- 图片来源：https://www.bilibili.com/video/BV1cr4y1671t?p=160&vd_source=f52d9488d7d3c21ed33580e4dce1a022
1. 分布式锁需要优化的问题
- 不可重入：同一个线程，依次执行两个方法都要获取同一把分布式锁，需要可重入锁
- 不可重试：获取锁只尝试一次就返回false，缺少重试机制
- 超时释放：业务执行时间长，导致锁释放，存在安全隐患
- 主从一致性：主从集群，数据同步存在时延，主机宕机，从机还没有完成同步锁数据
2. 可重入锁
- 判断持有锁是不是自己
- 是自己将上锁次数+1
- 释放锁一次，上锁次数-1
- 释放锁两次，上锁次数-1，为0，释放锁
利用HASH结构记录线程ID和重入次数
//org.redisson.RedissonLock#tryLockInnerAsync
<T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
internalLockLeaseTime = unit.toMillis(leaseTime);
return evalWriteAsync(getName(), 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(getName()), internalLockLeaseTime, getLockName(threadId));
}
// org.redisson.RedissonLock#unlockInnerAsync
protected RFuture<Boolean> unlockInnerAsync(long threadId) {
return 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.asList(getName(), getChannelName()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName(threadId));
}
3. 锁重试
- 不是单一的死循环，而是通过消息订阅和信号量的方式来节省资源，收到消息或信号量，才去重试获取锁
org.redisson.RedissonLock#tryLock(long, long, java.util.concurrent.TimeUnit)
@Override
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);
// lock acquired
if (ttl == null) {
return true;
}
time -= System.currentTimeMillis() - current;
if (time <= 0) {
acquireFailed(threadId);
return false;
}
current = System.currentTimeMillis();
// 订阅一个通知，这个通知就是锁释放时的通知 "redis.call('publish', KEYS[2], ARGV[1]); "
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(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;
}
// 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(threadId);
return false;
}
}
} finally {
unsubscribe(subscribeFuture, threadId);
}
// return get(tryLockAsync(waitTime, leaseTime, unit));
}
4. 锁超时
- 获取锁之后，利用watchDog定时任务去续约
//org.redisson.RedissonLock#tryAcquireAsync
//org.redisson.RedissonLock#scheduleExpirationRenewal
//org.redisson.RedissonLock#renewExpiration
//org.redisson.RedissonLock#scheduleExpirationRenewal
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 " + getName() + " expiration", e);
return;
}
if (res) {
// reschedule itself
// 递归调用自身，会不断的续约，什么时候取消续约。在锁释放时 cancelExpirationRenewal(threadId);
renewExpiration();
}
});
}
}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
ee.setTimeout(task);
}
private void scheduleExpirationRenewal(long threadId) {
ExpirationEntry entry = new ExpirationEntry();
// 续约时，锁的可重入
ExpirationEntry oldEntry = EXPIRATION_RENEWAL_MAP.putIfAbsent(getEntryName(), entry);
if (oldEntry != null) {
oldEntry.addThreadId(threadId);
} else {
entry.addThreadId(threadId);
renewExpiration();
}
}
5. 主从一致性
如下图所示：主节点宕机后，数据同步没有完成，导致新的主节点没有同步原主节点的数据，来上锁时，别的JVM上锁成功了。
所以不使用主从的形式来管理集群，使用同等节点的方式 redissonClient.getMultiLock()
- 所有节点要全部上锁
- 一个失败，其他要全部释放
- 全部上锁后，要续约一次，使首次上锁与末次上锁的释放时间保持一致
5.1 续约
@Override
org.redisson.RedissonMultiLock#tryLock(long, long, java.util.concurrent.TimeUnit)
public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
// try {
// return tryLockAsync(waitTime, leaseTime, unit).get();
// } catch (ExecutionException e) {
// throw new IllegalStateException(e);
// }
long newLeaseTime = -1;
if (leaseTime != -1) {
if (waitTime == -1) {
newLeaseTime = unit.toMillis(leaseTime);
} else {
newLeaseTime = unit.toMillis(waitTime)*2;
}
}
long time = System.currentTimeMillis();
long remainTime = -1;
if (waitTime != -1) {
remainTime = unit.toMillis(waitTime);
}
long lockWaitTime = calcLockWaitTime(remainTime);
int failedLocksLimit = failedLocksLimit();
List<RLock> acquiredLocks = new ArrayList<>(locks.size());
for (ListIterator<RLock> iterator = locks.listIterator(); iterator.hasNext();) {
RLock lock = iterator.next();
boolean lockAcquired;
try {
if (waitTime == -1 && leaseTime == -1) {
lockAcquired = lock.tryLock();
} else {
long awaitTime = Math.min(lockWaitTime, remainTime);
lockAcquired = lock.tryLock(awaitTime, newLeaseTime, TimeUnit.MILLISECONDS);
}
} catch (RedisResponseTimeoutException e) {
unlockInner(Arrays.asList(lock));
lockAcquired = false;
} catch (Exception e) {
lockAcquired = false;
}
if (lockAcquired) {
acquiredLocks.add(lock);
} else {
if (locks.size() - acquiredLocks.size() == failedLocksLimit()) {
break;
}
if (failedLocksLimit == 0) {
unlockInner(acquiredLocks);
if (waitTime == -1) {
return false;
}
failedLocksLimit = failedLocksLimit();
acquiredLocks.clear();
// reset iterator
while (iterator.hasPrevious()) {
iterator.previous();
}
} else {
failedLocksLimit--;
}
}
if (remainTime != -1) {
remainTime -= System.currentTimeMillis() - time;
time = System.currentTimeMillis();
if (remainTime <= 0) {
unlockInner(acquiredLocks);
return false;
}
}
}
if (leaseTime != -1) {
List<RFuture<Boolean>> futures = new ArrayList<>(acquiredLocks.size());
for (RLock rLock : acquiredLocks) {
RFuture<Boolean> future = ((RedissonLock) rLock).expireAsync(unit.toMillis(leaseTime), TimeUnit.MILLISECONDS);
futures.add(future);
}
for (RFuture<Boolean> rFuture : futures) {
rFuture.syncUninterruptibly();
}
}
return true;
}