AQS唤醒后继线程为什么是从后向前遍历

欧阳正山

private void unparkSuccessor(Node node) {
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/*
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* If status is negative (i.e., possibly needing signal) try& t5 h# W& R, f& {
* to clear in anticipation of signalling. It is OK if this" B) ?" [( G0 s- D/ e* f- L
* fails or if status is changed by waiting thread.
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int ws = node.waitStatus;" y6 E+ v" Q8 |5 {2 q( ]
if (ws < 0)8 a P0 z M$ }
compareAndSetWaitStatus(node, ws, 0); b( O$ m/ q0 M7 h b
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/*3 y% ~: ?4 C G5 y6 c
* Thread to unpark is held in successor, which is normally, U# H1 |& ?. j0 S& U8 p2 p: w C
* just the next node. But if cancelled or apparently null,
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* traverse backwards from tail to find the actual) n F! \: l0 Q0 B
* non-cancelled successor.
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*/
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Node s = node.next; ?# z+ D4 U/ |* w3 [% C0 y
if (s == null || s.waitStatus > 0) {( Z) Q$ u$ b! ^- y$ h4 p+ Y; R
s = null;. R4 C( x" b7 |, ]( \% P' _; R
for (Node t = tail; t != null && t != node; t = t.prev)0 u: `9 C" t* h K- A# p$ ^' M; C' e
if (t.waitStatus <= 0) B* X# v/ P- r ~1 j( @
s = t;
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if (s != null)1 K! Q3 C( M" c) m. m4 R( y- O" F
LockSupport.unpark(s.thread); }
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如果没有合法的数据，从后向前遍历，为什么这么做呢？从前向后不行吗？

lgd1152已获得悬赏 5 金钱最佳答案

因为从前往后会遇到空指针

AQS唤醒后继线程 为什么是从后向前遍历