大纲
1.Redisson的分布式锁简单总结
2.Redisson的Semaphore简介
3.Redisson的Semaphore源码剖析
4.Redisson的CountDownLatch简介
5.Redisson的CountDownLatch源码剖析
1.Redisson的分布式锁简单总结
(1)可重入锁RedissonLock
(2)公平锁RedissonFairLock
(3)联锁MultiLock
(4)红锁RedLock
(5)读写锁之读锁RedissonReadLock和写锁RedissonWriteLock
Redisson分布式锁包括：可重入锁、公平锁、联锁、红锁、读写锁。
(1)可重入锁RedissonLock
非公平锁，最基础的分布式锁，最常用的锁。
(2)公平锁RedissonFairLock
各个客户端尝试获取锁时会排队，按照队列的顺序先后获取锁。
(3)联锁MultiLock
可以一次性加多把锁，从而实现一次性锁多个资源。
(4)红锁RedLock
RedLock相当于一把锁。虽然利用了MultiLock包裹了多个小锁，但这些小锁并不对应多个资源，而是每个小锁的key对应一个Redis实例。只要大多数的Redis实例加锁成功，就可以认为RedLock加锁成功。RedLock的健壮性要比其他普通锁要好。
但是RedLock也有一些场景无法保证正确性，当然RedLock只要求部署主库。比如客户端A尝试向5个Master实例加锁，但仅仅在3个Maste中加锁成功。不幸的是此时3个Master中有1个Master突然宕机了，而且锁key还没同步到该宕机Master的Slave上，此时Salve切换为Master。于是在这5个Master中，由于其中有一个是新切换过来的Master，所以只有2个Master是有客户端A加锁的数据，另外3个Master是没有锁的。但继续不幸的是，此时客户端B来加锁，那么客户端B就很有可能成功在没有锁数据的3个Master上加到锁，从而满足了过半数加锁的要求，最后也完成了加锁，依然发生重复加锁。
(5)读写锁之读锁RedissonReadLock和写锁RedissonWriteLock
不同客户端线程的四种加锁情况：
情况一：先加读锁再加读锁，不互斥
情况二：先加读锁再加写锁，互斥
情况三：先加写锁再加读锁，互斥
情况四：先加写锁再加写锁，互斥
同一个客户端线程的四种加锁情况：
情况一：先加读锁再加读锁，不互斥
情况二：先加读锁再加写锁，互斥
情况三：先加写锁再加读锁，不互斥
情况四：先加写锁再加写锁，不互斥
2.Redisson的Semaphore简介
(1)Redisson的Semaphore原理图
Semaphore也是Redisson支持的一种同步组件。Semaphore作为一个锁机制，可以允许多个线程同时获取一把锁。任何一个线程释放锁之后，其他等待的线程就可以尝试继续获取锁。
(2)Redisson的Semaphore使用演示
public class RedissonDemo {public static void main(String[] args) throws Exception {//连接3主3从的Redis CLusterConfig config = new Config();...//SemaphoreRedissonClient redisson = Redisson.create(config);final RSemaphore semaphore = redisson.getSemaphore("semaphore");semaphore.trySetPermits(3);for (int i = 0; i < 10; i++) {new Thread(new Runnable() {public void run() {try {System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]尝试获取Semaphore锁");semaphore.acquire();System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]成功获取到了Semaphore锁，开始工作");Thread.sleep(3000);semaphore.release();System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]释放Semaphore锁");} catch (Exception e) {e.printStackTrace();}}}).start();}}
}
3.Redisson的Semaphore源码剖析
(1)Semaphore的初始化
(2)Semaphore设置允许获取的锁数量
(3)客户端尝试获取Semaphore的锁
(4)客户端释放Semaphore的锁
(1)Semaphore的初始化
public class Redisson implements RedissonClient {//Redis的连接管理器，封装了一个Config实例protected final ConnectionManager connectionManager;//Redis的命令执行器，封装了一个ConnectionManager实例protected final CommandAsyncExecutor commandExecutor;...protected Redisson(Config config) {this.config = config;Config configCopy = new Config(config);//初始化Redis的连接管理器connectionManager = ConfigSupport.createConnectionManager(configCopy);... //初始化Redis的命令执行器commandExecutor = new CommandSyncService(connectionManager, objectBuilder);...}@Overridepublic RSemaphore getSemaphore(String name) {return new RedissonSemaphore(commandExecutor, name);}...
