquiredLocks.clear();// reset iteratorwhile (iterator.hasPrevious()) {iterator.previous();}} else {// RedissonRedLock才会进入这个逻辑failedLocksLimit--;}}// 如果remainTime不为-1// remainTime=2000msif (remainTime != -1) {// 查看remainTime的剩余时间remainTime -= System.currentTimeMillis() - time;// 重置timetime = System.currentTimeMillis();// 如果保持时间也就是之前的waitTime小于0，也就是说超过了尝试获取锁时最长的等待时间，释放所有已获得的锁，并返回false，加锁失败if (remainTime <= 0) {unlockInner(acquiredLocks);return false;}}}// 如果没有超过尝试获取锁时最长等待时间，并且leaseTime不为-1if (leaseTime != -1) {// 创建了一个RFuture集合List<RFuture<Boolean>> futures = new ArrayList<>(acquiredLocks.size());for (RLock rLock : acquiredLocks) {//为每个锁设置过期时间，是一个异步的操作RFuture<Boolean> future = ((RedissonLock) rLock).expireAsync(unit.toMillis(leaseTime), TimeUnit.MILLISECONDS);futures.add(future);}for (RFuture<Boolean> rFuture : futures) {// 阻塞当前线程，同步等待每个异步操作的结果rFuture.syncUninterruptibly();}}return true;}