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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* If status is negative (i.e., possibly needing signal) try
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* to clear in anticipation of signalling. It is OK if this8 @' _/ V; j1 l6 ]/ ?* H+ l$ w9 Q) E
* fails or if status is changed by waiting thread.
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int ws = node.waitStatus;7 C4 x4 }2 R, C, o. V, x2 r
if (ws < 0)
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compareAndSetWaitStatus(node, ws, 0);
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/*
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* Thread to unpark is held in successor, which is normally0 S4 h# h9 H* h! }3 m5 R
* just the next node. But if cancelled or apparently null, j2 V; f% A8 b6 k+ h- A& l
* traverse backwards from tail to find the actual
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* non-cancelled successor./ p% S$ C2 I1 w) w; V
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Node s = node.next;9 ], P3 i/ k& W% J9 R
if (s == null || s.waitStatus > 0) {
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s = null;
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for (Node t = tail; t != null && t != node; t = t.prev)3 Q9 @3 _, C3 I7 h. z. |# D" j
if (t.waitStatus <= 0)* s9 E& o, b+ C* `
s = t;
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}
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if (s != null)+ r2 F" V @$ u& M7 z# ?) ~, T1 P* y
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