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前言

分布式锁主要是解决分布式系统下数据一致性的问题。在单机的环境下，应用是在同一进程下的，只需要保证单进程多线程环境中的线程安全性，通过 Java 提供的 volatile、ReentrantLock、synchronized 以及 concurrent 并发包下一些线程安全的类等就可以做到。

一、Redisson介绍

Redisson 是一个基于 Redis 的分布式 Java 客户端。它提供了丰富的功能和工具，帮助开发者在分布式系统中解决数据共享、并发控制和任务调度等问题。通过使用Redisson，开发者可以轻松地操作 Redis 的分布式对象（如集合、映射、队列等），实现可靠的分布式锁机制，以及管理和调度分布式环境中的任务和服务。

Redisson 的分布式锁的特点

● 线程安全：分布式锁可以确保在多线程和多进程环境下的数据一致性和可靠性。
● 可重入性： 同一个线程可以多次获取同一个锁，避免死锁的问题。
● 锁超时： 支持设置锁的有效期，防止锁被长时间占用而导致系统出现问题。
● 阻塞式获取锁： 当某个线程尝试获取锁时，如果锁已经被其他线程占用，则该线程可以选择等待直到锁释放。
● 无阻塞式获取锁： 当某个线程尝试获取锁时，如果锁已经被其他线程占用，则该线程不会等待，而是立即返回获取锁失败的信息。

redisson 实现分布式官网文档：https://github.com/redisson/redisson/wiki/8.-distributed-locks-and-synchronizers

二、Redisson的使用

Redisson 支持单点模式、主从模式、哨兵模式、集群模式，本文以单点模式为例说明。

1.1 引入依赖

<!-- redisson -->
<dependency><groupId>org.redisson</groupId><artifactId>redisson</artifactId><version>3.12.0</version>
</dependency>
<!-- redis -->
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-redis</artifactId><version>2.1.21.RELEASE</version>
</dependency>

1.2 编写配置

spring:redis:host: 192.168.57.129port: 6379

@Configuration
public class RedissonConfig {@Value("${spring.redis.host}")private String host;@Value("${spring.redis.port}")private String port;/*** 对所有redisson的使用都是通过redissonClient对象* @return*/@Bean(destroyMethod = "shutdown")public RedissonClient redissonClient() {Config config = new Config();config.useSingleServer().setAddress("redis://" + host + ":" + port);return Redisson.create(config);}
}

1.3 示例测试_01

@Autowired
private RedissonClient redissonClient;// redisson分布式锁的key
private static final String LOCK_TEST_KEY = "redisson_lock";// redisson分布式锁的key
private static int TICKET_NUMBER = 10;/*** 分布式锁测试用例* 模拟开启11个用户去抢车票*/
@Test
public void lockTest() {// 利用循环+多线程 模仿高并发请求for (int i = 0; i < 11; i++) {CompletableFuture.runAsync(() -> {if (TICKET_NUMBER > 0) {// 这里获取公平锁，遵循先进先出原则，方便测试RLock fairLock = redissonClient.getFairLock(LOCK_TEST_KEY);try {// 尝试加锁// waitTimeout 尝试获取锁的最大等待时间,超过这个值,则认为获取锁失败// leaseTime   锁的持有时间,超过这个时间锁会自动失效(值应设置为大于业务处理的时间，确保在锁有效期内业务能处理完)boolean lock = fairLock.tryLock(3000, 30, TimeUnit.MILLISECONDS);if (lock){log.info("线程:" + Thread.currentThread().getName() + "获得了锁");log.info("车票剩余数量:{}", --TICKET_NUMBER);}} catch (InterruptedException e) {e.printStackTrace();} finally {log.info("线程:" + Thread.currentThread().getName() + "准备释放锁");//注意: 无论出现任何情况都要主动解锁fairLock.unlock();}}else {log.info("车票已售罄！");}});try {// ->_-> 这里使当前方法占用的线程休息10秒，不要立即结束Thread.sleep(3000L);} catch (InterruptedException e) {e.printStackTrace();}}
}

日志信息输出

2023-11-18 15:27:00.834  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:00.835  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:9
2023-11-18 15:27:00.835  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:03.749  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:03.749  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:8
2023-11-18 15:27:03.749  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:06.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:06.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:7
2023-11-18 15:27:06.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:09.749  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:09.750  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:6
2023-11-18 15:27:09.750  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:12.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:12.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:5
2023-11-18 15:27:12.759  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:15.752  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:15.752  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:4
2023-11-18 15:27:15.752  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:18.762  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:18.762  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:3
2023-11-18 15:27:18.762  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:21.754  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:21.754  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:2
2023-11-18 15:27:21.754  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:24.763  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:24.763  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:1
2023-11-18 15:27:24.763  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:27.757  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6获得了锁
2023-11-18 15:27:27.757  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票剩余数量:0
2023-11-18 15:27:27.757  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 线程:ForkJoinPool.commonPool-worker-6准备释放锁
2023-11-18 15:27:30.753  INFO 5800 --- [onPool-worker-6] c.a.g.p.GulimallProductApplicationTests  : 车票已售罄！