}public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {private final SemaphorePubSub semaphorePubSub;final CommandAsyncExecutor commandExecutor;public RedissonSemaphore(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;this.semaphorePubSub = commandExecutor.getConnectionManager().getSubscribeService().getSemaphorePubSub();}...
}(2)Semaphore设置允许获取的锁数量
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {...@Overridepublic boolean trySetPermits(int permits) {return get(trySetPermitsAsync(permits));}@Overridepublic RFuture<Boolean> trySetPermitsAsync(int permits) {RFuture<Boolean> future = commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,//执行命令"get semaphore"，获取到当前的数值"local value = redis.call('get', KEYS[1]); " +"if (value == false) then " +//然后执行命令"set semaphore 3"//设置这个信号量允许客户端同时获取锁的总数量为3"redis.call('set', KEYS[1], ARGV[1]); " +"redis.call('publish', KEYS[2], ARGV[1]); " +"return 1;" +"end;" +"return 0;",Arrays.asList(getRawName(), getChannelName()),permits);if (log.isDebugEnabled()) {future.onComplete((r, e) -> {if (r) {log.debug("permits set, permits: {}, name: {}", permits, getName());} else {log.debug("unable to set permits, permits: {}, name: {}", permits, getName());}});}return future;}...
}首先执行命令"get semaphore"，获取到当前的数值。然后执行命令"set semaphore 3"，也就是设置这个信号量允许客户端同时获取锁的总数量为3。
(3)客户端尝试获取Semaphore的锁
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {...private final SemaphorePubSub semaphorePubSub;final CommandAsyncExecutor commandExecutor;public RedissonSemaphore(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;this.semaphorePubSub = commandExecutor.getConnectionManager().getSubscribeService().getSemaphorePubSub();}@Overridepublic void acquire() throws InterruptedException {acquire(1);}@Overridepublic void acquire(int permits) throws InterruptedException {if (tryAcquire(permits)) {return;}CompletableFuture<RedissonLockEntry> future = subscribe();commandExecutor.syncSubscriptionInterrupted(future);try {while (true) {if (tryAcquire(permits)) {return;}//获取Redisson的Semaphore失败，于是便调用本地JDK的Semaphore的acquire()方法，此时当前线程会被阻塞//之后如果Redisson的Semaphore释放了锁，那么当前客户端便会通过监听订阅事件释放本地JDK的Semaphore，唤醒被阻塞的线程，继续执行while循环//注意：getLatch()返回的是JDK的Semaphore = "new Semaphore(0)" ==> (state - permits)//首先调用CommandAsyncService.getNow()方法//然后调用RedissonLockEntry.getLatch()方法//接着调用JDK的Semaphore的acquire()方法commandExecutor.getNow(future).getLatch().acquire();}} finally {unsubscribe(commandExecutor.getNow(future));}}@Overridepublic boolean tryAcquire(int permits) {//异步转同步return get(tryAcquireAsync(permits));}@Overridepublic RFuture<Boolean> tryAcquireAsync(int permits) {if (permits < 0) {throw new IllegalArgumentException("Permits amount can't be negative");}if (permits == 0) {return RedissonPromise.newSucceededFuture(true);}return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,//执行命令"get semaphore"，获取到当前值"local value = redis.call('get', KEYS[1]); "+//如果semaphore的当前值不是false，且大于客户端线程申请获取锁的数量"if (value ~= false and tonumber(value) >= tonumber(ARGV[1])) then " +//执行"decrby semaphore 1"，将信号量允许获取锁的总数量递减1"local val = redis.call('decrby', KEYS[1], ARGV[1]); " +"return 1; " +"end; " +//如果semaphore的值变为0，那么客户端就无法获取锁了，此时返回false"return 0;",Collections.<Object>singletonList(getRawName()),permits//ARGV[1]默认是1);}...
}public class CommandAsyncService implements CommandAsyncExecutor {...@Overridepublic <V> V getNow(CompletableFuture<V> future) {try {return future.getNow(null);} catch (Exception e) {return null;}}...
}public class RedissonLockEntry implements PubSubEntry<RedissonLockEntry> {private final Semaphore latch;...public RedissonLockEntry(CompletableFuture<RedissonLockEntry> promise) {super();this.latch = new Semaphore(0);this.promise = promise;}public Semaphore getLatch() {return latch;}...
