The one difficulty I see in moving from the current Geronimo
classloading model to the OSGi model is dealing with the change in
granularity. In the current model, the granularity is at the jar level
and using the one-classloader-per-jar model, you get all of the classes
contained in the jar. This is generally the source of the various class
loading conflict we see with Geronimo. Different jars might contain
their own versions of classes, creating the conflict.
With the OSGi model, each bundle explicitly specifies its classloading
dependencies in the form of package imports and also specifies which
classes it makes available for others to use as package exports. Any
packages not specified as exports are private to the bundle scope and
are only directly visible to the bundle itself.
The bnd tools created by Peter Kriens is a good step toward converting
jars into bundles. It examines all of the class files contained in a
jar and figures out which files need to be imported from other source
and constructs the appropriate headers. This works ok if the bundle
doesn't need to export any packages. If exports are required, then
additional information about the jar is required.
There are a couple of additional downsides to the static analysys:
1) This type of analysis can detect which packages are required, but
doesn't tell you anything about which versions of the packages are
needed. This can probably be sorted out by coupling the analysis
information with information from the maven-style dependencies. That
is, your jar requires package org.apache.foo.bar and you specify you
have a dependency on org.apache.foo-1.3.2 which happens to export
org.apache.foo.bar package, so your bundle import dependency for
org.apache.foo.bar is set to the 1.3.2 version level.
2) It's not very good at determining dynamic classloading
dependencies. This also requires some additional metadata to indicate
certain packages are going to be required by the application.
Rick
David Jencks wrote:
Geronimo has been around for a while and despite the many good
features gbeans and the geronimo kernel are not catching on big time.
I think we want to consider taking action now to avoid ending up being
dragged down by supporting a dead container. Here are a few thoughts.
Actual problems with geronimo:
- gbeans are too restrictive. It's too hard to instantiate other
peoples components as gbeans. GBeans don't support common patterns
like factory methods, factory beans, etc etc, and require the
component to be instantiated directly by the gbean framework.
- it's too hard to get the classloaders to work. The most common
problem is a class cast exception due to loading the same jar in two
plugins. NoClassDefFound errors from an optional jar in a child
classloader are also really annoying.
Really good things about geronimo I haven't seen elsewhere (at least
in one place):
- gbean dependencies work across plugins. Dependencies are a unified
system, not per-plugin.
- gbean dependencies are resolved in the ancestors of a plugin, not
server wide. This means that you can't make a partially specified
dependency ambiguous by deploying additional plugins. I consider this
an extremely important feature for predictability.
- plugin dependencies allow assembly of a server from the explicit
dependencies which are normally the same as the maven dependencies.
Other projects and specs that have stuff we should look into:
maven. Maven has a lot better infrastructure for dealing with
dependency resolution from partial transitive dependency specification
than we do. We should look into using more of their infrastructure.
osgi. osgi has a lot of similarities to geronimo. The osgi
classloading model is getting a lot of people excited. The
import-bundle idea is pretty much the same as our classloader model
where every jar is a plugin. I don't know if people are really using
the allegedly recommended method of specifying imports and exports and
letting the osgi runtime figure out where they come from; this seems
worth investigating to me. Also, we get periodic inquiries about when
we are going to support osgi and the was ce folks get even more.
osgi blueprint service (rfc 124) This appears to be a simple wiring
framework for a single plugin. IIUC it uses the osgi service registry
for component dependencies between bundles.
xbean-spring. I'd be reluctant to try to implement a blueprint
service that didn't provide the xbean-spring capabilities really well
ee6 dependency injection. EE6 is going to have a pretty sophisticated
dependency injection service which we'll need to support anyway. We
should try to figure out how much of the core we can assemble using it.
Other great stuff we have:
xbean-reflect, xbean-finder, xbean-spring
These ideas have been floating around in my head for a long time and
I've chatted with various people about them occasionally. While more
discussion is certainly needed on everything here I need to do some
implementation to understand much more. So, what I'm planning to do:
Dave's crazy work plan...
- Try to use the osgi classloader. I think this involves putting the
classloader creation in Configuration into a service. Configurations
will turn into osgi bundles. I'll put the Kernel in the osgi
ServiceRegistry so the Configuration bundle activator should be able
to use it to resolve cross-plugin dependencies.
- try to figure out how maven dependency resolution fits into osgi.
- see if eclipse p2 is relevant for provisioning geronimo repositories
at this point I think geronimo would be running on osgi, still using
gbeans.
- look into relaxing the gbean framework so it is more
plugin-at-a-time rather than gbean-at-a-time
- see how that differs from the blueprint service, ee DI, and
xbean-spring. Try to support all of these at once.
Thoughts? Counter proposals? Anyone interested?
many thanks
david jencks
