[infinispan-dev] [Pull Request] Modular Classloading Compatibility

[Manik Surtani]

Hi guys

Sorry I've been absent from this thread for a while now (it's been growing faster than I've been able to deal with email backlog!)

Anyway, this is a very interesting discussion.  To summarise - as Pete did at some point - there are 2 goals here:

1.  Safe and intuitive use of an appropriate classloader
2.  Safe type system for return values.

I think the far more pressing concern is (1) so I'd like to focus on that.  If we think (2) is pressing enough a concern, we should spawn a separate thread and discuss there.

So, onto the issue of safe classloading.

1) Class loader per session/cache.

I like Jason/Sanne/Trustin's suggestions of a session-like contract, and specifically I think this is best achieved as a delegate to a cache, again as suggested elsewhere by Pete, etc.  E.g.,

	Cache<?, ?> myCache = cacheManager.getCache("myCache", myClassLoader);

and what is returned is something that delegates to the actual cache, making sure the TCCL is set and re-set appropriately.  The handle to the cache is effectively your "session" and each webapp, etc in an EE environment will have its own handle.  I propose using the TCCL as an internal implementation detail within this delegate, helps with making sure it is carefully managed and cleaned up while not re-engineering loads of internals.

I think EmbeddedCacheManager.getCache(String name, ClassLoader cl) is enough ... it is clear enough, and I don't see the need for overloaded getCache(name, classOfWhichClassLoaderIWishToUse).

2) Class loader per invocation.

I've been less than happy with this, not just because it pollutes the API but that it adds a layer of confusion.  If all use cases discussed can be solved with (1) above, then I'd prefer to just do that.

The way I see it, most user apps directly using Infinispan would only be exposed to a single class loader per cache reference (even if multiple references talk to the same cache).  

Frameworks, OTOH, are a bit tougher, Hibernate being a good example on this thread.  So this is a question for Galder - is it feasible to maintain several references to a cache, one for each app/persistence unit?

3) Can all OSGi requirements be handled by (1)? I would guess so, from what I have read here, since the class loader is explicitly passed in when getting a handle on a cache.

Cheers
Manik


On 27 Apr 2011, at 17:39, Jason T. Greene wrote:

> Available here:
> https://github.com/infinispan/infinispan/pull/278
> 
> The problem is basically that infinispan currently is using TCCL for all 
> class loading and resource loading. This has a lot of problems in 
> modular containers (OSGi, AS7, etc), where you dont have framework 
> implementation classes on the same classloader as user classes (that is 
> how they achieve true isolation)
> 
> You can read about this in more detail here:
> http://community.jboss.org/wiki/ModuleCompatibleClassloadingGuide
> 
> The patch in the pull request is a first step, and should fix many 
> issues, but it does not address all (there is still a lot of TCCL usage 
> spread out among cacheloaders and so on), and ultimately it's just a 
> work around. It should, however, be compatible in any other non-modular 
> environment.
> 
> Really the ultimate solution is to setup a proper demarcation between 
> what the user is supposed to provide, and what is expected to be bundled 
> with infinispan. Whenever there is something the user can provide a 
> class, then the API should accept a classloader to load that class from. 
> If it's a class that is just internal wiring of infinispan, then 
> Infinispan's defining classloader should always be used.
> 
> The one case I can think of in infnispan where TCCL really makes sense 
> is in the case of lazy deserialization from an EE application. However 
> that is only guaranteed to work when you are executing in a context that 
> has that style of contract (like in an EE component). There are many 
> other cases though where someone would expect it to work from a non-EE 
> context (e.g. from a thread pool). What you really want is the caller's 
> classloader, but that is not cheap to get at, so it's something that 
> should be supplied as part of the API interaction (in the case where 
> there is no EE context). Alternatively you can just just require that 
> folks push/pop TCCL, but users often get this wrong.
> 
> Thanks!
> 
> -- 
> Jason T. Greene
> JBoss, a division of Red Hat
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--
Manik Surtani
manik at jboss.org
twitter.com/maniksurtani

Lead, Infinispan
http://www.infinispan.org