}执行命令"get semaphore"，获取到semaphore的当前值。如果semaphore的当前值不是false，且大于客户端线程申请获取锁的数量。那么就执行"decrby semaphore 1"，将信号量允许获取锁的总数量递减1。
如果semaphore的值变为0，那么客户端就无法获取锁了，此时tryAcquire()方法返回false。表示获取semaphore的锁失败了，于是当前客户端线程便会通过本地JDK的Semaphore进行阻塞。
当客户端后续收到一个订阅事件把本地JDK的Semaphore进行释放后，便会唤醒阻塞线程继续while循环。在while循环中，会不断尝试获取这个semaphore的锁，如此循环往复，直到成功获取。
(4)客户端释放Semaphore的锁
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {...@Overridepublic void release() {release(1);}@Overridepublic void release(int permits) {get(releaseAsync(permits));}@Overridepublic RFuture<Void> releaseAsync(int permits) {if (permits < 0) {throw new IllegalArgumentException("Permits amount can't be negative");}if (permits == 0) {return RedissonPromise.newSucceededFuture(null);}RFuture<Void> future = commandExecutor.evalWriteAsync(getRawName(), StringCodec.INSTANCE, RedisCommands.EVAL_VOID,//执行命令"incrby semaphore 1""local value = redis.call('incrby', KEYS[1], ARGV[1]); " +"redis.call('publish', KEYS[2], value); ",Arrays.asList(getRawName(), getChannelName()),permits);if (log.isDebugEnabled()) {future.onComplete((o, e) -> {if (e == null) {log.debug("released, permits: {}, name: {}", permits, getName());}});}return future;}...
}//订阅semaphore不为0的事件，semaphore不为0时会触发执行这里的监听回调
public class SemaphorePubSub extends PublishSubscribe<RedissonLockEntry> {public SemaphorePubSub(PublishSubscribeService service) {super(service);}@Overrideprotected RedissonLockEntry createEntry(CompletableFuture<RedissonLockEntry> newPromise) {return new RedissonLockEntry(newPromise);}@Overrideprotected void onMessage(RedissonLockEntry value, Long message) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute != null) {runnableToExecute.run();}//将客户端本地JDK的Semaphore进行释放value.getLatch().release(Math.min(value.acquired(), message.intValue()));}
}//订阅锁被释放的事件，锁被释放为0时会触发执行这里的监听回调
public class LockPubSub extends PublishSubscribe<RedissonLockEntry> {public static final Long UNLOCK_MESSAGE = 0L;public static final Long READ_UNLOCK_MESSAGE = 1L;public LockPubSub(PublishSubscribeService service) {super(service);} @Overrideprotected RedissonLockEntry createEntry(CompletableFuture<RedissonLockEntry> newPromise) {return new RedissonLockEntry(newPromise);}@Overrideprotected void onMessage(RedissonLockEntry value, Long message) {if (message.equals(UNLOCK_MESSAGE)) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute != null) {runnableToExecute.run();}value.getLatch().release();} else if (message.equals(READ_UNLOCK_MESSAGE)) {while (true) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute == null) {break;}runnableToExecute.run();}//将客户端本地JDK的Semaphore进行释放value.getLatch().release(value.getLatch().getQueueLength());}}
}客户端释放Semaphore的锁时，会执行命令"incrby semaphore 1"。每当客户端释放掉permits个锁，就会将信号量的值累加permits，这样Semaphore信号量的值就不再是0了。然后通过publish命令发布一个事件，之后订阅了该事件的其他客户端都会对getLatch()返回的本地JDK的Semaphore进行加1。于是其他客户端正在被本地JDK的Semaphore进行阻塞的线程，就会被唤醒继续执行。此时其他客户端就可以尝试获取到这个信号量的锁，然后再次将这个Semaphore的值递减1。
4.Redisson的CountDownLatch简介
(1)Redisson的CountDownLatch原理图解
(2)Redisson的CountDownLatch使用演示
(1)Redisson的CountDownLatch原理图解
CountDownLatch的基本原理：要求必须有n个线程来进行countDown，才能让执行await的线程继续执行。如果没有达到指定数量的线程来countDown，会导致执行await的线程阻塞。
(2)Redisson的CountDownLatch使用演示
public class RedissonDemo {public static void main(String[] args) throws Exception {//连接3主3从的Redis CLusterConfig config = new Config();...//CountDownLatchfinal RedissonClient redisson = Redisson.create(config);RCountDownLatch latch = redisson.getCountDownLatch("myCountDownLatch");//1.设置可以countDown的数量为3latch.trySetCount(3);System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]设置了必须有3个线程执行countDown，进入等待中。。。");for (int i = 0; i < 3; i++) {new Thread(new Runnable() {public void run() {try {System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]在做一些操作，请耐心等待。。。。。。");Thread.sleep(3000);RCountDownLatch localLatch = redisson.getCountDownLatch("myCountDownLatch");localLatch.countDown();System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]执行countDown操作");} catch (Exception e) {e.printStackTrace();}}}).start();}latch.await();System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]收到通知，有3个线程都执行了countDown操作，可以继续往下执行");}
