I actually had a couple of issues during startup which seem to be a result of the classloader changes in 5.1.0.GA: 1. Caused by: java.lang.LinkageError: loader constraint violation: when resolving method "javax.xml.parsers.DocumentBuilder.setEntityResolver(Lorg/xml/sax/EntityResolver;)V" the class loader (instance of org/jboss/classloader/spi/base/BaseClassLoader) of the current class, org/springframework/beans/factory/xml/DefaultDocumentLoader, and the class loader (instance of ) for resolved class, javax/xml/parsers/DocumentBuilder, have different Class objects for the type org/xml/sax/EntityResolver used in the signature Our web app is deployed is as follows: default /lib saxon.jar /deploy /app.ear /app.war In order to resolve this issue (without disabling the WarClassLoaderDeployer) I had to copy saxon.jar to the WEB-INF/lib directory of app.war. If I moved it, I received other ClassNotFoundExceptions during startup. Not sure why this would be. 2. Caused by: java.lang.ClassCastException: org.apache.xerces.jaxp.DocumentBuilderFactoryImpl cannot be cast to javax.xml.parsers.DocumentBuilderFactory This is the error which lead me to this forum posting. We do not have xercesImpl as part of our app, only saxon.jar, which I moved there in order to get past the first issue. View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4250411#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...