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本文主要研究一下redisson的分布式锁

maven

     <dependency><groupId>org.redisson</groupId><artifactId>redisson</artifactId><version>3.8.1</version></dependency>
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实例

    @Testpublic void testDistributedLock(){Config config = new Config();
//        config.setTransportMode(TransportMode.EPOLL);config.useSingleServer().setAddress("redis://192.168.99.100:6379");RedissonClient redisson = Redisson.create(config);IntStream.rangeClosed(1,5).parallel().forEach(i -> {executeLock(redisson);});executeLock(redisson);}public void executeLock(RedissonClient redisson){RLock lock = redisson.getLock("myLock");boolean locked = false;try{LOGGER.info("try lock");locked = lock.tryLock();
//            locked = lock.tryLock(1,2,TimeUnit.MINUTES);LOGGER.info("get lock result:{}",locked);if(locked){TimeUnit.HOURS.sleep(1);LOGGER.info("get lock and finish");}}catch (Exception e){e.printStackTrace();}finally {LOGGER.info("enter unlock");if(locked){lock.unlock();}}}
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源码解析

RedissonLock.tryLock

redisson-3.8.1-sources.jar!/org/redisson/RedissonLock.java

    @Overridepublic boolean tryLock() {return get(tryLockAsync());}@Overridepublic RFuture<Boolean> tryLockAsync() {return tryLockAsync(Thread.currentThread().getId());}@Overridepublic RFuture<Boolean> tryLockAsync(long threadId) {return tryAcquireOnceAsync(-1, null, threadId);}private RFuture<Boolean> tryAcquireOnceAsync(long leaseTime, TimeUnit unit, final long threadId) {if (leaseTime != -1) {return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_NULL_BOOLEAN);}RFuture<Boolean> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_NULL_BOOLEAN);ttlRemainingFuture.addListener(new FutureListener<Boolean>() {@Overridepublic void operationComplete(Future<Boolean> future) throws Exception {if (!future.isSuccess()) {return;}Boolean ttlRemaining = future.getNow();// lock acquiredif (ttlRemaining) {scheduleExpirationRenewal(threadId);}}});return ttlRemainingFuture;}<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));}protected String getLockName(long threadId) {return id + ":" + threadId;}private void scheduleExpirationRenewal(final long threadId) {if (expirationRenewalMap.containsKey(getEntryName())) {return;}Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {@Overridepublic void run(Timeout timeout) throws Exception {RFuture<Boolean> future = renewExpirationAsync(threadId);future.addListener(new FutureListener<Boolean>() {@Overridepublic void operationComplete(Future<Boolean> future) throws Exception {expirationRenewalMap.remove(getEntryName());if (!future.isSuccess()) {log.error("Can't update lock " + getName() + " expiration", future.cause());return;}if (future.getNow()) {// reschedule itselfscheduleExpirationRenewal(threadId);}}});}}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);if (expirationRenewalMap.putIfAbsent(getEntryName(), new ExpirationEntry(threadId, task)) != null) {task.cancel();}}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));}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>() {@Overridepublic void operationComplete(Future<Long> future) throws Exception {if (!future.isSuccess()) {return;}Long ttlRemaining = future.getNow();// lock acquiredif (ttlRemaining == null) {scheduleExpirationRenewal(threadId);}}});return ttlRemainingFuture;}
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• 这里leaseTime没有设置的话，默认是-1，使用的是commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout()，默认为30秒
• tryLockInnerAsync使用的是一段lua脚本，该脚本有3个参数，第一个参数为KEYS数组，后面几个参数为ARGV数组的元素
• 这里key的值为调用方指定的这个redissonLock的名称，两个变量，第一个为leaseTime，第二个为锁的名称，使用redissonLock的id+线程id
• lua脚本第一个方法判断redissonLock的hashmap是否存在，如果不存在则创建，该hashmap有一个entry的key为锁名称，valude为1，之后设置该hashmap失效时间为leaseTime
• lua脚本第二个方法是在redissonLock的hashmap存在的情况下，将该锁名的value增1，同时设置失效时间为leaseTime
• 最后返回该redissonLock名称的key的ttl
• 执行成功之后判断ttl是否还有值，有的话则调用scheduleExpirationRenewal，防止lock未执行完就失效
• scheduleExpirationRenewal是注册一个延时任务，在internalLockLeaseTime / 3的时候触发，执行的方法是renewExpirationAsync，将该锁失效时间重置回internalLockLeaseTime
• scheduleExpirationRenewal里头给scheduleExpirationRenewal任务增加listener，如果设置成功之后还会再次递归调用scheduleExpirationRenewal重新注册延时任务
• tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId)方法是指定自动解锁时间时调用的方法，它与tryAcquireOnceAsync的区别在于，它对ttl的方回值采用long值来判断，如果是null，才执行延长失效时间的定时任务，而tryAcquireOnceAsync方法采用的是BooleanNullReplayConvertor，只要返回不是null，则返回true

