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前言

通过前篇文章 redis 分布式锁实现
我们发现简单做一把分布式锁没啥问题，但是针对以往的锁来说，还存在一下两点需要考虑。

• 1.一个线程如果多次重复拿锁，该如何实现重入
• 2.因为防止死锁设置了过期时间，那么假如锁的时间到期了，业务还没有执行完毕，导致新的业务进来造成的并发问题如何处理

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一、分布式重入锁

1、单机重入锁

在单机锁时代，需要支持重入主要是为了避免在单线程情况下可能出现的死锁问题，同时简化编程模型，提升代码的灵活性与可重用性。

• 避免死锁：如果一个线程在持有锁的情况下，再次尝试获取同一把锁，非重入锁会导致该线程一直等待自己释放锁，从而引发死锁问题。重入锁允许同一个线程多次获取同一把锁，这样就可以避免这种情况的发生。
• 简化递归调用场景：在递归方法中，方法会多次调用自己，而每次调用都需要通过同一个锁保护共享资源。重入锁能够确保这些调用不会因为锁的重复获取而出现阻塞情况。
• 支持锁的调用链：在面向对象编程中，一个持有锁的方法可能会调用对象中同样需要持有锁的其他方法。重入锁保证这些方法可以顺利执行而不会因为锁的竞争而阻塞。
• 增强灵活性：重入锁数据结构更复杂，可以记录获取锁的次数。这使得锁可以灵活用在较复杂的同步场景中。
  综上所述，重入锁通过允许同一线程多次获取同一把锁，避免了许多潜在的同步问题，使得同步代码的编写变得更加简单和可靠。

例如synchronized、ReentrantLock

2、redis重入锁

参考重入锁的设计思维，我们在实现redis重入锁，应该要遵循一下原则

• 互斥条件：实现锁的必要条件，标记是否有线程已占用，不同线程不能重复占用
• 线程信息：记录线程信息，来判断加锁的是不是同一个线程
• 重入次数：记录重入次数，再释放锁的时候，减少相应的次数

二、redisson实现重入锁

1、 添加依赖

<dependency><groupId>org.redisson</groupId><artifactId>redisson-spring-boot-starter</artifactId><version>3.16.3</version> <!-- 根据需要选择合适的版本 -->
</dependency>

2、 配置 Redisson 客户端

普通模式

import org.redisson.Redisson;
import org.redisson.api.RedissonClient;
import org.redisson.config.Config;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;@Configuration
public class RedissonConfig {@Beanpublic RedissonClient redissonClient() {Config config = new Config();config.useSingleServer().setAddress("redis://127.0.0.1:6379");return Redisson.create(config);}
}

集群模式

@Bean
public RedissonClient redissonClient() {Config config = new Config();config.useClusterServers().addNodeAddress("redis://127.0.0.1:7000", "redis://127.0.0.1:7001");return Redisson.create(config);
}

哨兵模式

@Bean
public RedissonClient redissonClient() {Config config = new Config();config.useSentinelServers().setMasterName("masterName").addSentinelAddress("redis://127.0.0.1:26379", "redis://127.0.0.1:26380");return Redisson.create(config);
}

3、 使用 Redisson 实现重入锁

import org.redisson.api.RLock;
import org.redisson.api.RedissonClient;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;@Service
public class MyService {@Autowiredprivate RedissonClient redissonClient;private String lock = "myLock";public void outerMethod() {RLock lock = redissonClient.getLock(lock);lock.lock();try {System.out.println("Outer method acquired lock");innerMethod();} finally {lock.unlock();System.out.println("Outer method released lock");}}private void innerMethod() {RLock lock = redissonClient.getLock(lock);lock.lock();try {System.out.println("Inner method acquired lock");} finally {lock.unlock();System.out.println("Inner method released lock");}}
}

4、 验证

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;@RestController
public class MyController {@Autowiredprivate MyService myService;@GetMapping("/test-lock")public String testLock() {myService.outerMethod();return "Lock tested successfully";}
}

5、运行项目

启动 Spring Boot 应用，并访问 http://localhost:8080/test-lock 以测试多次重入锁的实现。你应该能够在控制台上看到如下输出，表明锁多次重入的正确执行：

