分布式锁—2.Redisson的可重入锁二

大纲

1.Redisson可重入锁RedissonLock概述

2.可重入锁源码之创建RedissonClient实例

3.可重入锁源码之lua脚本加锁逻辑

4.可重入锁源码之WatchDog维持加锁逻辑

5.可重入锁源码之可重入加锁逻辑

6.可重入锁源码之锁的互斥阻塞逻辑

7.可重入锁源码之释放锁逻辑

8.可重入锁源码之获取锁超时与锁超时自动释放逻辑

9.可重入锁源码总结

4.可重入锁源码之WatchDog维持加锁逻辑

(1)异步执行完加锁的lua脚本会触发执行thenApply()里的回调

(2)根据执行加锁lua脚本的返回值来决定是否创建定时调度任务

(3)定时调度任务由Netty的时间轮机制来实现

(4)10秒后会执行定时任务并判断是否要再创建一个定时任务在10秒后执行

如果某个客户端上锁后，过了几分钟都没释放掉这个锁，而锁对应的key一开始的过期时间其实只设置了30秒而已，那么在这种场景下这个锁就不能在上锁的30秒后被自动释放。

RedissonLock中有一个WatchDog机制：

当客户端对myLock这个key成功加锁后，就会创建一个定时任务，这个定时任务会默认10秒后更新myLock这个key的过期时间为30秒。当然，前提是客户端一直持有myLock这个锁，还没有对锁进行释放。只要客户端一直持有myLock这个锁没有对其进行释放，那么就会不断创建一个定时任务，10秒后更新myLock这个key的过期时间。

下面详细介绍加锁成功后的定时任务，是如何每隔10秒去更新锁的过期时间的。

(1)异步执行完加锁的lua脚本会触发执行thenApply()里的回调

异步执行完RedissonLock.tryLockInnerAsync()方法里的加锁lua脚本内容后，会触发执行ttlRemainingFuture的thenApply()方法里的回调。其中加锁lua脚本会返回Long型的ttlRemaining变量，ttlRemainingFuture的thenApply()方法里的回调入参便是这个变量。

具体会先在RedissonLock的tryAcquireAsync()方法中，定义一个RFuture类型的名为ttlRemainingFuture的变量。这个变量封装了当前这个锁对应的key的剩余存活时间，单位毫秒，这个剩余存活时间是在执行完加锁lua脚本时返回的。

然后通过RFuture的thenApply()方法给ttlRemainingFuture添加一个回调。这样当lua脚本执行完成后，就会触发ttlRemainingFuture的thenApply()方法里的回调，而回调里的入参ttlRemaining就是执行加锁lua脚本的返回值。

public class RedissonLock extends RedissonBaseLock {protected long internalLockLeaseTime;protected final LockPubSub pubSub;final CommandAsyncExecutor commandExecutor;public RedissonLock(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;//与WatchDog有关的internalLockLeaseTime//通过命令执行器CommandExecutor可以获取连接管理器ConnectionManager//通过连接管理器ConnectionManager可以获取Redis的配置信息类Config//通过Redis的配置信息类Config可以获取lockWatchdogTimeout超时时间this.internalLockLeaseTime = commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout();this.pubSub = commandExecutor.getConnectionManager().getSubscribeService().getLockPubSub();}//加锁@Overridepublic void lock() {try {lock(-1, null, false);} catch (InterruptedException e) {throw new IllegalStateException();}}...private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {//线程ID，用来生成设置Hash的值long threadId = Thread.currentThread().getId();//尝试加锁，此时执行RedissonLock.lock()方法默认传入的leaseTime=-1Long ttl = tryAcquire(-1, leaseTime, unit, threadId);//ttl为null说明加锁成功if (ttl == null) {return;}      //加锁失败时的处理...}...private Long tryAcquire(long waitTime, long leaseTime, TimeUnit unit, long threadId) {//默认下waitTime和leaseTime都是-1，下面调用的get方法是来自于RedissonObject的get()方法//可以理解为异步转同步：将异步的tryAcquireAsync通过get转同步return get(tryAcquireAsync(waitTime, leaseTime, unit, threadId));}private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {RFuture<Long> ttlRemainingFuture;if (leaseTime != -1) {ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} else {//默认情况下，由于leaseTime=-1，所以会使用初始化RedissonLock实例时的internalLockLeaseTime//internalLockLeaseTime的默认值就是lockWatchdogTimeout的默认值，30秒ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime, TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);}CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {//异步执行完tryLockInnerAsync()里加锁lua脚本内容后//就会触发执行ttlRemainingFuture.thenApply()里的回调//加锁返回的ttlRemaining为null表示加锁成功if (ttlRemaining == null) {if (leaseTime != -1) {internalLockLeaseTime = unit.toMillis(leaseTime);} else {//传入加锁成功的线程ID，启动一个定时任务scheduleExpirationRenewal(threadId);}}return ttlRemaining;});return new CompletableFutureWrapper<>(f);}//默认情况下，外部传入的leaseTime=-1时，会取lockWatchdogTimeout的默认值=30秒<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {return evalWriteAsync(getRawName(), LongCodec.INSTANCE, command,"if (redis.call('exists', KEYS[1]) == 0) then " +"redis.call('hincrby', 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.singletonList(getRawName()),//锁的名字：KEYS[1]unit.toMillis(leaseTime),//过期时间：ARGV[1]，默认时为30秒getLockName(threadId)//ARGV[2]，值为UUID + 线程ID);}...
}