1.4 示例测试_02

@ResponseBody
@GetMapping("/hello")
public String hello(){//1、获取一把锁，只要锁的名字一样，就是同一把锁String lockKey = "my-lock";RLock lock = redissonClient.getLock(lockKey);//2、加锁阻塞式等待，默认加的锁都是30s。lock.lock();//10秒自动解锁，自动解锁时间一定要大于业务的执行时间。问题：在锁时间到了以后，不会自动续期。//lock.lock(10, TimeUnit.SECONDS);//最佳实战：省掉了整个续期操作。手动解锁//1）、lock.lock(30, TimeUnit.SECONDS);try {log.info("加锁成功，执行业务ing, 线程ID = {}", Thread.currentThread().getId());Thread.sleep(10000);}catch (Exception e){e.printStackTrace();}finally {//3、解锁 假设解锁代码没有运行，redisson会不会出现死锁log.info("释放锁, 线程ID = {}", Thread.currentThread().getId());lock.unlock();}return "hello";
}

浏览器执行两个 hello 请求，只有当第一个请求业务执行完，第二个才能正常执行，不然第二个处于阻塞式等待状态。

控制台打印日志信息

2023-11-18 16:01:00.784  INFO 3916 --- [io-10000-exec-4] c.a.g.product.web.IndexController        : 加锁成功，执行业务ing, 线程ID = 116
2023-11-18 16:01:10.785  INFO 3916 --- [io-10000-exec-4] c.a.g.product.web.IndexController        : 释放锁, 线程ID = 116
2023-11-18 16:01:10.794  INFO 3916 --- [io-10000-exec-2] c.a.g.product.web.IndexController        : 加锁成功，执行业务ing, 线程ID = 114
2023-11-18 16:01:20.794  INFO 3916 --- [io-10000-exec-2] c.a.g.product.web.IndexController        : 释放锁, 线程ID = 114

redisson 实现分布式锁解决了 redis 实现分布式锁的两个问题

1. 锁的自动续期，如果业务超长，运行期间自动给锁续上新的30s。不用担心业务时间长，锁自动过期被删掉。
2. 加锁的业务只要运行完成，就不会给当前锁续期，即使不手动解锁，锁默认在30s以后自动删除。

三、Redisson源码分析

redisson 这个框架的实现依赖了 Lua 脚本和 Netty，以及各种 Future 及FutureListener 的异步、同步操作转换，加锁和解锁过程中还巧妙地利用了 redis 的发布订阅功能。

2.1 加锁源码

无参加锁方法

@Override
public void lock() {try {lock(-1, null, false);} catch (InterruptedException e) {throw new IllegalStateException();}
}

有参加锁方法

@Override
public void lock(long leaseTime, TimeUnit unit) {try {lock(leaseTime, unit, false);} catch (InterruptedException e) {throw new IllegalStateException();}
}

2.2 看门狗机制

private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {long threadId = Thread.currentThread().getId();//尝试获取锁Long ttl = tryAcquire(leaseTime, unit, threadId);// lock acquiredif (ttl == null) {return;}RFuture<RedissonLockEntry> future = subscribe(threadId);if (interruptibly) {commandExecutor.syncSubscriptionInterrupted(future);} else {commandExecutor.syncSubscription(future);}try {while (true) {ttl = tryAcquire(leaseTime, unit, threadId);// lock acquiredif (ttl == null) {break;}// waiting for messageif (ttl >= 0) {try {future.getNow().getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);} catch (InterruptedException e) {if (interruptibly) {throw e;}future.getNow().getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);}} else {if (interruptibly) {future.getNow().getLatch().acquire();} else {future.getNow().getLatch().acquireUninterruptibly();}}}} finally {unsubscribe(future, threadId);}
}

//尝试获取锁
private Long tryAcquire(long leaseTime, TimeUnit unit, long threadId) {return get(tryAcquireAsync(leaseTime, unit, threadId));
}

异步的方式尝试获取锁

private <T> RFuture<Long> tryAcquireAsync(long leaseTime, TimeUnit unit, long threadId) {if (leaseTime != -1) {//如果我们传递了锁的超时时间，就发送给redis执行脚本，进行占锁，默认超时就是我们指定的时间。return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG);}//如果我们未指定锁的超时时间，就使用30*1000【LockWatchdogTimeout看门狗的默认时间】RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);//占锁成功ttlRemainingFuture.onComplete((ttlRemaining, e) -> {//发生异常直接返回，若无异常执行下面逻辑if (e != null) {return;}// lock acquiredif (ttlRemaining == null) {scheduleExpirationRenewal(threadId);}});return ttlRemainingFuture;
}

//默认自动续期时间30s，看门狗时间
private long lockWatchdogTimeout = 30 * 1000;

<T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {internalLockLeaseTime = unit.toMillis(leaseTime);return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,"if (redis.call('exists', KEYS[1]) == 0) then " +"redis.call('hset', 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.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));
}

重新设置超时时间

private 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);renewExpiration();}
}

开启定时任务，发送 LUA 脚本，锁的超时时间达到1/3就重新设为30s

private void renewExpiration() {ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());if (ee == null) {return;}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;}RFuture<Boolean> future = renewExpirationAsync(threadId);future.onComplete((res, e) -> {if (e != null) {log.error("Can't update lock " + getName() + " expiration", e);return;}if (res) {// reschedule itselfrenewExpiration();}});}//只要占锁成功，就会启动一个定时任务【重新给锁设置过期时间，新的过期时间就是看门狗的默认时间】，每隔10s都会自动再次续期，续成30s internalLockLeaseTime【看门狗时间】 / 3，10s}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);ee.setTimeout(task);
}

重新设置超时时间 LUA 脚本

protected RFuture<Boolean> renewExpirationAsync(long threadId) {return commandExecutor.evalWriteAsync(getName(), 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.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));}