共享模式与基于 Condition 的等待 / 通知机制实现（2）
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- UID
- 1066743
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共享模式与基于 Condition 的等待 / 通知机制实现（2）
第19行~第23行的代码是独占锁和共享锁最不一样的一个地方，我们再看独占锁acquireQueued的代码：
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| final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
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这意味着独占锁某个节点被唤醒之后，它只需要将这个节点设置成head就完事了，而共享锁不一样，某个节点被设置为head之后，如果它的后继节点是SHARED状态的，那么将继续通过doReleaseShared方法尝试往后唤醒节点，实现了共享状态的向后传播。
共享模式release实现流程上面讲了共享模式下acquire是如何实现的，下面再看一下release的实现流程，方法为releaseShared：
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| public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
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tryReleaseShared方法是子类实现的，如果tryReleaseShared成功，那么执行doReleaseShared()方法：
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| private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
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主要是两层逻辑：
- 头结点本身的waitStatus是SIGNAL且能通过CAS算法将头结点的waitStatus从SIGNAL设置为0，唤醒头结点的后继节点
- 头结点本身的waitStatus是0的话，尝试将其设置为PROPAGATE状态的，意味着共享状态可以向后传播
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