Java100705：Memory Consistency Error(内存一致错误)

 

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Hist:

100705, draft

 

 

 

当不同的thread访问同一个memory时,thread看到的的memory不一致(有差异), 这就是Memory Consistency Error. 也就是说thread A改变memory后,并不保证thread B能即刻看到这些变化. 实际就是同步的问题.

要避免Memory Consistency Error, 就必须要求thread A的变动memory操作必须比thread B的观察操作’发生的早’(happens-before). Happens-before 在JLS 17.4.5()中有说明, 引用如下:

Two actions can be ordered by a happens-before relationship. If one action happens-before another, then the first is visible to and ordered before the second.

If we have two actions x and y, we write hb(x, y) to indicate that x happens-before y.

• If x and y are actions of the same thread and x comes before y in program order, then hb(x, y).
• There is a happens-before edge from the end of a constructor of an object to the start of a finalizer for that object.
• If an action x synchronizes-with a following action y, then we also have hb(x, y).
• If hb(x, y) and hb(y, z), then hb(x, z).

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.