Today I'd like to talk about strange XML ClassCastExceptions one sometimes gets when deploying the same web application twice in the same Tomcat.
The problem occurs when you deploy for example a newer xerces library in your webapp's WEB-INF/lib directory. The problem has to do with the mechanism the XML factories are instantiated dynamically. I've always thought (and wondered) why this is so, because obviously the xerces classes have been loaded properly by the webapp classloader specific for the webapp.
After not having to deal with the problem in deegree 2 (the xerces was not specifically needed there, and the problem could be solved to remove it from WEB-INF/lib) I ran into the problem again with deegree 3, where xerces is a central and necessary library to parse XML Schemas.
I've found that there is a debug setting for the JVM -Djaxp.debug=true, which was essential for finding the problem. It showed that the ClassCastException did not actually occur when instantiating the DocumentBuilderFactory for the second time, but when XSLT was used. So what happened?
The built-in XSLT-solution in Java is xerces' counterpart, xalan. Like the built-in xerces this is an older version, apparently preconfigured to use the built-in xerces. I think what probably happens is that the used class is somewhere cached within the parent class loader (one of the global class loaders of Tomcat), but loaded with the webapp class loader of the first deployed webapp. Once the second webapp tries to use XSLT, it fails, because a different (and inaccessible) class loader was used to instantiate the parser in question. But that's pure speculation...
... which led to a hunch on my side. I thought that if each webapp had its own xalan as well, it might solve the problem, because the dynamic loading mechanism for XML factories prefers factories that can be loaded via SPI. So I deployed a recent xalan as well (plus the xalan serializer jar) and the problem was gone.
To conclude, it seems good practice to deploy recent versions of all needed (directly or indirectly) XML factories in each webapp. An indirect need for XSLT can be created as quickly as converting a OMElement to an XMLStream, so it's a good guess you need it.
PS: Another solution would be to deploy all the XML factory libs centrally in the Tomcat, but that solution has obvious disadvantages (having to fiddle with the Tomcat, not being able to use different versions across webapps).