redis分布式锁RedissonLock的实现细节解析

redis分布式锁RedissonLock的实现细节解析

redis分布式锁RedissonLock

简单使用

String key = "key-lock";
RLock lock = redisson.getLock(key);
lock.lock();
try {
    // TODO
} catch (Exception e){
    log.error(e.getMessage(), e);
} finally {
    lock.unlock();
}

String key = "key-tryLock";
long maxWaitTime = 3_000;
RLock lock = redisson.getLock(key);
if (lock.tryLock(maxWaitTime, TimeUnit.MILLISECONDS)){
    try {
        // TODO
    } catch (Exception e){
        log.error(e.getMessage(), e);
    } finally {
        lock.unlock();
    }
} else {
    log.debug("redis锁竞争失败");
}

流程图

多个线程节点锁竞争的正常流程如下图:

多个线程节点锁竞争,并出现节点下线的异常流程如下图:

源码解析

RedissonLock是可重入锁,使用redis的hash结构作为锁的标识存储,锁的名称作为hash的key,UUID + 线程ID作为hash的field,锁被重入的次数作为hash的value。如图所示:

private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException {
    long threadId = Thread.currentThread().getId();
    // 尝试获取锁,锁获取成功则ttl为null;获取失败则返回锁的剩余过期时间
    Long ttl = tryAcquire(leaseTime, unit, threadId);
    if (ttl == null) {
        return;
    }
    // 锁被其他线程占用而索取失败,使用线程通知而非自旋的方式等待锁
    // 使用redis的发布订阅pub/sub功能来等待锁的释放通知
    RFuture<RedissonLockEntry> future = subscribe(threadId);
    commandExecutor.syncSubscription(future);
    try {
        while (true) {
            ttl = tryAcquire(leaseTime, unit, threadId);
            // 尝试获取锁,锁获取成功则ttl为null;获取失败则返回锁的剩余过期时间
            if (ttl == null) {
                break;
            }
            if (ttl >= 0) {
                // 使用LockSupport.parkNanos方法线程休眠
                try {
                    getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                } catch (InterruptedException e) {
                    if (interruptibly) {
                        throw e;
                    }
                    getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                }
            } else {
                if (interruptibly) {
                    getEntry(threadId).getLatch().acquire();
                } else {
                    getEntry(threadId).getLatch().acquireUninterruptibly();
                }
            }
        }
    } finally {
        // 退出锁竞争(锁获取成功或者放弃获取锁),则取消锁的释放订阅
        unsubscribe(future, threadId);
    }
}

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

RedissonLock实现的是可重入锁,通过redis的hash结构实现,而非加单的set nx ex。为了实现原子性的复杂的加锁逻辑,而通过lua脚本实现。获取锁会有如下三种状态:

1、锁未被任何线程占用,则锁获取成功,返回null

2、锁被当前线程占用,则锁获取成功并进行锁的重入,对锁的重入计数+1,返回null

3、锁被其他线程占用,则锁获取失败,返回该锁的自动过期时间ttl

<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));
}

当锁因为被其他线程占用而 使用redis的发布订阅pub/sub功能,通过监听锁的释放通知(在其他线程通过RedissonLock释放锁时,会通过发布订阅pub/sub功能发起通知),等待锁被其他线程释放。通过如此的线程唤醒而非自旋的操作,提高了锁的效率。

public RFuture<E> subscribe(String entryName, String channelName) {
    AtomicReference<Runnable> listenerHolder = new AtomicReference<Runnable>();
    AsyncSemaphore semaphore = service.getSemaphore(new ChannelName(channelName));
    RPromise<E> newPromise = new RedissonPromise<E>() {
        @Override
        public boolean cancel(boolean mayInterruptIfRunning) {
            return semaphore.remove(listenerHolder.get());
        }
    };
    Runnable listener = new Runnable() {
        @Override
        public void run() {
            E entry = entries.get(entryName);
            if (entry != null) {
                entry.aquire();
                semaphore.release();
                entry.getPromise().onComplete(new TransferListener<E>(newPromise));
                return;
            }
            
            E value = createEntry(newPromise);
            value.aquire();
            
            E oldValue = entries.putIfAbsent(entryName, value);
            if (oldValue != null) {
                oldValue.aquire();
                semaphore.release();
                oldValue.getPromise().onComplete(new TransferListener<E>(newPromise));
                return;
            }
            
