从tryLock()源码来出发，解析Redisson的重试机制和看门狗机制

参考：bilibili

源码基于 redisson-3.27.2

tryLock() 只尝试获取一次
tryLock(waitTime，leaseTime..)：
  - 先尝试获取一次
  - 失败，订阅释放锁的消息
  - 在waitTime时间内得到订阅消息，在do..while循环内重复尝试获取锁
  - 在这些过程中，是一直不断的在判断waitTime是否到期
---------------------------------------------------------------------
  - 获取到锁
  - leaseTime = -1，执行看门狗机制
  - 创建一个ExpiratinEntry对象，存储了获取锁的线程ID
  - 将该ExpiratinEntry对象存放到EXPIRATION_RENEWAL_MAP这个静态的concurrentHashMap中
  - 开启递归延迟任务自动续约，每10s 续约 30s
---------------------------------------------------------------------
unlockAsync0(long threadId)：
  - 解决重入的问题，每释放一次锁，threadIds-Map 中该threadId 的 value--
  - 判断是否是最后一次释放锁
  - 取消ExpirationEntry中的Timeout定时任务
  - 将该ExpirationEntry 从 EXPIRATION_RENEWAL_MAP中移除

tryLock

public boolean tryLock() {
        return (Boolean)this.get(thi.tryLockAsync());
}
    
public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
        // 转换为毫秒
        long time = unit.toMillis(waitTime);
        long current = System.currentTimeMillis();
        long threadId = Thread.currentThread().getId();
        // 调用tryAcquire  获得ttl 
        // 若ttl 为null 代表获取锁成功
        // tryAcquire 最终调用的是 tryAcquireAsync() 可见该方法详解
        Long ttl = this.tryAcquire(waitTime, leaseTime, unit, threadId);
        // 获取锁成功
        if (ttl == null) {
            return true;
        } else {
            // waitTime - 获取锁消耗的时间
            time -= System.currentTimeMillis() - current;
            // 消耗时间 > 等待时间 ，返回获取失败
            if (time <= 0L) {
                this.acquireFailed(waitTime, unit, threadId);
                return false;
            } else {
                // 还存在等待时间，通过subscribe()订阅别人释放锁的信号
                // 在unlockInnerAsync()方法中，可以看到释放锁后发布消息的代码
                current = System.currentTimeMillis();
                CompletableFuture<RedissonLockEntry> subscribeFuture = this.subscribe(threadId);

                try {
                    // 订阅任务在此等待，但并不是无限等待，而是在剩余等待时间内等待
                    subscribeFuture.get(time, TimeUnit.MILLISECONDS);
                } catch (TimeoutException var21) {
                    if (!subscribeFuture.completeExceptionally(new RedisTimeoutException("Unable to acquire subscription lock after " + time + "ms. Try to increase 'subscriptionsPerConnection' and/or 'subscriptionConnectionPoolSize' parameters."))) {
                        subscribeFuture.whenComplete((res, ex) -> {
                            if (ex == null) {
                                // 取消订阅
                                this.unsubscribe(res, threadId);
                            }

                        });
                    }

                    this.acquireFailed(waitTime, unit, threadId);
                    return false;
                } catch (ExecutionException var22) {
                    this.acquireFailed(waitTime, unit, threadId);
                    return false;
                }

                // 等待时间内等到了释放锁的消息
                // 在do while 循环内，在还有剩余时间下 不断的尝试获取锁
                boolean var14;
                try {
                    time -= System.currentTimeMillis() - current;
                    if (time > 0L) {
                        boolean var16;
                        do {
                            long currentTime = System.currentTimeMillis();
                            ttl = this.tryAcquire(waitTime, leaseTime, unit, threadId);
                            if (ttl == null) {
                                var16 = true;
                                return var16;
                            }

                            time -= System.currentTimeMillis() - currentTime;
                            if (time <= 0L) {
                                this.acquireFailed(waitTime, unit, threadId);
                                var16 = false;
                                return var16;
                            }

