In chapter 17 of JLS, it introduce a concept: happens-before consistent.

A set of actions A is happens-before consistent if for all reads r in A, where W(r) is the write action seen by r, it is not the case that either hb(r, W(r)) or that there exists a write w in A such that w.v = r.v and hb(W(r), w) and hb(w, r)"

In my understanding, it equals to following words: ..., it is the case that neither ... nor ...

So my first two questions are:

• is my understanding right?
• what does "w.v = r.v" mean?

It also gives an Example: 17.4.5-1

Thread 1 Thread 2

B = 1; A = 2; 

r2 = A; r1 = B; 

In first execution order:

1: B = 1;

3: A = 2;

2: r2 = A;  // sees initial write of 0

4: r1 = B;  // sees initial write of 0

The order itself has already told us that two threads are executed alternately, so my third question is: what does left number mean?

In my understanding, the reason of both r2 and r1 can see initial write of 0 is both A and B are not volatile field. So my fourth quesiton is: whether my understanding is right?

In second execution order:

1: r2 = A;  // sees write of A = 2

3: r1 = B;  // sees write of B = 1

2: B = 1;

4: A = 2;

According to definition of happens-before consistency, it is not difficult to understand this execution order is happens-before consistent(if my first understanding is correct). So my fifth and sixth questions are: does it exist this situation (reads see writes that occur later) in real world? If it does, could you give me a real example?