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From Oded Arbel <>
Subject Re: Java databases as alternative to MySQL on OS X Server? (OT)
Date Tue, 29 Nov 2005 21:24:38 GMT
On Tuesday, 29 בNovember 2005 17:28, Caldarale, Charles R wrote:
> > From: Oded Arbel []
> > Subject: Re: Java databases as alternative to MySQL on OS X
> > Server? (OT)
> >
> > inter-thread communication in java is done through shared
> > memory - shared variables, but the Java memory sharing model
> > doesn't really share memory, Instead it uses thread local
> > storage to store copies of shared variables and when you
> > cross into or out of a synchronized block, the contents of
> > the variables are copied.
> That's one of the most bizarre and blatantly wrong descriptions of
> the Java memory model that I've ever read.  

Rather simplified and not accurate where accuracy would have caused me 
to get way off topic, but all in all not really incorrect.

> All Java objects reside 
> in the Java heap, which is shared across all threads of the JVM
> process and directly referenceable by all. 

Please read up on your Java memory model. What you describe is a nice 
abstraction that many Java developers have in their mind, but is 
technically not correct and can't be correct when you consider multiple 
processors, DMA, hyper-threading or even just plain old L2 CPU caches.


Also please note that JSR-133, once implemented, supposedly makes the 
whole discussion irrelevant as it forces read/write ordering. But 
AFAIK, only Sun's JVM 5.0 (1.5) implements it and I'm not sure about 
its correctness.

> Entering a synchronized block in a HotSpot-based JVM normally does
> not require any context switching, since the lock on an object is
> established via the platform's compare-and-exchange instruction; only
> if a conflict exists are kernel services required to suspend the
> conflicting thread.  

You are obviously disregarding the fact that Java is a multi-platform 
environment, and some platforms (notably the Mac OS-X which started the 
whole thread) does not provide such a mechanism. Are you an MS-Windows 
programmer by any chance ?

> No copying of local variable is ever
> performed; 

If it ever appeared that I was talking about 'local variables' when I 
wrote the word 'local' then I'm deeply sorry, and I here-by request 
that you please re-read my original post more carefully.
The word local in this case was used to denote the running thread's view 
on the shared variables, also knows as "thread local storage" (do look 
up that concept as well when you read about the Java memory mode, 

Oded Arbel
m-Wise mobile solutions

+972-9-9611212 (204)

When the student is ready the master appears.
	-- Japanese proverb

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