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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: Java databases as alternative to MySQL on OS X Server? (OT)
Date Tue, 29 Nov 2005 15:28:40 GMT
> From: Oded Arbel [mailto:oded@m-wise.com] 
> 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.  All Java objects reside in the
Java heap, which is shared across all threads of the JVM process and
directly referenceable by all.  Local variables for a Java method exist
in each thread's Java stack, which is normally not shared across threads
other than for garbage collection operations.  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.  (This is also true
of the JRockit and IBM JVMs, as well as several others.)  No copying of
local variable is ever performed; on some OS implementations, parameters
are copied when calling a kernel service that requires a context switch
but that occurs within the OS, not the JVM.

 - Chuck


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