了解JVM会发生 - 之前和重新排序(Understanding JVM Happens-before and reorder)

 我正在阅读关于内存模型的JLS规范， 17.4.5发生在订单之前 。 我不明白第一条规则： 
 
 
 
 “＃如果x和y是同一个线程的动作，并且x按照程序顺序在y之前，那么hb（x，y）。” 
 
 
 我们假设A和B是可以在多个线程之间共享的对象（类对象的实例）： 
 
int i=A.getNum();  // ActionA
int j=B.getNum();  // ActionB
 
 三个问题： 
 
 
 
 根据上述规则，是否意味着hb（ActionA，ActionB） ？ 
 
 
 如果对1的答案为真，那么这是否意味着根据规则之前发生的事情，ActionB无法在遵循JSR133内存模型的任何JVM的ActionA之前重新排序？ 
 
 
 如果1和2都是真的，那么ActionA和ActionB似乎不相关，为什么不能重新排序呢？ 只是为了这个规范？ 
 

I am reading the JLS spec on memory model, 17.4.5 Happens-before Order. I do not understand the first rule: 
 
 
 
"# If x and y are actions of the same thread and x comes before y in program order, then hb(x, y)."
 
 
Let's assume A an B are objects (instances of class object) that can be shared between multiple threads:
 
int i=A.getNum();  // ActionA
int j=B.getNum();  // ActionB
 
Three questions:
 
 
 
According to the above rule, does it mean hb(ActionA,ActionB)?
 
 
If the answer to 1 is true, does it mean according to happens-before rule, ActionB can not reordered to come before ActionA in any JVM that follows JSR133 memory model?
 
 
If 1 and 2 both are true, it seems that ActionA and ActionB are not relevant, why can not reorder them? just for this spec?
 

原文：https://stackoverflow.com/questions/6633983