                            currentTime = System.currentTimeMillis();
                            if (ttl >= 0L && ttl < time) {
                                ((RedissonLockEntry)this.commandExecutor.getNow(subscribeFuture)).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                            } else {
                                ((RedissonLockEntry)this.commandExecutor.getNow(subscribeFuture)).getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);
                            }

                            time -= System.currentTimeMillis() - currentTime;
                        } while(time > 0L);

                        this.acquireFailed(waitTime, unit, threadId);
                        var16 = false;
                        return var16;
                    }

                    this.acquireFailed(waitTime, unit, threadId);
                    var14 = false;
                } finally {
                    this.unsubscribe((RedissonLockEntry)this.commandExecutor.getNow(subscribeFuture), threadId);
                }

                return var14;
            }
        }
    }

tryAcquireAsync-获取锁 && 看门狗机制的方法

private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {
    RFuture ttlRemainingFuture;
    // 通过leaseTime 判断是否走看门狗机制，默认不传leaseTime，使用其默认值-1，也就是走else路径
    if (leaseTime > 0L) {
        ttlRemainingFuture = this.tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
    } else {
    // tryLockInnerAsync()是真正执行获取锁的方法，代码见下
        ttlRemainingFuture = this.tryLockInnerAsync(waitTime, this.internalLockLeaseTime, TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
    }

    // 看门狗机制的实现
    CompletionStage<Long> s = this.handleNoSync(threadId, ttlRemainingFuture);
    RFuture<Long> ttlRemainingFuture = new CompletableFutureWrapper(s);
    CompletionStage<Long> f = ttlRemainingFuture.thenApply((ttlRemaining) -> {
        // ttlRemaining == null 意味着获取锁成功
        if (ttlRemaining == null) {
            if (leaseTime > 0L) {
                this.internalLockLeaseTime = unit.toMillis(leaseTime);
            } else {
                // leaseTime = -1 调用scheduleExpirationRenewal()开启看门狗机制
                this.scheduleExpirationRenewal(threadId);
            }
        }

        return ttlRemaining;
    });
    return new CompletableFutureWrapper(f);
}

tryLockInnerAsync-lua脚本执行获取锁的方法

// 该方法就是执行lua的最终方法，获取锁成功，返回null，获取锁失败，返回锁的剩余有效时间
<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
    return this.commandExecutor.syncedEval(this.getRawName(), LongCodec.INSTANCE, command, 
    "if ((redis.call('exists', KEYS[1]) == 0)
    or (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(this.getRawName()), 
    new Object[]{unit.toMillis(leaseTime), this.getLockName(threadId)});
}

scheduleExpirationRenewal-看门狗机制实现的方法

protected void scheduleExpirationRenewal(long threadId) {
    ExpirationEntry entry = new ExpirationEntry();
    // putIfAbsent()，存在，返回旧的ExpirationEntry ，不存在，返回null
    // EXPIRATION_RENEWAL_MAP，类的静态变量，一个concurrentHash类型，存储了所有获得锁的实例
    ExpirationEntry oldEntry = (ExpirationEntry)EXPIRATION_RENEWAL_MAP.putIfAbsent(this.getEntryName(), entry);
    if (oldEntry != null) {
        // 该方法是记录该线程ID出现的次数，即可以理解为，该线程重入锁的次数
        oldEntry.addThreadId(threadId);
    } else {
        entry.addThreadId(threadId);

        try {
            // 开启续约
            this.renewExpiration();
        } finally {
            if (Thread.currentThread().isInterrupted()) {
                this.cancelExpirationRenewal(threadId);
            }

