[Tomcat-users] Why cant the the classes (in the jars) places in Tomcat/lib see the classes from the webapp/WEB-INF/lib.

http://tomcat.apache.org/tomcat-6.0-doc/class-loader-howto.html

Mark

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I did go through this before posting here. Its mentioned "All unpacked
classes and resources in |$CATALINA_HOME/lib|, as well as classes and
resources in JAR files are made visible through this class loader". But
theres no mention about the visibility of the webapp/WEB-INF/classes to
the classes in |$CATALINA_HOME/lib.So is there no way to configure the same?

You need to read up on class loader delegation and then re-read those docs.

Mark

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Mark's point is that classes in yourApp/WEB-INF/lib (or classes) will
never be visible to tomcat/lib. Read the classloader hierarchy as
documented to understand why.


p

Mark,
Pretty much no.

Tomcat can deserialize your webapp's sessions after a redeploy because
it uses the webapp's ClassLoader to resolve everything. In your case,
you have no way to get to the webapp's ClassLoader so you are pretty
much stuck.

What kind of serialization/deserialization are you writing? Why is this
running at the Tomcat level instead of within your webapp?

- -chris
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Hi,

I just got pinged by Reinwald (we're already on contact via email) regarding
this issue. I created the memcached-session-manager (
http://code.google.com/p/memcached-session-manager/ ) for which Reinwald is
trying to create a custom serializer/transcoder (memcached-session-manager,
msm, basically additionally uses memcached as additional session backup
store where session can be loaded from in the case of a tomcat/hardware
crash).

I just signed up here on nabble to be able to get into the thread, therefore
quoting is not that good / I'm top posting.

Now let me try to clarify how msm works regarding
serialization/classloading.

It is a Manager which is configured as usual via context.xml/server.xml. It
supports standard java serialization but also other pluggable serializers
like e.g. kryo (faster than java serialization). For kryo the manager
configuration accepts custom serializer class names. The basic kryo
serializer and the specified custom serializers are loaded during
Manager.init like this:

ClassLoader cl = manager.getContainer().getLoader().getClassLoader()
Class.forName( customSerializerClassName, true, cl )

The msm jar is placed in tomcat's lib dir, the kryo serializer jar, required
kryo jar (and friends) and all application related jars are living in the
webapps WEB-INF/lib directory.

This works, there's also a sample app on github (
https://github.com/magro/msm-sample-webapp ) with a runtime directory
containing a tomcat installation and a sample webapp (linked in webapps,
available after building the war).

@Reinwald: I assume you're trying to get kryo custom serializers up and
running, right? Have you placed kryo jars, msm-kryo-serializer jar and your
custom serializer jar / classes in your WEB-INF/lib directory?

Cheers,
Martin



Christopher Schultz-2 wrote:

@Martin :Actually I was trying to write my own Session Manager since i
wanted to store the session into mysql. The issue was when deserializing
Tomcat could not find the classes since the objects stored in the
sessions are related to my web-app.
Reading the docs I found another solution which worked in my case
http://wiki.apache.org/tomcat/HowTo#How_do_I_add_JARs_or_classes_to_the_common_classloader_without_adding_them_to_.24CATALINA_HOME.2BAC8-common.2BAC8-lib.3F

@ All : Also wanted to mentioned that the memcached-session-manager
which Martin has created is an awesome idea if you want to back up
sessions to memcache.

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Another option would be to extend PersistentManager to satisfy your
requirements, as it provides much of what you want. IIRC the main "issue"
for you was that swapOut/swapIn is done in the background thread right now.
If you extend PersistentManager you could reuse JDBCStore (and friends) -
this would also be a nice contribution to the tomcat community :-)

Cheers,
Martin

Will give it a thought. Will let you know how it goes :)

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I am writing a Tomcat Manager to custom handle sessions (custom jar is
placed in Tomcat/lib). When de-serializing ,the objects stored in the
sessions are related to my web-app.Hence was receiving the
classnotfound exception.
After reading the docs I figured that one work around would be to define
my web-app/lib in the common class loader section which can be defined
in catalina.properties.

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If I read the above properly, I think you're suggesting that the common class loader be responsible for loading classes from a webapp's WEB-INF/lib directory; that would be a complete disaster. Don't even think about it.

- Chuck


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Then how do you suggest going about it?

ClassLoader cl = manager.getContainer().getLoader().getClassLoader()
Class.forName( className, true, cl )

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Exactly as others have suggested. For example:
http://marc.info/?l=tomcat-user&m=129553509607733&w=2

- Chuck


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+1
That is pretty much how the FileStore and JDBCStore do it. Take a look
at the source for the exact details.

Mark

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