Memory Model (yeah, this again...)

I have a debate going on StackOverflow over an answer to this question:

http://stackoverflow.com/questions/300316/java-concurrency-scenario-do-i-=
need-synchronization-or-not#300394

I'm 'Ken G' in that forum.

My understanding of the problem/question:

1) A Single instance of a class with an instance variable (heap, IIRC) th=
at is a reference to a HashMap - private visibility, not volatile.
2) Multiple threads containing "readers" that get the value of the HashMa=
p reference via an unsynchronized accessor method .  Threads exit, new th=
reads get a HashMap reference as part of their startup via the method
3) The application refreshes with a new HashMap reference on some periodi=
c basis.
4) It is OK if existing reader threads have references to an "old" HashMa=
p as long as new threads get the reference to the "newest" HashMap.

Given those conditions, my understanding is that no synchronization is re=
quired for *new* reader threads to pick up the most recent value of the H=
ashMap reference.  With #4, there is no added value by synchronization or=
 declaring the private, defining reference "volatile".

The dissenting opinion is that "volatile" is required (or some other barr=
ier) to force the *application's* copy of the reference to be visible to =
other threads.

We've both been through section 17, and either we're not understanding it=
 or we're not agreeing on the requirement.

Would ya'll please take a look and point out if/where we're incorrect?

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
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