}
5.Redisson的CountDownLatch源码剖析
(1)CountDownLatch的初始化
(2)trySetCount()方法设置countDown的数量
(3)awati()方法进行阻塞等待
(4)countDown()方法对countDown的数量递减
(1)CountDownLatch的初始化
public class Redisson implements RedissonClient {//Redis的连接管理器，封装了一个Config实例protected final ConnectionManager connectionManager;//Redis的命令执行器，封装了一个ConnectionManager实例protected final CommandAsyncExecutor commandExecutor;...protected Redisson(Config config) {this.config = config;Config configCopy = new Config(config);//初始化Redis的连接管理器connectionManager = ConfigSupport.createConnectionManager(configCopy);... //初始化Redis的命令执行器commandExecutor = new CommandSyncService(connectionManager, objectBuilder);...}@Overridepublic RCountDownLatch getCountDownLatch(String name) {return new RedissonCountDownLatch(commandExecutor, name);}...
}public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {...private final CountDownLatchPubSub pubSub;private final String id;protected RedissonCountDownLatch(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.id = commandExecutor.getConnectionManager().getId();this.pubSub = commandExecutor.getConnectionManager().getSubscribeService().getCountDownLatchPubSub();}...
}(2)trySetCount()方法设置countDown的数量
trySetCount()方法的工作就是执行命令"set myCountDownLatch 3"。
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {...@Overridepublic boolean trySetCount(long count) {return get(trySetCountAsync(count));}@Overridepublic RFuture<Boolean> trySetCountAsync(long count) {return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,"if redis.call('exists', KEYS[1]) == 0 then " +"redis.call('set', KEYS[1], ARGV[2]); " +"redis.call('publish', KEYS[2], ARGV[1]); " +"return 1 " +"else " +"return 0 " +"end",Arrays.asList(getRawName(), getChannelName()),CountDownLatchPubSub.NEW_COUNT_MESSAGE,count);}...
}(3)awati()方法进行阻塞等待
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {...@Overridepublic void await() throws InterruptedException {if (getCount() == 0) {return;}CompletableFuture<RedissonCountDownLatchEntry> future = subscribe();try {commandExecutor.syncSubscriptionInterrupted(future);while (getCount() > 0) {// waiting for open state//获取countDown的数量还大于0，就先阻塞线程，然后再等待唤醒，执行while循环//其中getLatch()返回的是JDK的semaphore = "new Semaphore(0)" ==> (state - permits)commandExecutor.getNow(future).getLatch().await();}} finally {unsubscribe(commandExecutor.getNow(future));}}@Overridepublic long getCount() {return get(getCountAsync());}@Overridepublic RFuture<Long> getCountAsync() {//执行命令"get myCountDownLatch"return commandExecutor.writeAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.GET_LONG, getRawName());}...
}在while循环中，首先会执行命令"get myCountDownLatch"去获取countDown值。如果该值不大于0，就退出循环不阻塞线程。如果该值大于0，则说明还没有指定数量的线程去执行countDown操作，于是就会先阻塞线程，然后再等待唤醒来继续循环。
(4)countDown()方法对countDown的数量递减
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {...@Overridepublic void countDown() {get(countDownAsync());}@Overridepublic RFuture<Void> countDownAsync() {return commandExecutor.evalWriteNoRetryAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,"local v = redis.call('decr', KEYS[1]);" +"if v <= 0 then redis.call('del', KEYS[1]) end;" +"if v == 0 then redis.call('publish', KEYS[2], ARGV[1]) end;",Arrays.<Object>asList(getRawName(), getChannelName()),CountDownLatchPubSub.ZERO_COUNT_MESSAGE);}...
}countDownAsync()方法会执行decr命令，将countDown的数量进行递减1。如果这个值已经小于等于0，就执行del命令删除掉该CoutDownLatch。如果是这个值为0，还会发布一条消息：
publish redisson_countdownlatch__channel__{anyCountDownLatch} 0
