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From David Jencks <>
Subject Re: An idea for defining custom valves in config.xml
Date Mon, 06 Oct 2008 17:59:35 GMT

On Oct 6, 2008, at 10:35 AM, Jason Warner wrote:

> On Mon, Oct 6, 2008 at 11:56 AM, David Jencks  
> <> wrote:
> On Oct 6, 2008, at 7:22 AM, Jason Warner wrote:
>> On Fri, Oct 3, 2008 at 6:55 PM, David Jencks  
>> <> wrote:
>> On Oct 3, 2008, at 12:51 PM, Jason Warner wrote:
>>>   Hey all.  I'm working on an idea for allowing custom valves to  
>>> be defined in config.xml.  Currently this isn't possible since the  
>>> tomcat classloader would not contain the custom classes for the  
>>> valve.  I've create a jira for tracking this issue [1] and it  
>>> contains a few links to workarounds.  IMHO, The solution we should  
>>> be looking for is a way to add classes to a module without having  
>>> to undeploy, modify the module config, and redeploying.
>> People have suggested stuff like this before.  IMO it pretty much  
>> goes against the fundamental idea of geronimo of having fairly  
>> fixed plugins with only a few knobs to turn to adjust things in  
>> config.xml and
>> Why is changing the classloader contents in config.xml a good  
>> idea?  What is so hard about redeploying the app if you want to  
>> change its classloader significantly?  If you want to change a  
>> class in the app you have to redeploy it.... why is this situation  
>> different?
>> The specific instance I have in mind for this change is using a  
>> custom valve for tomcat, so I think the scope really should be  
>> limited to just the tomcat module.  I can't think of another  
>> instance where this would be useful, so it's probably not necessary  
>> or desirable to expand it further.  I believe this situation is  
>> different because the structure of geronimo is causing a disconnect  
>> between the functionality of tomcat and the functionality of tomcat  
>> as it is embedded in geronimo.  As Don just said in the middle of  
>> my typing this, I don't believe we should expect the average user  
>> to have to rebuild one of our modules to add something that can be  
>> added in a much simpler way within tomcat itself.
> Could you explain more about the circumstances for this custom  
> valve?  Is it intended to be for every app deployed on this tomcat  
> server instance rather than for one particular app?  Will it work if  
> it is in a child classloader of the tomcat plugin classloader?
>     When a valve is added to the tomcat valve chain, it becomes part  
> of the request processing pipeline.  Every request that is made to  
> that tomcat server instance passes through this valve chain as it's  
> processed regardless of whether the valve will act upon it or not.   
> It's possible that a single web app will be the only app to use the  
> valve, and for that instance it is already possible to define the  
> valve in the context of the web app rather than the tomcat server.   
> We need to be able to define a valve as part of tomcat server  
> instance as well, though, to be consistent with tomcat.  Currently  
> we can only define the valves on the per web app basis.
>     I don't think this would work in a child classloader of the  
> tomcat plugin classloader.  When we start up the tomcat module now,  
> the currently defined valves are processed and added to the engine.   
> The custom valves would need to be added to the valves already in  
> the tomcat engine to be available in the way described previously.   
> Once the valves were added to the engine (which would be using the  
> tomcat classloader, I believe) the class def not found issues we  
> currently see would pop back up.  For this to work, the custom valve  
> classes and the tomcat engine would need to share the same  
> classloader.

Could you try this to be sure?  I would hope that tomcat would use a  
TCCL or supplied classloader for loading components rather than  
something like TomcatEngine.class.getClassLoader() which I believe is  
what you are suggesting it does.

One example of an inconvenient tomcat configuration is the app-per- 
port sample where we set up a whole additional tomcat server in a  
child configuration.  I think all the server components in that  
example are also in a standard tomcat server but its a similar  
situation to what I'm thinking of here in terms of configuring a  
tomcat server in a child classloader.

> At the moment I would MUCH rather see us make it easier for users to  
> deploy new/different/modified tomcat servers (and other plugins)  
> than introduce a hack to modify classloaders of existing plugins.   
> Our customization story is already  too complicated, IMO we don't  
> need to glue on more bits that don't actually fit well.
> IMO the best end result for users is to have a new tomcat plugin  
> with the needed extra jars and valve configuration.  Lets look for a  
> way to make it really easy for our users to get there.
> I agree that a whole new plugin with all desired functionality  
> included would be best for users.  Any ideas how to make this easier  
> than it currently is?  Perhaps the attribute idea mentioned by Joe  
> could serve as a temporary solution until we can come up with  
> something better.
> How would you deal with this in an osgi or spring environment?

If anyone knows how osgi deals with situations like this I'd find it  
really helpful in considering alternative directions.
david jencks

> thanks
> david jencks
>> Thanks!
>> thanks
>> david jencks
>>> I think this can be done by allowing a user to indicate jars that  
>>> should be loaded by a module within the config.xml.  These jars  
>>> can then be added to the module's classloader for use by the  
>>> module.  I'm not extremely familiar with how our classloader  
>>> works, but I've taken a look through the code and I think the  
>>> ability to add to the classloader can be implemented without too  
>>> much difficulty.  I'm not quite sure what type of scope to give  
>>> this change, though.  Should I leave it as a change aimed solely  
>>> at tomcat valves or should it be expanded to encompass any  
>>> configuration?  I realize this is only a rough idea of what i plan  
>>> to do, but I'm still working out the details of how to proceed.   
>>> I'm hoping for some feedback on what I intend to do and possibly  
>>> some alternate ideas if anyone has some.
>>> Thanks!
>>> [1]
>>> -- 
>>> ~Jason Warner
>> -- 
>> ~Jason Warner
> -- 
> ~Jason Warner

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