java - Does 'volatile' guarantee that any thread reads the most recently written value?

Question

From the book Effective Java:

While the volatile modifier performs no mutual exclusion, it guarantees that any thread that reads the field will see the most recently written value

SO and many other sources claim similar things.

Is this true?

I mean really true, not a close-enough model, or true only on x86, or only in Oracle JVMs, or some definition of "most recently written" that's not the standard English interpretation...

Other sources (SO example) have said that volatile in Java is like acquire/release semantics in C++. Which I think do not offer the guarantee from the quote.

I found that in the JLS 17.4.4 it 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)." But I don't quite understand.

There are quite some sources for and against this, so I'm hoping the answer is able to convince that many of those (on either side) are indeed wrong - for example reference or spec, or counter-example code.

Answer 1

Is this true?

I mean really true, not a close-enough model, or true only on x86, or only in Oracle JVMs, or some definition of "most recently written" that's not the standard English interpretation...

Yes, at least in the sense that a correct implementation of Java gives you this guarantee.

Unless you are using some exotic, experimental Java compiler/JVM (*), you can essentially take this as true.

From JLS 17.4.5:

A write to a volatile field (§8.3.1.4) happens-before every subsequent read of that field.

---

(*) As Stephen C points out, such an exotic implementation that doesn't implement the memory model semantics described in the language spec can't usefully (or even legally) be described as "Java".