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