Outer method acquired lock
Inner method acquired lock
Inner method released lock
Outer method released lock

三、redisson分布式锁分析

1、获取锁对象

RLock lock = redissonClient.getLock(lock);public RLock getLock(String name) {return new RedissonLock(this.connectionManager.getCommandExecutor(), name, this.id);
}public RedissonLock(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;this.internalLockLeaseTime = commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout();this.pubSub = commandExecutor.getConnectionManager().getSubscribeService().getLockPubSub();
}public RedissonBaseLock(CommandAsyncExecutor commandExecutor, String name) {super(commandExecutor, name);this.commandExecutor = commandExecutor;this.id = commandExecutor.getConnectionManager().getId();// 默认锁释放时间 30sthis.internalLockLeaseTime = commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout();this.entryName = id + ":" + name;
}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);
}

2、 加锁

org.redisson.RedissonLock#tryLock

    @Overridepublic boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {// 等待时间转成MSlong time = unit.toMillis(waitTime);long current = System.currentTimeMillis();// 当前线程long threadId = Thread.currentThread().getId();// 尝试获取锁 返回空标识回去锁Long ttl = tryAcquire(waitTime, leaseTime, unit, threadId);// lock acquiredif (ttl == null) {return true;}time -= System.currentTimeMillis() - current;// 超时，拿锁失败 返回falseif (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}current = System.currentTimeMillis();// 订阅解锁消息，见org.redisson.pubsub.LockPubSub#onMessage// 订阅锁释放事件，并通过await方法阻塞等待锁释放，有效的解决了无效的锁申请浪费资源的问题：// 基于信息量，当锁被其它资源占用时，当前线程通过 Redis 的 channel 订阅锁的释放事件，一旦锁释放会发消息通知待等待的线程进行竞争// 当 this.await返回false，说明等待时间已经超出获取锁最大等待时间，取消订阅并返回获取锁失败// 当 this.await返回true，进入循环尝试获取锁CompletableFuture<RedissonLockEntry> subscribeFuture = subscribe(threadId);try {subscribeFuture.get(time, TimeUnit.MILLISECONDS);} catch (TimeoutException e) {if (!subscribeFuture.cancel(false)) {subscribeFuture.whenComplete((res, ex) -> {if (ex == null) {unsubscribe(res, threadId);}});}acquireFailed(waitTime, unit, threadId);return false;} catch (ExecutionException e) {acquireFailed(waitTime, unit, threadId);return false;}try {time -= System.currentTimeMillis() - current// 超时，拿锁失败 返回false;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}// 自旋获取锁while (true) {long currentTime = System.currentTimeMillis();// 再次加锁ttl = tryAcquire(waitTime, leaseTime, unit, threadId);// lock acquired// 获得锁if (ttl == null) {return true;}time -= System.currentTimeMillis() - currentTime;// 超时，拿锁失败 返回false;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}// waiting for messagecurrentTime = System.currentTimeMillis();// 下面的阻塞会在释放锁的时候，通过订阅发布及时relaseif (ttl >= 0 && ttl < time) {// 如果锁的超时时间小于等待时间，通过SemaphorerelaseryAcquire阻塞锁的释放时间commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);} else {// 否则，通过Semaphore的tryAcquire阻塞传入的最大等待时间commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);}time -= System.currentTimeMillis() - currentTime;if (time <= 0) {acquireFailed(waitTime, unit, threadId);return false;}}} finally {// 取消订阅unsubscribe(commandExecutor.getNow(subscribeFuture), threadId);}}

尝试获取锁

    private Long tryAcquire(long waitTime, long leaseTime, TimeUnit unit, long threadId) {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 > 0) {// 释放时间同步ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} else {// 如果没有传入锁的释放时间，默认 internalLockLeaseTime = 30000 MSttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime,TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);}CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {// lock acquired// 如果返回null说明抢到了锁或者是可重入 否则直接返回还有多久过期if (ttlRemaining == null) {if (leaseTime > 0) {// 释放时间 赋值给 internalLockLeaseTimeinternalLockLeaseTime = unit.toMillis(leaseTime);} else {scheduleExpirationRenewal(threadId);}}// 没有抢到直接返回return ttlRemaining;});return new CompletableFutureWrapper<>(f);}<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()), unit.toMillis(leaseTime), getLockName(threadId));}