(2)根据执行加锁lua脚本的返回值来决定是否创建定时调度任务

当执行加锁lua脚本的返回值ttlRemaining为null时，则表明锁获取成功。

如果锁获取成功，且指定锁的过期时间，即leaseTime不是默认的-1，那么此时并不会创建定时任务。如果锁获取成功，且没指定锁的过期时间，即leaseTime是默认的-1，那么此时才会创建定时调度任务，并且是根据Netty的时间轮来实现创建。

所以调用scheduleExpirationRenewal()方法会创建一个定时调度任务，只要锁还被客户端持有，定时任务就会不断更新锁对应的key的过期时间。

public class RedissonLock extends RedissonBaseLock {...private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {RFuture<Long> ttlRemainingFuture;if (leaseTime != -1) {ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} else {ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime, TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);}CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {//异步执行完tryLockInnerAsync()里的内容后，就会执行ttlRemainingFuture.thenApply()里的逻辑//加锁返回的ttlRemaining为null表示加锁成功if (ttlRemaining == null) {if (leaseTime != -1) {//如果指定了锁的过期时间，并不会启动定时任务internalLockLeaseTime = unit.toMillis(leaseTime);} else {//传入加锁成功的线程ID，启动一个定时任务scheduleExpirationRenewal(threadId);}}return ttlRemaining;});return new CompletableFutureWrapper<>(f);}...
}