        }
    }

}

renewExpiration-看门狗续约的实现方法

private void renewExpiration() {
    ExpirationEntry ee = (ExpirationEntry)EXPIRATION_RENEWAL_MAP.get(this.getEntryName());
    if (ee != null) {
        Timeout task = this.getServiceManager().newTimeout(new TimerTask() {
            public void run(Timeout timeout) throws Exception {
                ExpirationEntry ent = (ExpirationEntry)RedissonBaseLock.EXPIRATION_RENEWAL_MAP.get(RedissonBaseLock.this.getEntryName());
                if (ent != null) {
                    Long threadId = ent.getFirstThreadId();
                    if (threadId != null) {
                        // renewExpirationAsync() 刷新锁的有效期
                        CompletionStage<Boolean> future = RedissonBaseLock.this.renewExpirationAsync(threadId);
                        // whenComplete 是 CompletionStage 的一个方法，用于在异步操作完成后执行一个回调函数
                        // res是异步操作的结果，Boolean类型，e是操作过程中抛出的异常
                        future.whenComplete((res, e) -> {
                            if (e != null) {
                                RedissonBaseLock.log.error("Can't update lock {} expiration", RedissonBaseLock.this.getRawName(), e);
                                RedissonBaseLock.EXPIRATION_RENEWAL_MAP.remove(RedissonBaseLock.this.getEntryName());
                            } else {
                                if (res) {
                                    // 递归调用自己
                                    RedissonBaseLock.this.renewExpiration();
                                } else {
                                    RedissonBaseLock.this.cancelExpirationRenewal((Long)null);
                                }

                            }
                        });
                    }
                }
            }
            // 该任务延时 internalLockLeaseTime / 3L 执行，即10s后执行
        }, this.internalLockLeaseTime / 3L, TimeUnit.MILLISECONDS);
        ee.setTimeout(task);
    }
}

renewExpirationAsync-续约的lua脚本方法

protected CompletionStage<Boolean> renewExpirationAsync(long threadId) {
    return this.evalWriteAsync(this.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(this.getRawName()), this.internalLockLeaseTime, this.getLockName(threadId));
}

unlockAsync0-释放锁方法

private RFuture<Void> unlockAsync0(long threadId) {
    //调用释放锁的lua方法
    CompletionStage<Boolean> future = this.unlockInnerAsync(threadId);
    CompletionStage<Void> f = future.handle((opStatus, e) -> {
        // 取消看门狗续约的方法
        this.cancelExpirationRenewal(threadId);
        if (e != null) {
            if (e instanceof CompletionException) {
                throw (CompletionException)e;
            } else {
                throw new CompletionException(e);
            }
        } else if (opStatus == null) {
            IllegalMonitorStateException cause = new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: " + this.id + " thread-id: " + threadId);
            throw new CompletionException(cause);
        } else {
            return null;
        }
    });
    return new CompletableFutureWrapper(f);
}

unlockInnerAsync-释放锁的lua脚本方法

    protected RFuture<Boolean> unlockInnerAsync(long threadId) {
        return this.evalWriteAsync(this.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]); 
        // 发布释放锁的消息
        redis.call('publish', KEYS[2], ARGV[1]); 
        return 1; end; 
        return nil;", 
        Arrays.asList(this.getRawName(), this.getChannelName()), new Object[]{LockPubSub.UNLOCK_MESSAGE, this.internalLockLeaseTime, this.getLockName(threadId)});
    }

cancelExpirationRenewal-取消看门狗续约的方法

protected void cancelExpirationRenewal(Long threadId) {
    ExpirationEntry task = (ExpirationEntry)EXPIRATION_RENEWAL_MAP.get(this.getEntryName());
    if (task != null) {
        if (threadId != null) {
            // 将threadIds Map中对应的threadID-key 的 value值 -1
            task.removeThreadId(threadId);
        }

        // 如果是最后一次解锁  || threadIdS Map 直接为空 意味着是彻底释放掉锁了，在这取消看门狗的续约
        if (threadId == null || task.hasNoThreads()) {
            Timeout timeout = task.getTimeout();
            if (timeout != null) {
                // 取消定时任务
                timeout.cancel();
            }
            // EXPIRATION_RENEWAL_MAP中移除该ExpirationEntry 
            EXPIRATION_RENEWAL_MAP.remove(this.getEntryName());
        }

    }
}