LUA脚本分析

# 加锁
"if (redis.call('exists', KEYS[1]) == 0) then " # 判断我的锁是否存在 =0 为不存在 没人抢占锁
"redis.call('hincrby', KEYS[1], ARGV[2], 1); " + # 设置锁并计数重入次数 
"redis.call('pexpire', KEYS[1], ARGV[1]); " + # 设置过期时间 30S
"return nil; " + 
"end; " +
# 重入
"if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " + # 进入该逻辑说明有线程抢占了锁 继续判断是否同一个线程 ==1 为同一线程
"redis.call('hincrby', KEYS[1], ARGV[2], 1); " + # 重入次数 + 1
"redis.call('pexpire', KEYS[1], ARGV[1]); " + # 设置超时时间
"return nil; " +
"end; " +
"return redis.call('pttl', KEYS[1]);", // 前面2个if都没进，说明锁被抢占并且不是同一线程，直接返回过期时间

3、订阅

订阅锁状态，挂起唤醒线程
org.redisson.RedissonLock#subscribe

public CompletableFuture<E> subscribe(String entryName, String channelName) {AsyncSemaphore semaphore = service.getSemaphore(new ChannelName(channelName));CompletableFuture<E> newPromise = new CompletableFuture<>();semaphore.acquire().thenAccept(c -> {if (newPromise.isDone()) {semaphore.release();return;}E entry = entries.get(entryName);if (entry != null) {entry.acquire();semaphore.release();entry.getPromise().whenComplete((r, e) -> {if (e != null) {newPromise.completeExceptionally(e);return;}newPromise.complete(r);});return;}E value = createEntry(newPromise);value.acquire();E oldValue = entries.putIfAbsent(entryName, value);if (oldValue != null) {oldValue.acquire();semaphore.release();oldValue.getPromise().whenComplete((r, e) -> {if (e != null) {newPromise.completeExceptionally(e);return;}newPromise.complete(r);});return;}// 创建监听，释放锁，会发送消息RedisPubSubListener<Object> listener = createListener(channelName, value);CompletableFuture<PubSubConnectionEntry> s = service.subscribeNoTimeout(LongCodec.INSTANCE, channelName, semaphore, listener);newPromise.whenComplete((r, e) -> {if (e != null) {s.completeExceptionally(e);}});s.whenComplete((r, e) -> {if (e != null) {value.getPromise().completeExceptionally(e);return;}value.getPromise().complete(value);});});return newPromise;}

org.redisson.pubsub.PublishSubscribe#createListener

    private RedisPubSubListener<Object> createListener(String channelName, E value) {// 创建监听，当监听到消息回来的时候，进入onMessage进行处理RedisPubSubListener<Object> listener = new BaseRedisPubSubListener() {@Overridepublic void onMessage(CharSequence channel, Object message) {if (!channelName.equals(channel.toString())) {return;}PublishSubscribe.this.onMessage(value, (Long) message);}};return listener;}

org.redisson.pubsub.LockPubSub#onMessage

    @Overrideprotected void onMessage(RedissonLockEntry value, Long message) {if (message.equals(UNLOCK_MESSAGE)) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute != null) {runnableToExecute.run();}// 释放 Semaphorevalue.getLatch().release();} else if (message.equals(READ_UNLOCK_MESSAGE)) {while (true) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute == null) {break;}runnableToExecute.run();}value.getLatch().release(value.getLatch().getQueueLength());}}