(3)定时调度任务由Netty的时间轮机制来实现

Redisson的WatchDog机制底层并不是调度线程池，而是Netty时间轮。

scheduleExpirationRenewal()方法会创建一个定时调度任务TimerTask交给HashedWheelTimer，10秒后执行。

public abstract class RedissonBaseLock extends RedissonExpirable implements RLock {...protected void scheduleExpirationRenewal(long threadId) {ExpirationEntry entry = new ExpirationEntry();ExpirationEntry oldEntry = EXPIRATION_RENEWAL_MAP.putIfAbsent(getEntryName(), entry);if (oldEntry != null) {oldEntry.addThreadId(threadId);} else {entry.addThreadId(threadId);try {//创建一个更新过期时间的定时调度任务renewExpiration();} finally {if (Thread.currentThread().isInterrupted()) {cancelExpirationRenewal(threadId);}}}}//更新过期时间private void renewExpiration() {ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (ee == null) {return;}//使用了Netty的定时任务机制：HashedWheelTimer + TimerTask + Timeout//创建一个更新过期时间的定时调度任务，下面会调用MasterSlaveConnectionManager.newTimeout()方法//即创建一个定时调度任务TimerTask交给HashedWheelTimer，10秒后执行Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {@Overridepublic void run(Timeout timeout) throws Exception {...}}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);ee.setTimeout(task);}...
}public class MasterSlaveConnectionManager implements ConnectionManager {private HashedWheelTimer timer;//Netty的时间轮...@Overridepublic Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {try {//给Netty的时间轮添加任务taskreturn timer.newTimeout(task, delay, unit);} catch (IllegalStateException e) {if (isShuttingDown()) {return DUMMY_TIMEOUT;}throw e;}}...
}//Netty的时间轮HashedWheelTimer
public class HashedWheelTimer implements Timer {private final HashedWheelBucket[] wheel;private final Queue<HashedWheelTimeout> timeouts = PlatformDependent.newMpscQueue();private final Executor taskExecutor;...@Overridepublic Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {...//启动Worker线程start();...//封装定时调度任务TimerTask实例到HashedWheelTimeout实例HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);timeouts.add(timeout);return timeout;}...private final class Worker implements Runnable {private long tick;//轮次...@Overridepublic void run() {...do {final long deadline = waitForNextTick();if (deadline > 0) {int idx = (int) (tick & mask);processCancelledTasks();HashedWheelBucket bucket = wheel[idx];//将定时调度任务HashedWheelTimeout转换到时间轮分桶里transferTimeoutsToBuckets();//处理时间轮分桶HashedWheelBucket里的到期任务bucket.expireTimeouts(deadline);tick++;}} while (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_STARTED);...}private void transferTimeoutsToBuckets() {for (int i = 0; i < 100000; i++) {HashedWheelTimeout timeout = timeouts.poll();...long calculated = timeout.deadline / tickDuration;timeout.remainingRounds = (calculated - tick) / wheel.length;final long ticks = Math.max(calculated, tick); // Ensure we don't schedule for past.int stopIndex = (int) (ticks & mask);//对mask取模HashedWheelBucket bucket = wheel[stopIndex];bucket.addTimeout(timeout);}}...}private static final class HashedWheelBucket {...public void expireTimeouts(long deadline) {HashedWheelTimeout timeout = head;//process all timeoutswhile (timeout != null) {...//对定时调度任务timeout进行过期处理timeout.expire();...}}...}private static final class HashedWheelTimeout implements Timeout, Runnable {private final HashedWheelTimer timer;...public void expire() {...//执行定时调度任务，让Executor执行HashedWheelTimeout的run方法timer.taskExecutor.execute(this);...}@Overridepublic void run() {...//执行定时调度任务task.run(this);...}...}...
}

(4)10秒后会执行定时任务并判断是否要再创建一个定时任务在10秒后执行

该定时任务TimerTask实例会调用RedissonBaseLock的renewExpirationAsync()方法去执行lua脚本，renewExpirationAsync()方法会传入获取到锁的线程ID。

在lua脚本中，会通过Redis的hexists命令判断在key为锁名的Hash值里，field为UUID + 线程ID的映射是否存在。其中KEYS[1]就是锁的名字如myLock，ARGV[2]为UUID + 线程ID。

如果存在，说明获取锁的线程还在持有锁，并没有对锁进行释放。那么就通过Redis的pexpire命令，设置key的过期时间为30秒。

异步执行lua脚本完后，会传入布尔值触发future.whenComplete()方法的回调，回调中会根据布尔值来决定是否继续递归调用renewExpiration()方法。

也就是说，如果获取锁的线程还在持有锁，那么就重置锁的过期时间为30秒，并且lua脚本会返回1，接着在future.whenComplete()方法的回调中，继续调用RedissonBaseLock的renewExpiration()方法重新创建定时调度任务。

如果获取锁的线程已经释放了锁，那么lua脚本就会返回0，接着在future.whenComplete()方法的回调中，会调用RedissonBaseLock的cancelExpirationRenewal()方法执行清理工作。

