17.4.5 Happens-before Order
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 (§12.6)
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.
最后是说有happens-before关系的两个action在实现中未必会按照那样发生，因为之前可能还存在合法的变序操作
http://java.sun.com/docs/books/jls/third_edition/html/memory.html
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