Class loading in Tomcat (and other web containers) is done by using a tree of 
classloaders[1]. Higher level classloaders cannot access lower level classes as 
they are invisible to them. The Tomcat distribution of Ontopia places 
ontopia.jar at the 'common' level (apache-tomcat/common/lib) for all webapps to 
use it. Individual webapps can contain additional classes at the 'webapp' level.

A typical usecase could be that a webapp defines classes to be used, e.g. as 
web-editor actions or logic:externalFunction. The current version of Ontopia 
cannot reach these implementations because it uses the wrong classloader in 
Class.forName invokes.

The default implementation of Class.forName uses 
this.getClass().getClassLoader() as classloader[2]. In the Ontopia Tomcat 
distribution, this will be the 'common' level classloader. If the call was made 
from a webapp, it cannot access classes at the 'webapp' level. The fix to this 
issue is to replace Class.forName(String) occurrences with 
Class.forName(String, boolean, ClassLoader) with the classloader used by the 
current thread as argument. This would preserve a possible lower level 
classloader from an individual webapp.

[1]: http://tomcat.apache.org/tomcat-5.5-doc/class-loader-howto.html
[2]: 
http://download.oracle.com/javase/6/docs/api/java/lang/Class.html#forName(java.l
ang.String,%20boolean,%20java.lang.ClassLoader)