public abstract class RedissonBaseLock extends RedissonExpirable implements RLock {...protected void scheduleExpirationRenewal(long threadId) {ExpirationEntry entry = new ExpirationEntry();ExpirationEntry oldEntry = EXPIRATION_RENEWAL_MAP.putIfAbsent(getEntryName(), entry);if (oldEntry != null) {oldEntry.addThreadId(threadId);} else {entry.addThreadId(threadId);try {//创建一个更新过期时间的定时调度任务renewExpiration();} finally {if (Thread.currentThread().isInterrupted()) {cancelExpirationRenewal(threadId);}}}}//更新过期时间private void renewExpiration() {ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (ee == null) {return;}//使用了Netty的定时任务机制：HashedWheelTimer + TimerTask + Timeout//创建一个更新过期时间的定时调度任务，下面会调用MasterSlaveConnectionManager.newTimeout()方法//即创建一个定时调度任务TimerTask交给HashedWheelTimer，10秒后执行Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {@Overridepublic 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;}//异步执行lua脚本去更新锁的过期时间RFuture<Boolean> future = renewExpirationAsync(threadId);future.whenComplete((res, e) -> {if (e != null) {log.error("Can't update lock " + getRawName() + " expiration", e);EXPIRATION_RENEWAL_MAP.remove(getEntryName());return;}//res就是执行renewExpirationAsync()里的lua脚本的返回值if (res) {//重新调度自己renewExpiration();} else {//执行清理工作cancelExpirationRenewal(null);}});}}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);ee.setTimeout(task);}protected RFuture<Boolean> renewExpirationAsync(long threadId) {//其中KEYS[1]就是锁的名字如myLock，ARGV[2]为UUID+线程ID；return evalWriteAsync(getRawName(), 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.singletonList(getRawName()),internalLockLeaseTime, getLockName(threadId));}}protected void cancelExpirationRenewal(Long threadId) {ExpirationEntry task = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (task == null) {return;}if (threadId != null) {task.removeThreadId(threadId);}if (threadId == null || task.hasNoThreads()) {Timeout timeout = task.getTimeout();if (timeout != null) {timeout.cancel();}EXPIRATION_RENEWAL_MAP.remove(getEntryName());}}...
}

注意：如果持有锁的机器宕机了，就会导致该机器上的锁的WatchDog不执行了。从而锁的key会在30秒内自动过期，释放掉锁。此时其他客户端最多再等30秒就可获取到这把锁了。

5.可重入锁源码之可重入加锁逻辑

(1)核心一是key的命名：第一层key是锁名，第二层field是UUID + 线程ID

(2)核心二是Redis的exists命令、hexists命令、hincrby命令

public class Application {public static void main(String[] args) throws Exception {Config config = new Config();config.useClusterServers().addNodeAddress("redis://192.168.1.110:7001");//创建RedissonClient实例RedissonClient redisson = Redisson.create(config);//获取可重入锁RLock lock = redisson.getLock("myLock");//首次加锁lock.lock();//重入加锁lock.lock();//重入加锁lock.lock();lock.unlock();lock.unlock();lock.unlock();...}
}public class RedissonLock extends RedissonBaseLock {...<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {return evalWriteAsync(getRawName(), LongCodec.INSTANCE, command,"if (redis.call('exists', KEYS[1]) == 0) then " +"redis.call('hincrby', 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.singletonList(getRawName()),//锁的名字：KEYS[1]unit.toMillis(leaseTime),//过期时间：ARGV[1]，默认时为30秒getLockName(threadId)//ARGV[2]，值为UUID + 线程ID);}...
}

实现可重入加锁的核心就是加锁的lua脚本。

(1)核心一是key的命名：第一层key是锁名，第二层field是UUID+线程ID

KEYS[1]的值就是锁的名字，ARGV[2]的值就是客户端UUID + 线程ID。由于第二层field包含了线程ID，所以可以区分申请加锁的线程是否在重入。对在key为锁名的Hash值中，field为UUID + 线程ID的value值进行递增1操作。

(2)核心二是Redis的exists命令、hexists命令、hincrby命令

判断锁是否存在，使用的是Redis的exists命令。判断锁是否正在被线程重入，则使用的是Redis的hexists命令。对field映射的value值进行递增，使用的是Redis的hincrby命令。

所以，持有锁的线程每重入锁一次，就会对field映射的value值递增1。比如：hincrby key UUID:ThreadID 1。

6.可重入锁源码之锁的互斥阻塞逻辑

(1)获取锁失败时会执行Redis的pttl命令返回锁的剩余存活时间

(2)ttlRemainingFuture的回调方法发现ttlRemaining不是null

(3)RedissonLock的tryAcquire()方法会返回这个剩余存活时间ttl

(4)RedissonLock的lock()方法会对tryAcquire()方法返回的ttl进行判断

(5)RedissonLock的lock()方法在ttl不为null时利用Semaphore进行阻塞

(1)获取锁失败时会执行Redis的pttl命令返回锁的剩余存活时间

在执行加锁的lua脚本中：首先通过"exists myLock"，判断是否存在myLock这个锁key，发现已经存在。接着判断"hexists myLock 另一个UUID:另一个Thread的ID"，发现不存在。于是执行"pttl myLock"并返回，也就是返回myLock这个锁的剩余存活时间。

