一直对happens-before的理解有些模糊，看了网上的一些解读也是似懂非懂。怎么办？直接看看The Java® Language Specification中是如何解释的，准确、权威。
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).
The wait methods of class Object (§17.2.1) have lock and unlock actions associated with them; their happens-before relationships are defined by these associated actions.
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.
For example, the write of a default value to every field of an object constructed by a thread need not happen before the beginning of that thread, as long as no read ever observes that fact.
More specifically, if two actions share a happens-before relationship, they do not necessarily have to appear to have happened in that order to any code with which they do not share a happens-before relationship. Writes in one thread that are in a data race with reads in another thread may, for example, appear to occur out of order to those reads.
The happens-before relation defines when data races take place.
A set of synchronization edges, S, is sufficient if it is the minimal set such that the transitive closure of S with the program order determines all of the happens-before edges in the execution. This set is unique.
It follows from the above definitions that: