On Mon, Oct 11, 2010 at 18:52, Jason Reilly <[email protected]> wrote:
> Hi,
>
> I wrote a Taverna2 plugin to call into a set of web services & am
> getting the following exception:
>
> org.apache.axiom.om.OMException: java.lang.ClassNotFoundException:
> org.apache.axiom.om.impl.llom.factory.OMLinkedListImplFactory
>
> at
> org.apache.axiom.om.OMAbstractFactory.getOMFactory(OMAbstractFactory.java:61)
This getOMFactory() method does not seem to be written for systems
with more than one classloader, such as in Taverna's plugin system.
public static OMFactory getOMFactory() {
if (defaultOMFactory != null) {
return defaultOMFactory;
}
String omFactory;
try {
omFactory = System.getProperty(OM_FACTORY_NAME_PROPERTY);
if (omFactory == null || "".equals(omFactory)) {
omFactory = DEFAULT_OM_FACTORY_CLASS_NAME;
}
} catch (SecurityException e) {
omFactory = DEFAULT_OM_FACTORY_CLASS_NAME;
}
try {
defaultOMFactory = (OMFactory)
Class.forName(omFactory).newInstance();
..
return defaultOMFactory;
}
Class.forName() - loading a class by string from the callers
classloader - would not work very well.
The reason is that OMAbstractFactory is in axiom-api, and
OMLinkedListImplFactory in axiom-impl - but in Raven (Taverna's plugin
system) each Maven module gets a dedicated classloader that can only
see classes from its own dependencies. So this would have worked with
the good-old java -classpath a.jar:b.jar - but not within a
hierarchical plugin system, where Class.forName() will use the
classloader of axiom-api to look for
org.apache.axiom.om.impl.llom.factory.OMLinkedListImplFactory -
unsuccessfully.
It seems OMAbstractFactory does not allow a way to set this class
except as a system property, but that classname would still only be
loaded from the axiom-api classloader. The 'correct' way to load
classes dynamically in a plugin environment would be to use
Thread.currentThread().getContextClassLoader(), a class loader passed
in to the getOMFactory() method - or allow the setting of the instance
directly using a setter method. As the field "defaultOMFactory" is
private, it would be tricky for you to make a subclass of
OMAbstractFactory to fix this.
I guess unfortunately Axiom is not your product, and you just want to
use it - so fixing that sourcecode is probably not your main
priority...
I see from your stack trace that you are the one using the factory -
so in theory you could do new OMLinkedListImplFactory(); directly
instead of calling getOMFactory - but if other code in Axiom tries to
use the getOMFactory() method it would probably fall over later.
From what I can see calls to getOMFactory() happens at various places
inside Axiom, so it will probably not work - but worth a try.
There's two hacks you can do to work around this.
1) Modify the POM file of axiom-api and axiom-impl to force a circular
dependency - so axiom-api depends on axiom-impl. Not sure if this
would work. You would need to use different version numbers and deploy
these modified versions to your Maven repository.
2) Merge the two JARs into an axiom.jar - and deploy this separately
with its own POM. This POM should not depend on axiom-api or
axiom-impl - but should depend on everything else those two have in
their POM. Again you would need to deploy this yourself - you should
indicate strongly in the version name that this is your own version -
like 1.2.8-renci
Once Taverna moves to use OSGi for its plugin system (post 2.3) it
should be easier to avoid these kind of problems - you could have just
a single OSGi bundle for your plugin, which could include both
axiom-api, axiom-impl and your own code on the same classloader.
--
Stian Soiland-Reyes, myGrid team
School of Computer Science
The University of Manchester
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