Redission分布式锁加锁原理源码解析

IT云清

发布于 2020-01-13 16:29:06

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发布于 2020-01-13 16:29:06

文章被收录于专栏：IT云清

追踪一下redission加锁的实现源码，并详细介绍核心加锁代码lua脚本的执行原理和过程。

1.获取锁

这里是我们自己实现，调用redission的方法，获取锁，然后加锁。lock.lock(expireTime, timeUnit)是关键，我们追进去。

    /**
     * 获取锁，如果没有主动调用unlock解锁，expireTime后会自动释放
     * @param lockKey
     * @param expireTime 如果没有调用unlock解锁，expireTime 后自动释放
     * @param timeUnit 时间单位
     * @return
     */
    public RLock lock(String lockKey,Integer expireTime,TimeUnit timeUnit){
        RLock lock = redisson.getLock(lockKey);
        lock.lock(expireTime, timeUnit);
        logger.info("【Redisson lock】success to acquire lock for [ "+lockKey+" ],expire time:"+expireTime+timeUnit);
        return lock;
    }

进入 lock.lock(expireTime, timeUnit);

    /**
     * Acquires the lock.
     *
     * <p>If the lock is not available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until the
     * lock has been acquired.
     *
     * If the lock is acquired, it is held until <code>unlock</code> is invoked,
     * or until leaseTime milliseconds have passed
     * since the lock was granted - whichever comes first.
     *
     * @param leaseTime the maximum time to hold the lock after granting it,
     *        before automatically releasing it if it hasn't already been released by invoking <code>unlock</code>.
     *        If leaseTime is -1, hold the lock until explicitly unlocked.
     * @param unit the time unit of the {@code leaseTime} argument
     *
     */
    void lock(long leaseTime, TimeUnit unit);

进入实现方法

    @Override
    public void lock(long leaseTime, TimeUnit unit) {
        try {
            lockInterruptibly(leaseTime, unit);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

进入 lockInterruptibly(leaseTime, unit);

    @Override
    public void lockInterruptibly(long leaseTime, TimeUnit unit) throws InterruptedException {
        long threadId = Thread.currentThread().getId();
        Long ttl = tryAcquire(leaseTime, unit, threadId);
        // lock acquired
        if (ttl == null) {
            return;
        }

        RFuture<RedissonLockEntry> future = subscribe(threadId);
        commandExecutor.syncSubscription(future);

        try {
            while (true) {
                ttl = tryAcquire(leaseTime, unit, threadId);
                // lock acquired
                if (ttl == null) {
                    break;
                }

                // waiting for message
                if (ttl >= 0) {
                    getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                } else {
                    getEntry(threadId).getLatch().acquire();
                }
            }
        } finally {
            unsubscribe(future, threadId);
        }
//        get(lockAsync(leaseTime, unit));
    }

进入Long ttl = tryAcquire(leaseTime, unit, threadId);

    private Long tryAcquire(long leaseTime, TimeUnit unit, long threadId) {
        return get(tryAcquireAsync(leaseTime, unit, threadId));
    }

进入

    private <T> RFuture<Long> tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId) {
        if (leaseTime != -1) {
            return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
        }
        RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
        ttlRemainingFuture.addListener(new FutureListener<Long>() {
            @Override
            public void operationComplete(Future<Long> future) throws Exception {
                if (!future.isSuccess()) {
                    return;
                }

                Long ttlRemaining = future.getNow();
                // lock acquired
                if (ttlRemaining == null) {
                    scheduleExpirationRenewal(threadId);
                }
            }
        });
        return ttlRemainingFuture;
    }

进入核心tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG)

这里是加锁的核心方法：

    <T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
    	//时间转化为毫秒值
        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));
    }

这里最终的是执行了一段具有原子性的lua脚本，由CommandAsynExecutor执行；这个锁最终持久化到redis时，使用的是hash类型的key field value; 这里注意下最后一行几个参数的对应关系： getName()), internalLockLeaseTime, getLockName(threadId) 分别是key[1],ARGV[1],ARGV[2];

getName()是逻辑锁名称，例如：我们发起锁的一方传递的锁名称 productId1672822；
internalLockLeaseTime是毫秒单位的锁过期时间；
getLockName则是锁对应的线程级别的名称，因为支持相同线程可重入，不同线程不可重入，所以这里的锁的生成方式是：UUID+":"threadId

Lua脚本中的执行分为以下三步：

1:exists检查redis中是否存在锁名称；如果不存在，则获取成功；同时把逻辑锁名称KEYS[1]，线程级别的锁名称[ARGV[2]，value=1,设置到redis。并设置逻辑锁名称的过期时间ARGV[2]，返回；
2:如果检查到存在KEYS[1],[ARGV[2],则说明获取成功，此时会自增对应的value值，记录重入次数；并更新锁的过期时间
3：key不存，直接返回key的剩余过期时间（-2）

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原始发表：2019/08/29 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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