Re: Thread safe instance variables

["Larry Loen" <lwloen@xxxxxxxxxx>]

>I think I made the assumption that Daniel is talking about
>Servlets, where it makes sense to avoid synchronization and
>not Swing.

Well, in servlets, I am of the opinion that synchronization can be kept
largely or entirely optional.  The way I would work around the
"multi-threading" of the servlet object architecture is that I create a
"shadow" object (one-to-one with each servlet I need).  At servlet entry,
I construct the shadow object normally or perhaps fetch (using synchronize)
from some stash of recently used shadow objects.  I then initialize the
shadow's instance variables with whatever I need.  It would typically
include stuff passed in, like the output and input objects, and so on.
This is how I think the servlet architecture should have been defined to
start with.  I then do something like shadowobject.run() to do the real
work of the servlet.

Since the object that does the real work "sits" on the stack, it is
inherently single-threaded (my code is started up in a single thread, no
matter who else is logically capable of being launched in the underlying
original Servlet object itself).  It is different in detail, but it is
functionally identical to implementing a run() method for a Runnable
object.

Perhaps someone can show that this is a bad idea, but it has worked for
what I have done.  This restores normal object-oriented rules at any rate
-- I can treat instance variables in the shadow object normally and I'm
back to normal considerations between the styles 1, 2, or 3 of my original
note if I need more than the original thread to get work done.

Now, if those "shadow" objects have to deal with state-ful beans of some
kind, beans that live over the course of multiple servlet invocations, then
I'll probably be synchronizing to reach those.  Ditto for anything that for
some reason has to sit in the servlet object.  For the rest, I think I can
be synchronize free and yet not have hard-to-think-about thread safety
considerations.

All this, as long as one can confine most or all of the work to the shadow
object.  What am I missing, here, that this would not usually work?


Larry W. Loen  -   Senior Linux, Java, and iSeries Performance Analyst
                          Dept HP4, Rochester MN