            RedisPubSubListener<Object> listener = createListener(channelName, value);
            service.subscribe(LongCodec.INSTANCE, channelName, semaphore, listener);
        }
    };
    semaphore.acquire(listener);
    listenerHolder.set(listener);    
    return newPromise;
}

由于是可重入锁则需要在释放锁的时候做订阅通知,因此释放锁的操作同样是lua脚本实现。锁的释放会有如下三个状态:

1、等待释放的锁不存在或者不是当前线程持有,返回null

2、等待释放的锁被当前线程持有,且该锁当前被重入多次,则锁的重入计数-1,返回0

3、等待释放的锁被当前线程持有,且该锁当前未被重入,则锁的删除并发布该锁释放的订阅通知,返回1

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

Watchdog

RedissonLock为了避免应用获取锁后宕机,因为没人来释放锁而导致死锁情况的出现,默认每次锁的占用只有30秒的时间(org.redisson.config.Config#lockWatchdogTimeout = 30 * 1000)。

于是便有了Watchdog设计,由独立的线程定时给未释放的锁续期,默认锁有效期的三分之一的时长即每10秒给锁自动续期。

private void renewExpiration() {
    ExpirationEntry ee = EXPIRATION_RENEWAL_MAP.get(getEntryName());
    if (ee == null) {
        return;
    }
    
    // 默认10秒钟后执行锁续期任务
    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 " + getName() + " expiration", e);
                    return;
                }
                // 如果锁续期成功,则10秒钟后再次续期
                if (res) {
                    renewExpiration();
                }
            });
        }
    }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);    
    ee.setTimeout(task);
}
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));
}

Redisson 几种锁

1. 可重入锁(Reentrant Lock)

Redisson的分布式可重入锁RLock Java对象实现了java.util.concurrent.locks.Lock接口,同时还支持自动过期解锁。

public void testReentrantLock(RedissonClient redisson){ 
        RLock lock = redisson.getLock("anyLock");
        try{
            // 1. 最常见的使用方法
            //lock.lock();
 
            // 2. 支持过期解锁功能,10秒钟以后自动解锁, 无需调用unlock方法手动解锁
            //lock.lock(10, TimeUnit.SECONDS);
 
            // 3. 尝试加锁,最多等待3秒,上锁以后10秒自动解锁
            boolean res = lock.tryLock(3, 10, TimeUnit.SECONDS);
            if(res){    //成功
                // do your business
 
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
 
    }

Redisson同时还为分布式锁提供了异步执行的相关方法:

public void testAsyncReentrantLock(RedissonClient redisson){
        RLock lock = redisson.getLock("anyLock");
        try{
            lock.lockAsync();
            lock.lockAsync(10, TimeUnit.SECONDS);
            Future<Boolean> res = lock.tryLockAsync(3, 10, TimeUnit.SECONDS);
 
            if(res.get()){
                // do your business
 
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
 
    }

2. 公平锁(Fair Lock)

Redisson分布式可重入公平锁也是实现了java.util.concurrent.locks.Lock接口的一种RLock对象。在提供了自动过期解锁功能的同时,保证了当多个Redisson客户端线程同时请求加锁时,优先分配给先发出请求的线程。

public void testFairLock(RedissonClient redisson){ 
        RLock fairLock = redisson.getFairLock("anyLock");
        try{
            // 最常见的使用方法
            fairLock.lock();
 
            // 支持过期解锁功能, 10秒钟以后自动解锁,无需调用unlock方法手动解锁
            fairLock.lock(10, TimeUnit.SECONDS);
 
            // 尝试加锁,最多等待100秒,上锁以后10秒自动解锁
            boolean res = fairLock.tryLock(100, 10, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            fairLock.unlock();
        }
 
    }

Redisson同时还为分布式可重入公平锁提供了异步执行的相关方法:

RLock fairLock = redisson.getFairLock("anyLock");
fairLock.lockAsync();
fairLock.lockAsync(10, TimeUnit.SECONDS);
Future<Boolean> res = fairLock.tryLockAsync(100, 10, TimeUnit.SECONDS);

3. 联锁(MultiLock)

