What does “subsequent read” mean in the context of volatile variables?

Question

Java memory visibility documentation says that:

A write to a volatile field happens-before every subsequent read of that same field.

Answer 1

This is more a definition of what will not happen rather than what will happen.

Essentially it is saying that once a write to an atomic variable has happened there cannot be any other thread that, on reading the variable, will read a stale value.

Consider the following situation.

• Thread A is continuously incrementing an atomic value a.

• Thread B occasionally reads A.a and exposes that value as a non-atomic b variable.

• Thread C occasionally reads both A.a and B.b.

Given that a is atomic it is possible to reason that from the point of view of C, b may occasionally be less than a but will never be greater than a.

If a was not atomic no such guarantee could be given. Under certain caching situations it would be quite possible for C to see b progress beyond a at any time.

This is a simplistic demonstration of how the Java memory model allows you to reason about what can and cannot happen in a multi-threaded environment. In real life the potential race conditions between reading and writing to data structures can be much more complex but the reasoning process is the same.