Need help with servlet thread safety... 2005-04-11 - By Brett Zamora
Back
Hi...
I'm trying to build a JSP/Servlet application that will retrieve, add,
update, and delete records from a database. I've recently encountered some
of the complications that can be seen if a servlet is not written to be
thread safe.
First I'd like to say that I've been reading and reading on this issue and I
thought I had a pretty good grasp on the subject, however, I still cannot
eliminate the condition I am experiencing.
The problem I'm seeing is when I first start up my application server,
either Tomcat or Resin, and I run my application, which retrieves records to
be displayed from the database, invalid data is displayed in some of the
fields. Basically, the instance variables for the bean class are being
trashed.
I can only create this condition the first time after I stop and restart the
application server. After the first occurrence, it does not seem to
continue. This doesn't mean the condition doesn't exist, just that I can't
seem to recreate it.
I have set up a sample of the problem at http://www.zware.net/simpleDemo
It is a very basic, very small sample amount of code that does what I
outlined on the simple demo page. I've also provided the source code, so
that anybody that might be willing to help me, could see how I am attempting
to synchronize my code to avoid the condition I am seeing.
In a nutshell, I create a bean for the user record. I then synchronize the
bean and call a method to retrieve the data. Before exiting the synchronize
block, I add the bean to the session object. I then exit the synchronize
block and forward control to the jsp page to display the data.
As I said, I can only cause the condition to occur in a very specific way,
however, I'm inclined to believe that I'm doing something drastically wrong
and I am afraid to continue developing anything until I completely
understand how to avoid the multi-threading problem that I seem to be
experiencing.
I would appreciate any feedback that anybody could provide on what I've done
wrong. Please don't slam me for code structure. This simple app was put
together to be readable and to solicit some assistance with my
use/understanding of how to synchronize an object.
Synchronization seems pretty straight-forward, conceptually, however, I
cannot resolve this issue. I've even gone so far as to use the implements
SingleThreadModel on the SimpleCtl class, but this still does not resolve
the condition.
I'm wondering if it could be my JDBC driver? My backend database is MySql
and I've tried a couple of version 3.x JDBC drivers, but again, I cannot
resolve the problem.
Any help is greatly appreciated.
Regards,
Brett
Sample: http://www.zware.net/simpleDemo
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