RedissonLock.unlock

redisson-3.8.1-sources.jar!/org/redisson/RedissonLock.java

    @Overridepublic void unlock() {try {get(unlockAsync(Thread.currentThread().getId()));} catch (RedisException e) {if (e.getCause() instanceof IllegalMonitorStateException) {throw (IllegalMonitorStateException)e.getCause();} else {throw e;}}//        Future<Void> future = unlockAsync();
//        future.awaitUninterruptibly();
//        if (future.isSuccess()) {
//            return;
//        }
//        if (future.cause() instanceof IllegalMonitorStateException) {
//            throw (IllegalMonitorStateException)future.cause();
//        }
//        throw commandExecutor.convertException(future);}@Overridepublic RFuture<Void> unlockAsync(final long threadId) {final RPromise<Void> result = new RedissonPromise<Void>();RFuture<Boolean> future = unlockInnerAsync(threadId);future.addListener(new FutureListener<Boolean>() {@Overridepublic void operationComplete(Future<Boolean> future) throws Exception {if (!future.isSuccess()) {cancelExpirationRenewal(threadId);result.tryFailure(future.cause());return;}Boolean opStatus = future.getNow();if (opStatus == null) {IllegalMonitorStateException cause = new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: "+ id + " thread-id: " + threadId);result.tryFailure(cause);return;}if (opStatus) {cancelExpirationRenewal(null);}result.trySuccess(null);}});return result;}protected RFuture<Boolean> unlockInnerAsync(long threadId) {return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,"if (redis.call('exists', KEYS[1]) == 0) then " +"redis.call('publish', KEYS[2], ARGV[1]); " +"return 1; " +"end;" +"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.unlockMessage, internalLockLeaseTime, getLockName(threadId));}String getChannelName() {return prefixName("redisson_lock__channel", getName());}void cancelExpirationRenewal(Long threadId) {ExpirationEntry task = expirationRenewalMap.get(getEntryName());if (task != null && (threadId == null || task.getThreadId() == threadId)) {expirationRenewalMap.remove(getEntryName());task.getTimeout().cancel();}}
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• unlockInnerAsync通过lua脚本来释放锁，该lua使用两个key，一个是redissonLock名称，一个是channelName
• 该lua使用的变量有三个，一个是pubSub的unlockMessage，默认为0，一个是internalLockLeaseTime，默认为commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout()，一个是锁名称
• 如果该redissonLock不存在，则直接发布unlock消息返回1；如果该锁不存在则返回nil；
• 如果该锁存在则将其计数-1，如果counter大于0，则重置下失效时间，返回0；如果counter不大于0，则删除该redissonLock锁，发布unlockMessage，返回1；如果上面条件都没有命中返回nil
• unlockAsync里头对unlockInnerAsync注册了FutureListener，主要是调用cancelExpirationRenewal，取消掉scheduleExpirationRenewal任务

LockPubSub

redisson-3.8.1-sources.jar!/org/redisson/pubsub/LockPubSub.java

public class LockPubSub extends PublishSubscribe<RedissonLockEntry> {public static final Long unlockMessage = 0L;@Overrideprotected RedissonLockEntry createEntry(RPromise<RedissonLockEntry> newPromise) {return new RedissonLockEntry(newPromise);}@Overrideprotected void onMessage(RedissonLockEntry value, Long message) {if (message.equals(unlockMessage)) {Runnable runnableToExecute = value.getListeners().poll();if (runnableToExecute != null) {runnableToExecute.run();}value.getLatch().release();}}}
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• 接收到unlockMessage的时候，会调用RedissonLockEntry的listener，然后触发latch的release
• tryAcquireOnceAsync这个方法默认没有创建LockPubSub，而且没有指定自动解锁时间，则定时任务会一直延长失效时间，这个可能存在锁一直没释放的风险

小结

加锁有如下注意事项：

• 加锁需要设置超时时间，防止出现死锁
• 加锁以及设置超时时间的时候，需要保证两个操作的原子性，因而最好使用lua脚本或者使用支持NX以及EX的set方法
• 加锁的时候需要把加锁的调用方信息，比如线程id给记录下来，这个在解锁的时候需要使用
• 对于加锁时长不确定的任务，为防止任务未执行完导致超时被释放，需要对尚未运行完的任务延长失效时间

解锁有如下注意事项：

• 解锁一系列操作(判断key是否存在，存在的话删除key等)需要保证原子性，因而最好使用lua脚本
• 解锁需要判断调用方是否与加锁时记录的是否一致，防止锁被误删
• 如果有延续失效时间的延时任务，在解锁的时候，需要终止掉该任务

doc

• 一分钟实现分布式锁
• 这才是真正的分布式锁
• 漫画：什么是分布式锁？
• Redis分布式锁的正确实现方式（Java版）
• 基于Redis的分布式锁到底安全吗（上）？
• 基于Redis的分布式锁到底安全吗（下）？
• Redis分布式锁的正确实现姿势
• Redisson分布式锁浅析