同步块中的操作是否对其他线程可见？

Question

I have read this Related Question and the answer help a lot, but there is still a question.

I heard that changes of shared fields in a synchronized block is guaranteed visible to other threads.

For codes like this:

Thread1:

synchronized(lock) {
   obj.field1 = value1;
}

Thread2:

synchronized(lock) {
   System.out.println(obj.field1);
}

Assuming thread1 precedes thread2, it is said that according to the Java Specification, it may go like this:

hb(write to obj.field1 in threadOne, unlock in threadOne) AND 
hb(unlock in threadOne, lock in threadTwo) AND 
hb(lock in threadTwo, read from obj.field in threadTwo)

hb stands for happens-before in Java Specification, and it guaranteed the visibility.

And because " If hb(x, y) and hb(y, z), then hb(x, z) ", we get:

hb(write to obj.field1 in threadOne, read from obj.field1 in threadTwo) 

My question is on the first line:

hb(write to obj.field1 in threadOne, unlock in threadOne) 

In Java Specification, I only found:

An unlock action on monitor m synchronizes-with all subsequent lock actions on m (where "subsequent" is defined according to the synchronization order).

That indicates the unlock action happens-before subsequent lock actions. But I cannot find words like:

An action in synchronized block happens-before the unlock action.

So is it correct? And where can I find it ?

Answer 1

All actions of the same thread are ordered by the happens-before relation.

JLS 17.4.5:

If x and y are actions of the same thread and x comes before y in program order, then hb(x, y).

Also note, that happens-before is transitive, thus if the write within the synchronized block happens before unlock, and unlock happens before a lock on another thread, then the write happens before any action in the synchronized block in the other thread.

Answer 2

The best explanation here is that unlock action has a happens-before edge with the last action in the thread. It's called program order. It's described in the documentation

Among all the inter-thread actions performed by each thread t, the program order of t is a total order that reflects the order in which these actions would be performed according to the intra-thread semantics of t.