Redisson的RedissonMultiLock对象可以将多个RLock对象关联为一个联锁,每个RLock对象实例可以来自于不同的Redisson实例。

public void testMultiLock(RedissonClient redisson1,
RedissonClient redisson2, RedissonClient redisson3){ 
        RLock lock1 = redisson1.getLock("lock1");
        RLock lock2 = redisson2.getLock("lock2");
        RLock lock3 = redisson3.getLock("lock3"); 
        RedissonMultiLock lock = new RedissonMultiLock(lock1, lock2, lock3);
 
        try {
            // 同时加锁:lock1 lock2 lock3, 所有的锁都上锁成功才算成功。
            lock.lock();
 
            // 尝试加锁,最多等待100秒,上锁以后10秒自动解锁
            boolean res = lock.tryLock(100, 10, TimeUnit.SECONDS);
 
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
 
    }

4. 红锁(RedLock)

Redisson的RedissonRedLock对象实现了Redlock介绍的加锁算法。该对象也可以用来将多个RLock

对象关联为一个红锁,每个RLock对象实例可以来自于不同的Redisson实例。

 public void testRedLock(RedissonClient redisson1,
      RedissonClient redisson2, RedissonClient redisson3){ 
        RLock lock1 = redisson1.getLock("lock1");
        RLock lock2 = redisson2.getLock("lock2");
        RLock lock3 = redisson3.getLock("lock3"); 
        RedissonRedLock lock = new RedissonRedLock(lock1, lock2, lock3);
      try {
            // 同时加锁:lock1 lock2 lock3, 红锁在大部分节点上加锁成功就算成功。
            lock.lock();
 
            // 尝试加锁,最多等待100秒,上锁以后10秒自动解锁
            boolean res = lock.tryLock(100, 10, TimeUnit.SECONDS);
 
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
 
    }

5. 读写锁(ReadWriteLock)

Redisson的分布式可重入读写锁RReadWriteLock Java对象实现了java.util.concurrent.locks.ReadWriteLock接口。同时还支持自动过期解锁。该对象允许同时有多个读取锁,但是最多只能有一个写入锁。

RReadWriteLock rwlock = redisson.getLock("anyRWLock");
// 最常见的使用方法
rwlock.readLock().lock();
// 或
rwlock.writeLock().lock();
 
// 支持过期解锁功能
// 10秒钟以后自动解锁
// 无需调用unlock方法手动解锁
rwlock.readLock().lock(10, TimeUnit.SECONDS);
// 或
rwlock.writeLock().lock(10, TimeUnit.SECONDS);
 
// 尝试加锁,最多等待100秒,上锁以后10秒自动解锁
boolean res = rwlock.readLock().tryLock(100, 10, TimeUnit.SECONDS);
// 或
boolean res = rwlock.writeLock().tryLock(100, 10, TimeUnit.SECONDS);
...
lock.unlock();

6. 信号量(Semaphore)

Redisson的分布式信号量(Semaphore)Java对象RSemaphore采用了与java.util.concurrent.Semaphore相似的接口和用法。

RSemaphore semaphore = redisson.getSemaphore("semaphore");
semaphore.acquire();
//或
semaphore.acquireAsync();
semaphore.acquire(23);
semaphore.tryAcquire();
//或
semaphore.tryAcquireAsync();
semaphore.tryAcquire(23, TimeUnit.SECONDS);
//或
semaphore.tryAcquireAsync(23, TimeUnit.SECONDS);
semaphore.release(10);
semaphore.release();
//或
semaphore.releaseAsync();

7. 可过期性信号量(PermitExpirableSemaphore)

Redisson的可过期性信号量(PermitExpirableSemaphore)实在RSemaphore对象的基础上,为每个信号增加了一个过期时间。每个信号可以通过独立的ID来辨识,释放时只能通过提交这个ID才能释放。

RPermitExpirableSemaphore semaphore = redisson.getPermitExpirableSemaphore("mySemaphore");
String permitId = semaphore.acquire();
// 获取一个信号,有效期只有2秒钟。
String permitId = semaphore.acquire(2, TimeUnit.SECONDS);
// ...
semaphore.release(permitId);

8. 闭锁(CountDownLatch)

Redisson的分布式闭锁(CountDownLatch)Java对象RCountDownLatch采用了与java.util.concurrent.CountDownLatch相似的接口和用法。

以上为个人经验,希望能给大家一个参考,也希望大家多多支持趣讯吧。

发布于 2021-06-19 06:22:23
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