(2)ttlRemainingFuture的回调方法发现ttlRemaining不是null

当执行加锁的lua脚本发现加锁不成功，返回锁的剩余存活时间时，ttlRemainingFuture的回调方法发现ttlRemaining不是null，于是便不会创建定时调度任务在10秒之后去检查锁是否还被申请的线程持有，而是会将返回的剩余存活时间封装到RFuture中并向上返回。

(3)RedissonLock的tryAcquire()方法会返回这个剩余存活时间ttl

主要会通过RedisObject的get()方法去获取RFuture中封装的ttl值。其中异步转同步就是将异步的tryAcquireAsync()方法通过get()方法转同步。

(4)RedissonLock的lock()方法会对tryAcquire()方法返回的ttl进行判断

如果当前线程是第一次加锁，那么ttl一定是null。如果当前线程是多次加锁(可重入锁)，那么ttl也一定是null。如果当前线程加锁没成功，锁被其他机器或线程占用，那么ttl就是执行加锁lua脚本时获取到的锁对应的key的剩余存活时间。

如果ttl是null，说明当前线程加锁成功，于是直接返回。如果ttl不是null，说明当前线程加锁不成功，于是会执行阻塞逻辑。

(5)RedissonLock的lock()方法在ttl不为null时利用Semaphore进行阻塞

如果ttl不为null，即加锁不成功，那么会进入while(true)死循环。在死循环内，再次执行RedissonLock的tryAcquire()方法尝试获取锁。如果再次获取锁时返回的ttl为null，即获取到锁，就退出while循环。如果再次获取锁时返回的ttl不为null，则说明其他客户端或线程还持有锁。

于是就利用同步组件Semaphore进行阻塞等待一段ttl的时间。阻塞等待一段时间后，会继续执行while循环里的逻辑，再次尝试获取锁。以此循环往复，直到获得锁。

public class RedissonLock extends RedissonBaseLock {protected long internalLockLeaseTime;protected final LockPubSub pubSub;final CommandAsyncExecutor commandExecutor;public RedissonLock(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;//与WatchDog有关的internalLockLeaseTime//通过命令执行器CommandExecutor可以获取连接管理器ConnectionManager//通过连接管理器ConnectionManager可以获取Redis的配置信息类Config//通过Redis的配置信息类Config可以获取lockWatchdogTimeout超时时间this.internalLockLeaseTime = commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout();this.pubSub = commandExecutor.getConnectionManager().getSubscribeService().getLockPubSub();}...//加锁@Overridepublic void lock() {try {lock(-1, null, false);} catch (InterruptedException e) {throw new IllegalStateException();}}private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {//线程ID，用来生成设置Hash的值long threadId = Thread.currentThread().getId();//尝试加锁，此时执行RedissonLock.lock()方法默认传入的leaseTime=-1Long ttl = tryAcquire(-1, leaseTime, unit, threadId);//ttl为null说明加锁成功if (ttl == null) {return;}//加锁失败时的处理CompletableFuture<RedissonLockEntry> future = subscribe(threadId);if (interruptibly) {commandExecutor.syncSubscriptionInterrupted(future);} else {commandExecutor.syncSubscription(future);}try {while (true) {//再次尝试获取锁ttl = tryAcquire(-1, leaseTime, unit, threadId);//返回的ttl为null，获取到锁，就退出while循环if (ttl == null) {break;}//返回的ttl不为null，则说明其他客户端或线程还持有锁//那么就利用同步组件Semaphore进行阻塞等待一段ttl的时间if (ttl >= 0) {try {commandExecutor.getNow(future).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);} catch (InterruptedException e) {if (interruptibly) {throw e;}commandExecutor.getNow(future).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);}} else {if (interruptibly) {commandExecutor.getNow(future).getLatch().acquire();} else {commandExecutor.getNow(future).getLatch().acquireUninterruptibly();}}}} finally {unsubscribe(commandExecutor.getNow(future), threadId);}}...private Long tryAcquire(long waitTime, long leaseTime, TimeUnit unit, long threadId) {//默认下waitTime和leaseTime都是-1，下面调用的get方法是来自于RedissonObject的get()方法//可以理解为异步转同步：将异步的tryAcquireAsync通过get转同步return get(tryAcquireAsync(waitTime, leaseTime, unit, threadId));}private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {RFuture<Long> ttlRemainingFuture;if (leaseTime != -1) {ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} else {//默认情况下，由于leaseTime=-1，所以会使用初始化RedissonLock实例时的internalLockLeaseTime//internalLockLeaseTime的默认值就是lockWatchdogTimeout的默认值，30秒ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime, TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);}CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {//加锁返回的ttlRemaining为null表示加锁成功if (ttlRemaining == null) {if (leaseTime != -1) {internalLockLeaseTime = unit.toMillis(leaseTime);} else {scheduleExpirationRenewal(threadId);}}return ttlRemaining;});return new CompletableFutureWrapper<>(f);}//默认情况下，外部传入的leaseTime=-1时，会取lockWatchdogTimeout的默认值=30秒<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {return evalWriteAsync(getRawName(), LongCodec.INSTANCE, command,"if (redis.call('exists', KEYS[1]) == 0) then " +"redis.call('hincrby', 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.singletonList(getRawName()),//锁的名字：KEYS[1]unit.toMillis(leaseTime),//过期时间：ARGV[1]，默认时为30秒getLockName(threadId)//ARGV[2]，值为UUID + 线程ID);}...
}public class RedissonLockEntry implements PubSubEntry<RedissonLockEntry> {private final Semaphore latch;...public Semaphore getLatch() {return latch;}...
}

