Multi processor issue 2006-12-08 - By Caldarale, Charles R
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> From: Marziou, Gael [mailto:gael.marziou@(protected)]
> Subject: Multi processor issue
>
> We have recently switched a stable web application from JRun
> 4 to Tomcat 5.5.20 on Windows 2003 + Sun's JRE 1.5.0_08,
> running some testing for months, but when we moved to
> production environment we immediately experienced errors
> IllegalStateException: No modifications are allowed
> to a locked ParameterMap.
>
> We made code reviews of our application code and Tomcat's code and did
> not find any thread safety issues.
Look again.
> So, I don't believe it's a Tomcat, I would rather think to an H/W, or
> JRE or OS issue.
Welcome to the world of multi-processors (which I've been working with
since the 1960s.) The problem is almost undoubtedly in your application
- we run Tomcat on 16x and 32x systems without any such issues.
Within Tomcat, request objects are normally processed by a single thread
at a time, yet you note that two threads are accessing this particular
request - where did the second one come from? Is your app starting
extra threads? (Perhaps unknowingly, by using some third-party
package.)
Since requests are typically handled by a single thread, there is little
or no synchronization done during manipulation of references associated
with them, such as the parameter map. If your app is using multiple
threads to handle a request, it needs to provide its own synchronization
when calling methods of the request that manipulate data associated with
the request - even when just reading, since the other thread may be
changing it concurrently.
- Chuck
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