If Thread B wishes to see changes Thread A makes, can only the last change be to a volatile variable as opposed to all?

问题

I've looked at this answer, and it states how:

Under the new memory model, when thread A writes to a volatile variable V, and thread B reads from V, any variable values that were visible to A at the time that V was written are guaranteed now to be visible to B.

Therefore, given the example:

public class Main {
    static int value = -1;
    static volatile boolean read;

    public static void main(String[] args) {
        Thread a = new Thread(() -> {
            value = 1;
            read = true;
        });

        Thread b = new Thread(() -> {
            while (!read);

            System.out.println("Value: " + value);
        });

        a.start();
        b.start();
    }
}

Is the change to value (from -1 to 1) guaranteed to be visible to thread b despite the non-volatility of value (only read)?

If so, given a bunch of changes that are performed with the intention of being visible to another thread, is there any purpose in making any variable changed other than the last one volatile?

回答1:

Yes, the change to value is guaranteed to be visible to thread b.

JLS 17.4.4. Synchronization Order says:

• A write to a volatile variable v (§8.3.1.4) synchronizes-with all subsequent reads of v by any thread (where "subsequent" is defined according to the synchronization order).

JLS 17.4.5. Happens-before Order says:

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).

Bullet 1 says that value = 1 happens-before read = true.
Bullet 3 says that read = true happens-before !read.
Bullet 1 says that !read happens-before "Value: " + value.
Bullet 4 says that value = 1 happens-before "Value: " + value.

来源：https://stackoverflow.com/questions/52509663/if-thread-b-wishes-to-see-changes-thread-a-makes-can-only-the-last-change-be-to