7.可重入锁源码之释放锁逻辑

(1)宕机自动释放锁

(2)线程主动释放锁的流程

(3)主动释放锁的lua脚本分析

(1)宕机自动释放锁

如果获取锁的所在机器宕机，那么10秒后检查锁是否还在被线程持有的定时调度任务就没了。于是锁对应Redis里的key，就会在最多30秒内进行过期删除。之后，其他的客户端就能成功获取锁了。

当然，前提是创建锁的时候没有设置锁的存活时间leaseTime。如果指定了leaseTime，那么就不会存在10秒后检查锁的定时调度任务。此时获取锁的所在机器宕机，那么锁对应的key会在最多leaseTime后过期。

(2)线程主动释放锁的流程

主动释放锁调用的是RedissonLock的unlock()方法。在RedissonLock的unlock()方法中，会调用get(unlockAsync())方法。也就是首先调用RedissonBaseLock的unlockAsync()方法，然后再调用RedissonObject的get()方法。

其中unlockAsync()方法是异步化执行的方法，释放锁的操作就是异步执行的。而RedisObject的get()方法会通过RFuture同步等待获取异步执行的结果，可以将get(unlockAsync())理解为异步转同步。

在RedissonBaseLock的unlockAsync()方法中：首先会调用RedissonLock的unlockInnerAsync()方法进行异步释放锁，然后当完成释放锁的处理后，再通过异步去取消定时调度任务。

public class Application {public static void main(String[] args) throws Exception {Config config = new Config();config.useClusterServers().addNodeAddress("redis://192.168.1.110:7001");//创建RedissonClient实例RedissonClient redisson = Redisson.create(config);//获取可重入锁RLock lock = redisson.getLock("myLock");lock.lock();//获取锁lock.unlock();//释放锁...}
}public class RedissonLock extends RedissonBaseLock {...@Overridepublic void unlock() {...//异步转同步//首先调用的是RedissonBaseLock的unlockAsync()方法//然后调用的是RedissonObject的get()方法get(unlockAsync(Thread.currentThread().getId()));...}//异步执行释放锁的lua脚本protected RFuture<Boolean> unlockInnerAsync(long threadId) {return evalWriteAsync(getRawName(), 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.asList(getRawName(), getChannelName()),//KEYS[1] + KEYS[2]LockPubSub.UNLOCK_MESSAGE,//ARGV[1]internalLockLeaseTime,//ARGV[2]getLockName(threadId)//ARGV[3]);}...
}public abstract class RedissonBaseLock extends RedissonExpirable implements RLock {...@Overridepublic RFuture<Void> unlockAsync(long threadId) {//异步执行释放锁的lua脚本RFuture<Boolean> future = unlockInnerAsync(threadId);CompletionStage<Void> f = future.handle((opStatus, e) -> {//取消定时调度任务cancelExpirationRenewal(threadId);if (e != null) {throw new CompletionException(e);}if (opStatus == null) {IllegalMonitorStateException cause = new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: " + id + " thread-id: " + threadId);throw new CompletionException(cause);}return null;});return new CompletableFutureWrapper<>(f);}...
}abstract class RedissonExpirable extends RedissonObject implements RExpirable {...RedissonExpirable(CommandAsyncExecutor connectionManager, String name) {super(connectionManager, name);}...
}public abstract class RedissonObject implements RObject {protected final CommandAsyncExecutor commandExecutor;protected String name;protected final Codec codec;public RedissonObject(Codec codec, CommandAsyncExecutor commandExecutor, String name) {this.codec = codec;this.commandExecutor = commandExecutor;if (name == null) {throw new NullPointerException("name can't be null");}setName(name);}...protected final <V> V get(RFuture<V> future) {//下面会调用CommandAsyncService.get()方法return commandExecutor.get(future);}...
}public class CommandAsyncService implements CommandAsyncExecutor {...@Overridepublic <V> V get(RFuture<V> future) {if (Thread.currentThread().getName().startsWith("redisson-netty")) {throw new IllegalStateException("Sync methods can't be invoked from async/rx/reactive listeners");}try {return future.toCompletableFuture().get();} catch (InterruptedException e) {future.cancel(true);Thread.currentThread().interrupt();throw new RedisException(e);} catch (ExecutionException e) {throw convertException(e);}}...
}

