【源码分析】从源码层面深度剖析Redisson实现分布式锁的原理

快速入门

引入redisson依赖

<dependency>
    <groupId>org.redisson</groupId>
    <artifactId>redisson</artifactId>
    <version>3.16.0</version>
</dependency>

编写测试代码

public class RedissonTest {

    private static RedissonClient redissonClient;

    static {
        Config config=new Config();
        config.useSingleServer().setAddress("redis://127.0.0.1:6379");
        redissonClient= Redisson.create(config);
    }

    public static void main(String[] args) throws InterruptedException {
        RLock rLock=redissonClient.getLock("updateData");
        //最多等待100秒、上锁10s以后自动解锁
        if(rLock.tryLock(100,10, TimeUnit.SECONDS)){
            System.out.println("获取锁成功");
        }
        Thread.sleep(2000);
        rLock.unlock();

        redissonClient.shutdown();
    }
}

lock方法会一直重试，tryLock方法会尝试，在一定时间内放弃重试。加了leaseTime，没有看门狗机制

private void redissonDoc() throws InterruptedException {
    //1. 普通的可重入锁
    RLock lock = redissonClient.getLock("generalLock");

    // 拿锁失败时会不停的重试
    // 具有Watch Dog 自动延期机制 默认续30s 每隔30/3=10 秒续到30s
    lock.lock();

    // 尝试拿锁10s后停止重试,返回false
    // 具有Watch Dog 自动延期机制 默认续30s
    boolean res1 = lock.tryLock(10, TimeUnit.SECONDS);

    // 拿锁失败时会不停的重试
    // 没有Watch Dog ，10s后自动释放
    lock.lock(10, TimeUnit.SECONDS);

    // 尝试拿锁100s后停止重试,返回false
    // 没有Watch Dog ，10s后自动释放
    boolean res2 = lock.tryLock(100, 10, TimeUnit.SECONDS);

    //2. 公平锁 保证 Redisson 客户端线程将以其请求的顺序获得锁
    RLock fairLock = redissonClient.getFairLock("fairLock");

    //3. 读写锁 没错与JDK中ReentrantLock的读写锁效果一样
    RReadWriteLock readWriteLock = redissonClient.getReadWriteLock("readWriteLock");
    readWriteLock.readLock().lock();
    readWriteLock.writeLock().lock();
}

实现原理

RedissonLock#lock()，无参数默认leaseTime是-1

    private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {
        long threadId = Thread.currentThread().getId();
        Long ttl = tryAcquire(-1, leaseTime, unit, threadId);
        // lock acquired
        if (ttl == null) {
            return;
        }
        //...
    }

RedissonLock#tryAcquire

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

RedissonLock#tryAcquireAsync，如果租约时间是-1，获取internalLockLeaseTime；如果不等于-1，获取该时间。租约默认时间是在Config的构造函数中设置的，lockWatchdogTimeout。如果续约时间设置为-1，且获取锁成功，开启看门狗。

    private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {
        RFuture<Long> ttlRemainingFuture;
        if (leaseTime != -1) {
            ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
        } else {
            ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime,
                    TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
        }
        ttlRemainingFuture.onComplete((ttlRemaining, e) -> {
            if (e != null) {
                return;
            }

            // lock acquired
            if (ttlRemaining == null) {
                if (leaseTime != -1) {
                    internalLockLeaseTime = unit.toMillis(leaseTime);
                } else {
                    scheduleExpirationRenewal(threadId);
                }
            }
        });
        return ttlRemainingFuture;
    }

RedissonBaseLock#scheduleExpirationRenewal，判断是否存在EXPIRATION_RENEWAL_MAP，不存在则执行 renewExpiration();，存在则直接返回。考虑到了可重入锁。

    protected 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();
        }
    }

RedissonBaseLock#renewExpiration，开启定时任务。每次间隔租期的1/3时间执行

    private void renewExpiration() {
        ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());
        if (ee == null) {
            return;
        }
        
        Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
            @Override
            public 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 " + getRawName() + " expiration", e);
                        EXPIRATION_RENEWAL_MAP.remove(getEntryName());
                        return;
                    }
                    
                    if (res) {
                        // reschedule itself
                        renewExpiration();
                    }
                });
            }
        }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
        
        ee.setTimeout(task);
    }

使用lua进行续约

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

RedissonLock#tryLockInnerAsync，使用lua脚本进行加锁。

    <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));
    }

RedissonLock#unlockInnerAsync，使用lua脚本进行解锁

protected RFuture<Boolean> unlockInnerAsync(long threadId) {
    return evalWriteAsync(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]); " +
                    "redis.call('publish', KEYS[2], ARGV[1]); " +
                    "return 1; " +
                    "end; " +
                    "return nil;",
            Arrays.asList(getRawName(), getChannelName()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName(threadId));
}

RedissonLock处理锁竞争。subscribeFuture.await(time, TimeUnit.MILLISECONDS)在指定时间内获取锁失败，返回false。

    public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
        //...
        
        time -= System.currentTimeMillis() - current;
        if (time <= 0) {
            acquireFailed(waitTime, unit, threadId);
            return false;
        }
        
        current = System.currentTimeMillis();
        RFuture<RedissonLockEntry> subscribeFuture = subscribe(threadId);
        if (!subscribeFuture.await(time, TimeUnit.MILLISECONDS)) {
            if (!subscribeFuture.cancel(false)) {
                subscribeFuture.onComplete((res, e) -> {
                    if (e == null) {
                        unsubscribe(subscribeFuture, threadId);
                    }
                });
            }
            acquireFailed(waitTime, unit, threadId);
            return false;
        }
        
         try {
            time -= System.currentTimeMillis() - current;
            if (time <= 0) {
                acquireFailed(waitTime, unit, threadId);
                return false;
            }
        
            while (true) {
                long currentTime = System.currentTimeMillis();
                ttl = tryAcquire(waitTime, leaseTime, unit, threadId);
                // lock acquired
                if (ttl == null) {
                    return true;
                }

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

                // waiting for message
                currentTime = System.currentTimeMillis();
                if (ttl >= 0 && ttl < time) {
                    subscribeFuture.getNow().getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                } else {
                    subscribeFuture.getNow().getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);
                }

                time -= System.currentTimeMillis() - currentTime;
                if (time <= 0) {
                    acquireFailed(waitTime, unit, threadId);
                    return false;
                }
            }
        } finally {
            unsubscribe(subscribeFuture, threadId);
        }
    }

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