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From: chris derham <chris@derham.me.uk>
Date: Thu, 9 Aug 2012 07:41:18 -0300
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 <CAG6aRdXx7U5h-MX6pbEx+6Z_dsgzPB84OYX6YADtSU90-5kzFw@mail.gmail.com>
Subject: Re: ClassCastException org.apache.jasper.runtime.ELContextImpl cannot
 be cast to org.apache.jasper.el.ELContextImpl
To: Tomcat Users List <users@tomcat.apache.org>
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| I am 95% certain of what is happenning but haven't tested this. It will
help your understanding if you read the EL specification, specifically the
section in the Javadoc regarding how implementations are discovered.
| Tomcat implements these rules. It is possible for an EL implementation to
be loaded from an application. Everything will be fine EL-wise for the
application that shipped with the EL implementation but things will
| fall apart for any other application that tries to use EL since the
implementation classes will not be visible.

Mark,

I don't understand one key component of your response. I don't pretend to
understand the internals of tomcat, but I thought that I had a good grasp
on how things were supposed to work. Please forgive me if I ask something
stupid.

You seem to say that the EL implementation will be loaded from the app2, as
it has an implementation present in its lib. You then say that this won't
be visible to app 1, presumably because this class is loaded into the
app2's class loader. That bit matches up with my understanding on app class
loaders in web containers - each app has its own, and they are separate
preventing leakage from one app to another. The bit I don't really
understand is when app 1 tries to run some EL code, surely the same
mechanism should kick in and load an EL implementation. I get that app 1
doesn't have an impl in its lib folder, but I would expect the call to be
passed to the tomcat shared class loader, whereby tomcat's EL impl is
loaded from CATALINA_BASE/lib. You seem to imply that once tomcat has
loaded a EL impl into the app2's classloader, that this some how breaks or
dirties the tomcat shared classloader so that it can no longer work the way
I describe above. Could you spare a minute to explain where my
understanding falters?

Thanks

Chris

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