(3)主动释放锁的lua脚本分析

首先判断在key为锁名的Hash值中，field为UUID + 线程ID的映射是否存在。如果不存在，则表示锁已经被释放了，直接返回。如果存在，则在key为锁名的Hash值中对field为UUID + 线程ID的value值递减1。即调用Redis的hincrby命令，进行递减1处理。

接着对递减1后的结果进行如下判断：如果递减1后的结果大于0，则表示线程还在持有锁。这对应于持有锁的线程多次重入锁，此时需要重置锁的过期时间为30秒。如果递减1后的结果小于0，则表示线程不再持有锁，于是删除锁对应的key，并且通过Redis的publish命令，发布一个事件。

public class RedissonLock extends RedissonBaseLock {...//异步执行释放锁的lua脚本protected RFuture<Boolean> unlockInnerAsync(long threadId) {return evalWriteAsync(getRawName(), 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.asList(getRawName(), getChannelName()),//KEYS[1] + KEYS[2]LockPubSub.UNLOCK_MESSAGE,//ARGV[1]internalLockLeaseTime,//ARGV[2]getLockName(threadId)//ARGV[3]);}...
}

8.可重入锁源码之获取锁超时与锁超时自动释放逻辑

(1)尝试获取锁超时

(2)锁超时自动释放

针对如下代码方式去获取锁，如果超过60秒获取不到锁，就自动放弃获取锁，不会进行永久性阻塞。如果获取到锁之后，锁在10秒内没有被主动释放，那么就会自动释放锁。

public class Application {public static void main(String[] args) throws Exception {Config config = new Config();config.useClusterServers().addNodeAddress("redis://192.168.1.110:7001");//创建RedissonClient实例RedissonClient redisson = Redisson.create(config);//获取可重入锁RLock lock = redisson.getLock("myLock");boolean res = lock.tryLock(60, 10, TimeUnit.SECONDS);//尝试获取锁if (res) lock.unlock();//尝试获取锁成功，则释放锁...}
}

(1)尝试获取锁超时

RedissonLock的tryLock()方法，会传入waitTime和leaseTime尝试获取锁。其中waitTime就是最多可以等待多少时间去获取锁，比如60秒。leaseTime就是获取锁后锁的自动过期时间，比如10秒。

如果第一次获取锁的时间超过了等待获取锁的最大时间waitTime，那么就会返回false。

如果第一次获取锁的时间还没超过等待获取锁的最大时间waitTime，那么就进入while循环，不断地再次尝试获取锁。

如果再次尝试获取锁成功，那么就返回true。如果再次尝试获取锁失败，那么计算还剩下多少时间可以继续等待获取锁。也就是使用time去自减每次尝试获取锁的耗时以及自减每次等待的耗时。

