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From: Lew <noone@lewscanon.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Strange Socket problem
Date: Sat, 03 Mar 2012 23:21:19 -0800
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x wrote:
> But, the volatile keyword does not provide any kind of access control. It just
> tells the JVM "hey, if this variable is changed by thread A, the changed value
> must be visible for thread B, so don't cache it too aggressively'.

It does more than that.

All writes from thread A prior to the write to the volatile variable are 
visible from thread B after the latter reads that variable.

However I agree that multiple views of the same volatile variable introduce 
danger, as in the idiom

  if (volaVar != null)
  {
    volarVar.doSomething();
  }

because there's a threat window between the conditional check for non-'null' 
and the dereference to the method call where another thread could set the 
variable to a different value.

There's also a threat when you use two volatile variables to control a code block:

  if (volaVar1.equals(SOME_VALUE) && volaVar2.equals(DIFFERENT_VALUE)) ...

because 'volaVar1' can change by the time you test the second clause.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg