Why not just let SQL do the update? Do the following:
update my_table set counter_field = counter_field + 1 (where clause here if needed)
(off the top of my head, my syntax might be off)
In other words, why not let the database handle the concurrency issue? That's kind of what they're there for (partly anyway).
Malia Noori wrote:
Thank you for replying, but I would appreciate it if you could expand on your explanation. Here is my original post:
Actually, the data that I am modifying requires a transaction and
synchronization. It increments a counter stored in the database. So, I have to do a select to get the current value, increment the counter, and then insert the new value. So if two threads are accessing it at the same time, the counter will not be properly incremented. What's puzzling is
that
method level synchronization does not work while synchronizing on a block
of
code inside a method works.
Regardless of what persistence mechanism I use (i.e. Hibernate, EJB, etc), I
have to synchronize on the method that increments the counter stored in the
database. I can't avoid this due to business logic. Please let me know if
there is alternative to synchronization.
Thanks, Malia
-----Original Message-----
From: QM [mailto:[EMAIL PROTECTED] Sent: Thursday, September 30, 2004 3:01 PM
To: Tomcat Users List
Subject: Re: method level synchronization doesn't work
On Thu, Sep 30, 2004 at 01:14:26PM -0400, Malia Noori wrote: : I thought Tomcat instantiates only 1 instance of JSP servlet to handle all : requests. Isn't that the reason why it's not suggested to have instance : variables?
Close, but not quite. ;)
The servlet spec permits a container to create as many instances of a servlet class as it sees fit. The same goes for creating and destroying a servlet object several times over a context (webapp) lifetime.
That said, a servlet's main methods -- doGet(), doPost(), service() -- may be called concurrently for the same servlet object. These methods are thus meant to be thread-safe, and as such they shouldn't modify object instance variables. (It's fine to call such variables, as long as they are read-only or at least treated as such.)
: If it is the case that 2 instance of JSP servlets gets created : to handle each request, what's the best object to synchronize on?
I missed your original post, so I can say only this: when you find yourself painted into a corner, it's time to revisit the design stage.
-QM
-- Frank W. Zammetti Founder and Chief Software Architect Omnytex Technologies http://www.omnytex.com
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