Stephane and I chatted about this on irc, so I wanted to post up the
notes.
The issues Stephane points are big deal. This is a problem Paul
Hammant first pointed out to me a the last OSCon, and I have been
thinking about how to address for a while. Of course I never brought
it up for discussion here :(
In the long run (for Geronimo 2.0), I'm currently leaning towards
using OSGI to solve this problem. OSGI is the framework that eclipse
plugins use, and they have really solved the class loader problems.
This would require a major architectural change so I don't think it
can be done in 1.x. For those of you that are interested, the
current OSGi r3 spec chapter 4 describes the class loader
architecture. The next yet to be published r4 spec expands on the
model addressing some of the bad assumptions (e.g., furure versions
of any library are backwards compatible with all previous versions).
In the short term, I like Stephane's idea of adding an optional child
first delegation class loader to applications (not just wep
application). This would help applications avoid conflicts with
Geronimo system classes. Anyone want to take a look at implementing
this?
-dain
On Jun 14, 2005, at 8:18 AM, Stephane Bailliez wrote:
Hi all,
I'd like to clarify a couple of points about the Geronimo
ClassLoader architecture. I have had a discussion about it with
Gianny Damour on IRC, so i'm bringing the discussion here to have
some feedback about the existing implementation and my thoughts.
It looks like as of now the CL architecture is exposing its
internal implementation to applications [1] if I'm reading correctly.
Say for example, if Geronimo is using component 1.x it will be
visible to applications.
There are 2 aspects:
1. if the application uses component 2.x which is incompatible with
1.x you could then be in a blocker situation.
2. if the application incidentally forgets to ship with component
1.x, in a normal world, this would end up with a CNFE. In our case
it would simply mask it to the user, actually run by accident..and
may blow up later, say when upgrading the apps, or Geronimo.
3. some component unfortunately autoconfigure based on the presence
or absence of another component (a good example is ActiveMQ
autoconfiguring itself with derby persistence if available in the
classpath). I don't like too much autoconfiguration, as it will
often lead to problems later. The rational being to put as little
configuration as possible and thus forgetting to say 'even if you
find that component don't do that, use this instead' (for activemq
it would be explicity configuring vmpersistence). My rational is to
avoid magic. A component should always start the same way.
To be more general:
In the OSS J2EE CL land, we have as of now 2 different hierarchies:
- JBoss with its Unified ClassLoader [2]
AFAIK what has been said publicly to defend this design was
originally to 'please' users and to avoid them the dreaded
ClassCast or ClassNotFound, VeryErrors,etc..... by stuffing as much
components as they could find into a gigantic 'unified' pot. This
led to a terrible thing which any serious user has been fighting
against in production...not mentioning that classloader isolation
settings have been changing between micro releases.
- JOnAS
It has a similar classloader hierarchy of Geronimo's [3] (or vice-
versa) but is unfortunately exposing its implementation to
applications. There is now a very serious problems with deployment
of Hibernate based apps which use ASM and the ASM version used
within JOnAS internals...which are incompatible.
Web applications (WARs) deployed in those J2EE containers generally
can change the delegation model from the java2 delegation model
(parent first) to a servlet 2.3+ model (current first), but this is
insufficient in itself because it is only for the WAR part..you
could very well have this problem in the EJB tiers .. so you need
to be able to change the model type as well in the EJB tier.
Being able to change the delegation model of the EJB tiers of
course does not prevent actually having the case described in (2)
and (3) happening. Unless we can filter for sure what can be
delegated which is not so straightforward to me.
Indeed, it does not mean filtering out to allow 'only' J2EE apis
and implementation. In your app, you may perfectly need visibility
of components located in the boostrap classpath or in jre/lib/ext
(for example a crypto provider, etc...)
Shouldn't the classloader architecture have a child classloader to
load internals (just like it does for the database driver)
Any thoughts on that ? I would really like to avoid having a
release with a ClassLoader architecture that is already known to
expose the users to some serious problems. Otherwise this may lead
to tightly coupled components that will be very hard to decouple
later (if not now).
Any comment is welcome.
Cheers,
Stephane
[1] http://chariotsolutions.com/geronimo/elements-classloaders.html
[2] http://wiki.jboss.org/wiki/Wiki.jsp?page=ClassLoadingConfiguration
[3] http://www.redhat.com/docs/manuals/rhaps/jonas-guide/s1-
conclusion.html