4、锁续期

redisson watchDog 使用时间轮技术，请参考时间轮算法分析
org.redisson.RedissonBaseLock#scheduleExpirationRenewal

    protected void scheduleExpirationRenewal(long threadId) {ExpirationEntry entry = new ExpirationEntry();// 放入EXPIRATION_RENEWAL_MAP 这个MAP中ExpirationEntry oldEntry = EXPIRATION_RENEWAL_MAP.putIfAbsent(getEntryName(), entry);// 第一次进来，里面没有if (oldEntry != null) {// 如果其他线程来抢占这个锁，进入将线程ID保存至ExpirationEntry的threadIds这个Map中oldEntry.addThreadId(threadId);} else {// 将线程ID保存至ExpirationEntry的threadIds这个Map中entry.addThreadId(threadId);try {// 执行renewExpiration();} finally {if (Thread.currentThread().isInterrupted()) {cancelExpirationRenewal(threadId);}}}}

org.redisson.RedissonBaseLock#renewExpiration

  private void renewExpiration() {ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (ee == null) {return;}// 开启时间轮，时间是10s之后执行我们的TimerTask任务Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {@Overridepublic void run(Timeout timeout) throws Exception {// 从EXPIRATION_RENEWAL_MAP中拿到锁的对象，有可能在定时的时候被移除取消ExpirationEntry ent = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (ent == null) {return;}Long threadId = ent.getFirstThreadId();if (threadId == null) {return;}// 给锁续期CompletionStage<Boolean> future = renewExpirationAsync(threadId);future.whenComplete((res, e) -> {// 异常报错，从Map移除if (e != null) {log.error("Can't update lock " + getRawName() + " expiration", e);EXPIRATION_RENEWAL_MAP.remove(getEntryName());return;}// 如果返回的是1 代表线程还占有锁，递归调用自己if (res) {// 递归再次加入时间轮// reschedule itselfrenewExpiration();} else {// 所不存在，这取消任务，移除相关MAP信息cancelExpirationRenewal(null);}});}}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);// 设置Timeoutee.setTimeout(task);}

org.redisson.connection.MasterSlaveConnectionManager#newTimeout

    private HashedWheelTimer timer;@Overridepublic Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {try {// 添加进入时间轮return timer.newTimeout(task, delay, unit);} catch (IllegalStateException e) {if (isShuttingDown()) {return DUMMY_TIMEOUT;}throw e;}}

    protected CompletionStage<Boolean> renewExpirationAsync(long threadId) {return evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,"if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " + // 如果当前锁存在"redis.call('pexpire', KEYS[1], ARGV[1]); " +  // 续期"return 1; " +"end; " +"return 0;",Collections.singletonList(getRawName()),internalLockLeaseTime, getLockName(threadId));}

5、释放锁

org.redisson.RedissonLock#unlock

    @Overridepublic void unlock() {try {get(unlockAsync(Thread.currentThread().getId()));} catch (RedisException e) {if (e.getCause() instanceof IllegalMonitorStateException) {throw (IllegalMonitorStateException) e.getCause();} else {throw e;}}}

org.redisson.RedissonBaseLock.unlockAsync

  @Overridepublic RFuture<Void> unlockAsync(long threadId) {// 进入释放锁逻辑RFuture<Boolean> future = unlockInnerAsync(threadId);CompletionStage<Void> f = future.handle((opStatus, e) -> {// 移除ExpirationEntry中的threadId 并且移除 EXPIRATION_RENEWAL_MAP中的ExpirationEntry watchDog就不会继续续期  cancelExpirationRenewal(threadId);// 异常处理if (e != null) {throw new CompletionException(e);}// 不存在锁信息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);}

org.redisson.RedissonBaseLock.unlockInnerAsync

protected RFuture<Boolean> unlockInnerAsync(long threadId) {return evalWriteAsync(getName(), 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(getName(), getChannelName()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName(threadId));
}

释放锁LUA

"if (redis.call('hexists', KEYS[1], ARGV[3]) == 0) then " +   # 锁已经释放"return nil;" +"end; " +"local counter = redis.call('hincrby', KEYS[1], ARGV[3], -1); " +  #重入次数减一"if (counter > 0) then " +"redis.call('pexpire', KEYS[1], ARGV[2]); " + # 重入次数还不为0 那说明占有锁，设置过期时间"return 0; " +"else " +"redis.call('del', KEYS[1]); " +    # 重入次数为0，释放锁"redis.call('publish', KEYS[2], ARGV[1]); " + # 发布订阅事件，唤醒其它线程，可以去竞争锁了"return 1; " +"end; " +"return nil;",

6、流程图