Java内存模型:易变变量和发生之前 [英] Java memory model: volatile variables and happens-before

问题描述

我想说明 happens-before 关系如何与 volatile 变量一起使用.让我们有以下变量:

I'd like to clarify how happens-before relation works with volatile variables. Let we have the following variables:

public static int i, iDst, vDst;
public static volatile int v;

和线程A:

i = 1;
v = 2;

和线程B:

vDst = v;
iDst = i;

以下语句是否符合Java内存模型(JMM)??如果不正确，什么是正确的解释?

Are the following statements correct in accordance with Java memory model (JMM)? If not, what would be correct interpretation?

• i = 1总是在发生之前 v = 2
仅在JMM中
• v = 2 在...之前发生 vDst = v 如果v = 2实际上在时间上早于vDst = v之前，则JMM中的
• i = 1 发生在 iDst = i(并且iDst将可预测地分配为1)
• 否则i = 1和iDst = i之间的顺序不确定，并且iDst的结果值也不确定

• i = 1 always happens-before v = 2
• v = 2 happens-before vDst = v in JMM only if it's actually happens before in time
• i = 1 happens-before iDst = i in JMM (and iDst will be predictably assigned 1) if v = 2 actually happens before vDst = v in time
• Otherwise order between i = 1 and iDst = i is undefined and resulting value of iDst is undefined as well

逻辑错误:

JMM中没有挂钟时间"的概念，我们应该依靠同步顺序作为v = 2和vDst = v的排序指南.有关更多详细信息，请参见所选答案.

There is no "wall clock time" concept in JMM, and we should rely on synchronization order as an ordering guide for v = 2 and vDst = v. See the chosen answer for further details.

推荐答案

• i = 1总是在发生之前 v = 2
• i = 1 always happens-before v = 2

是的.通过JLS部分 17.4.5 ，

True. By JLS section 17.4.5,

如果 x 和 y 是同一线程的动作，并且 x 按照程序顺序排在 y 之前，然后是 hb(x，y).

If x and y are actions of the same thread and x comes before y in program order, then hb(x, y).

---

仅在JMM中
• v = 2 在...之前发生 vDst = v 如果v = 2实际上在时间上早于vDst = v之前，则JMM中的
• i = 1 发生在 iDst = i(并且iDst将可预测地分配为1)

---

• v = 2 happens-before vDst = v in JMM only if it's actually happens before in time
• i = 1 happens-before iDst = i in JMM (and iDst will be predictably assigned 1) if v = 2 actually happens before vDst = v in time

错.先发生先后顺序并不能保证事情在物理上先于先发生.在JLS的同一部分，

False. The happens-before order does not make guarantees about things happening before each other in physical time. From the same section of the JLS,

应注意，两个动作之间存在先发生后关系，并不一定意味着它们必须在实现中按此顺序进行.如果重新排序产生的结果与合法执行相符，则不合法.

It should be noted that the presence of a happens-before relationship between two actions does not necessarily imply that they have to take place in that order in an implementation. If the reordering produces results consistent with a legal execution, it is not illegal.

但是，如果v = 2出现在v = 2 vDst = v和i = 1 happens-before iDst = i之前，则可以保证v = 2 c3>按照同步顺序，通常是误认为实时顺序的执行同步动作的总顺序.

It is, however, guaranteed that v = 2 happens-before vDst = v and i = 1 happens-before iDst = i if v = 2 comes before vDst = v in the synchronization order, a total order over the synchronization actions of an execution that is often mistaken for the real-time order.

• 否则i = 1和iDst = i之间的顺序不确定，并且iDst的结果值也不确定
• Otherwise order between i = 1 and iDst = i is undefined and resulting value of iDst is undefined as well

如果vDst = v按照同步顺序排在v = 2之前，则是这种情况，但实际时间没有计算在内.

This is the case if vDst = v comes before v = 2 in the synchronization order, but actual time doesn't come into it.

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