Redisson可重入锁底层原理
早睡蛋
# Redisson加锁解锁方法
public void deduct() {
//加锁
RLock lock = redissonClient.getLock("lock");
lock.lock();
try {
//1.查询库存信息
String stock = redisTemplate.opsForValue().get("stock");
//2.判断库存是否充足
if (stock != null && stock.length() != 0) {
Integer st = Integer.valueOf(stock);
if (st > 0) {
//3.扣减库存
redisTemplate.opsForValue().set("stock", String.valueOf(--st));
}
}
} finally {
//解锁
lock.unlock();
}
}
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# RLock的lock()方法
我们点开lock方法，发现和ReentrantLock一样，RLock接口也是继承自Lock接口
查阅RLock接口，发现实现类几个，其中一个为RedissonBaseLock，我们用的RedissonLock就是继承自RedissonBaseLock
RedissonLock实现的就是Lock接口中的lock方法
@Override
public void lock() {
try {
lock(-1, null, false);
} catch (InterruptedException e) {
throw new IllegalStateException();
}
}
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实现原理：
- ReentrantLock基于AQS
- RedissonLock基于Redis
# Redisson的lock()方法
Redisson的lock()方法里面都调用了本类的lock方法
@Override
public void lock() {
try {
lock(-1, null, false);
} catch (InterruptedException e) {
throw new IllegalStateException();
}
}
@Override
public void lock(long leaseTime, TimeUnit unit) {
try {
lock(leaseTime, unit, false);
} catch (InterruptedException e) {
throw new IllegalStateException();
}
}
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本类的lock方法：
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;
}
CompletableFuture<RedissonLockEntry> future = subscribe(threadId);
pubSub.timeout(future);
RedissonLockEntry entry;
if (interruptibly) {
entry = commandExecutor.getInterrupted(future);
} else {
entry = commandExecutor.get(future);
}
try {
while (true) {
ttl = tryAcquire(-1, leaseTime, unit, threadId);
// lock acquired
if (ttl == null) {
break;
}
// waiting for message
if (ttl >= 0) {
try {
entry.getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {
if (interruptibly) {
throw e;
}
entry.getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
}
} else {
if (interruptibly) {
entry.getLatch().acquire();
} else {
entry.getLatch().acquireUninterruptibly();
}
}
}
} finally {
unsubscribe(entry, threadId);
}
// get(lockAsync(leaseTime, unit));
}
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这里我们重点关注tryAcquire()方法，在ReentrantLock中，他的底层也是经过tryAcquire()方法实现的，但是ReentrantLock是基于AQS，所以具体实现方法有点不一样。
tryAcquire()方法代码：
private Long tryAcquire(long waitTime, long leaseTime, TimeUnit unit, long threadId) {
return get(tryAcquireAsync(waitTime, leaseTime, unit, threadId));
}
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这里又调用了tryAcquireAsync()方法，继续查阅tryAcquireAsync()代码：
private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {
RFuture<Long> ttlRemainingFuture;
if (leaseTime > 0) {
ttlRemainingFuture = tryLockInnerAsync(waitTime, leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
} else {
ttlRemainingFuture = tryLockInnerAsync(waitTime, internalLockLeaseTime,
TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
}
CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {
// lock acquired
if (ttlRemaining == null) {
if (leaseTime > 0) {
internalLockLeaseTime = unit.toMillis(leaseTime);
} else {
scheduleExpirationRenewal(threadId);
}
}
return ttlRemaining;
});
return new CompletableFutureWrapper<>(f);
}
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tryAcquireAsync()方法执行了一个tryLockInnerAsync()尝试异步加锁的方法，我们继续查阅，发现也是用了一个Lua脚本来实现的。
<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));
}
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# Redisson加锁的自动续期原理
上述加锁过程，在tryAcquireAsync()方法内，有一个scheduleExpirationRenewal()方法，直译就是定时过期时间重置，并传入了一个threadId。我们进入此方法：
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);
try {
renewExpiration();
} finally {
if (Thread.currentThread().isInterrupted()) {
cancelExpirationRenewal(threadId);
}
}
}
}
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其中最核心的方法就是renewExpiration()方法，直译为重置过期时间。进入
renewExpiration()方法内部，我们发现，这里用了一个定时器，这个定时器和我们之前用java的util报下的Timer定时器很像。
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;
}
CompletionStage<Boolean> future = renewExpirationAsync(threadId);
future.whenComplete((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();
} else {
cancelExpirationRenewal(null);
}
});
}
}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
ee.setTimeout(task);
}
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进入newTimeout()方法内部，可看到使用了timer去调用定时任务，这个timer是netty里的时间轮提供的，并不是JUC里面的timer。不过以前的版本确实用的是JUC的timer。
@Override
public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
try {
return timer.newTimeout(task, delay, unit);
} catch (IllegalStateException e) {
if (isShuttingDown()) {
return DUMMY_TIMEOUT;
}
throw e;
}
}
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不过这个timer并不是我们研究的重点，我们回到上一级renewExpiration()代码中，其中有另外一个方法调用，renewExpirationAsync方法底层就是通过Lua脚本进行自动续期的。
protected CompletionStage<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));
}
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# Redisson的unlock()方法
@Override
public 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);
}
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进入unlockAsync()方法内部，这里调用了一个unlockInnerAsync方法直译为异步解锁。
@Override
public RFuture<Void> unlockAsync(long threadId) {
RFuture<Boolean> future = unlockInnerAsync(threadId);
CompletionStage<Void> f = future.handle((opStatus, e) -> {
cancelExpirationRenewal(threadId);
if (e != null) {
throw new CompletionException(e);
}
if (opStatus == null) {
IllegalMonitorStateException cause = new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: "
+ id + " thread-id: " + threadId);
throw new CompletionException(cause);
}
return null;
});
return new CompletableFutureWrapper<>(f);
}
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继续进入unlockInnerAsync()方法内，发现也是通过Lua脚本来实现的。
rotected 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));
}
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# 测试Redisson的自动续期和可重入
可重入锁：
分布式锁和同步器文档：文档 (opens new window)
官方描述：
大家都知道，如果负责储存这个分布式锁的Redisson节点宕机以后，而且这个锁正好处于锁住的状态时，这个锁会出现锁死的状态。为了避免这种情况的发生，Redisson内部提供了一个监控锁的看门狗，它的作用是在Redisson实例被关闭前，不断的延长锁的有效期。默认情况下，看门狗的检查锁的超时时间是30秒钟，也可以通过修改Config.lockWatchdogTimeout (opens new window)来另行指定。
我们也可以使用最简单的锁吗，通过设置过期时间：
// 加锁以后10秒钟自动解锁
// 无需调用unlock方法手动解锁
lock.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 = lock.tryLock(100, 10, TimeUnit.SECONDS);
if (res) {
try {
...
} finally {
lock.unlock();
}
}
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同时还有异步执行方法：
RLock lock = redisson.getLock("anyLock");
lock.lockAsync();
lock.lockAsync(10, TimeUnit.SECONDS);
Future<Boolean> res = lock.tryLockAsync(100, 10, TimeUnit.SECONDS);
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测试：
我们让我们的业务代码加锁之后睡眠1000秒，然后我们可以发现，锁的时间会不断地重置，实现了自动续期。