发生在Java内存模型中的关系之前 [英] Happens-Before relation in Java Memory Model
问题描述
关于JLS ch17 主题和锁定，它说如果一个动作发生在另一个动作之前，然后第一个动作在第二个动作之前可见并且在第我想知道：
Regarding JLS ch17 Threads and Locks, it says "if one action happens-before another, then the first is visible to and ordered before the second"; I wonder:
（1）说之前订购是什么意思？因为即使action_a发生在action_b之前，action_a可以在action_b之后的某些实现中执行，对吗？
(1) What does it really mean by saying "ordered before"? Because even if action_a happens-before action_b, action_a can be executed after action_b in some implementation, right?
（2）如果action_a发生在action_b之前，是否意味着action_a绝不能看到action_b？或者action_a可能看到或看不到action_b？
(2) If action_a happens-before action_b, does it mean action_a MUST NOT see action_b? Or action_a may see or may not see action_b?
（3）如果action_a没有发生 - 在action_b之前，而action_b没有发生 - 在action_a之前，是否意味着action_a可能看到或看不到action_b？
(3) If action_a does NOT happen-before action_b, and action_b does NOT happen-before action_a, does it mean action_a may see or may not see action_b?
（4）之前没有任何循环发生，对吧？
(4) There could not be any cyclic happens-before, right?
任何答案都将不胜感激：）
Any answer would be appreciated :)
推荐答案
（1）它的真正含义是什么说之前订购？因为即使action_a发生在action_b之前，action_a可以在action_b之后执行，在某些实现中，对吗？
(1) What does it really mean by saying "ordered before"? Because even if action_a happens-before action_b, action_a can be executed after action_b in some implementation, right?
发生之前 - 之前是因果，而不是时间关系。 action_a 在 action_b 之前进行因果排序，无论它是否真正在它之前执行。然而，在实践中，运行时很难在没有时间顺序的情况下保持因果关系。查看我之前的问题，其中详细介绍了该问题。因果关系。
Happens-before is a causal, not a temporal relationship. action_a is causally ordered before action_b, whether or not it actually executes before it. In practice, however, a runtime would have a really hard time maintaning the causality without temporal order. Check out my earlier question which goes into some detail on the subject of causality.
（2）如果action_a发生在action_b之前，是否意味着action_a绝不能看到action_b？或者action_a可能看到或看不到action_b？
(2) If action_a happens-before action_b, does it mean action_a MUST NOT see action_b? Or action_a may see or may not see action_b?
行动的可见性有一个明确的整体顺序。这由指定格式良好的执行的部分处理。因此，对于 a 和 b 中的任何两个操作， b 或 b都可以看到 a 到 a ，或者以上都不是。理解格式良好的执行概念的一个很好的解读是 Java内存模型示例：好的，坏的和丑的。
There is a definite overall order of the visibility of actions to one other. This is dealt with by the section specifying well-formed executions. Therefore, for any two actions a and b, either a is visible to b, or b to a, or none of the above. A good read to understand the notion of well-formed executions is Java Memory Model Examples: Good, Bad, and Ugly.
（3）如果action_a没有发生 - 在action_b之前，并且action_b没有发生-before action_a，是否意味着action_a可能会看到或看不到action_b？
(3) If action_a does NOT happen-before action_b, and action_b does NOT happen-before action_a, does it mean action_a may see or may not see action_b?
是的，两者都是可能的。无论如何都无法保证。
Yes, either is possible. There is no guarantee either way.
（4）之前没有任何循环发生，对吧？
(4) There could not be any cyclic happens-before, right?
发生前必须强加部分排序，而排序的关键属性是无循环。
Happens-before must impose a partial ordering, and the key property of ordering is no loops.
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