如果发现time小于0，表示等待了waitTime都无法获取锁，则返回false。如果发现time大于0，继续下一轮while循环，尝试获取锁 + time自减耗时。

public class RedissonLock extends RedissonBaseLock {...@Overridepublic boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {long time = unit.toMillis(waitTime);//最多可以等待多少时间去获取锁long current = System.currentTimeMillis();//current是第一次尝试获取锁之前的时间戳long threadId = Thread.currentThread().getId();Long ttl = tryAcquire(waitTime, leaseTime, unit, threadId);//尝试获取锁成功if (ttl == null) {return true;}//第一次尝试获取锁失败//当前时间减去第一次获取锁之前的时间戳，就是这次尝试获取锁的耗费时间，使用time进行自减time -= System.currentTimeMillis() - current;//如果第一次获取锁的时间超过了waitTime等待获取锁的最大时间，那么就会直接返回获取锁失败if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}//如果第一次获取锁的时间还没达到waitTime等待获取锁的最大时间current = System.currentTimeMillis();CompletableFuture<RedissonLockEntry> subscribeFuture = subscribe(threadId);try {subscribeFuture.toCompletableFuture().get(time, TimeUnit.MILLISECONDS);} catch (ExecutionException | TimeoutException e) {if (!subscribeFuture.cancel(false)) {subscribeFuture.whenComplete((res, ex) -> {if (ex == null) {unsubscribe(res, threadId);}});}acquireFailed(waitTime, unit, threadId);return false;}try {time -= System.currentTimeMillis() - current;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}while (true) {long currentTime = System.currentTimeMillis();ttl = tryAcquire(waitTime, leaseTime, unit, threadId);//再次尝试获取锁成功if (ttl == null) {return true;}//剩余可用等待的时间，使用time去自减每次尝试获取锁的耗时time -= System.currentTimeMillis() - currentTime;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}//返回的ttl不为null，则说明其他客户端或线程还持有锁//那么就利用同步组件Semaphore进行阻塞等待一段ttl的时间currentTime = System.currentTimeMillis();if (ttl >= 0 && ttl < time) {commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);} else {commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);}//使用time去自减每次等待的耗时time -= System.currentTimeMillis() - currentTime;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}}} finally {unsubscribe(commandExecutor.getNow(subscribeFuture), threadId);}}...
}

(2)锁超时自动释放

当使用RedissonLock.tryAcquire方法尝试获取锁时，传入的leaseTime不是-1，而是一个指定的锁存活时间，那么锁超时就会自动被释放。

当leaseTime不是-1时：

一.锁的过期时间就不是lockWatchdogTimeout=30秒，而是leaseTime

二.执行加锁lua脚本成功后，不会创建一个定时调度任务在10秒后检查锁

总结：当指定了leaseTime后，那么锁最多在leaseTime后，就必须被删除。因为此时没有定时调度任务去检查锁释放还在被线程持有并更新过期时间。所以，锁要么被主动删除，要么在存活leaseTime后自动过期。

public class RedissonLock extends RedissonBaseLock {...private Long tryAcquire(long waitTime, long leaseTime, TimeUnit unit, long threadId) {//默认下waitTime和leaseTime都是-1，下面调用的get方法是来自于RedissonObject的get()方法//可以理解为异步转同步：将异步的tryAcquireAsync通过get转同步return get(tryAcquireAsync(waitTime, leaseTime, unit, threadId));}private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {RFuture<Long> ttlRemainingFuture;if (leaseTime != -1) {ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} else {//默认情况下，由于leaseTime=-1，所以会使用初始化RedissonLock实例时的internalLockLeaseTime//internalLockLeaseTime的默认值就是lockWatchdogTimeout的默认值，30秒ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime, TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);}CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {//加锁返回的ttlRemaining为null表示加锁成功if (ttlRemaining == null) {if (leaseTime != -1) {internalLockLeaseTime = unit.toMillis(leaseTime);} else {scheduleExpirationRenewal(threadId);}}return ttlRemaining;});return new CompletableFutureWrapper<>(f);}...
}

9.可重入锁源码总结

(1)加锁

(2)WatchDog维持加锁

(3)可重入锁

(4)锁互斥

(5)手动释放锁

(6)宕机自动释放锁

(7)尝试加锁超时

(8)超时锁自动释放

(1)加锁

在Redis里设置两层Hash数据结构，默认的过期时间是30秒。第一层Hash值的key是锁名，第二层Hash值的key是UUID + 线程ID。涉及Redis的exists命令、hexists命令、hincrby命令。

(2)WatchDog维持加锁