Debugging strange unmarshalling problems when using jaxb —02 September 2012

The other day I had a very strange unmarshalling issue with jaxb: unmarshalling (deserializing) from an XML file and then marshalling (serializing) the same object to another XML file resulted in different content. There were no errors thrown in the process and I had no idea as to why.

Finally I found a way to debug it on this page http://jaxb.java.net/guide/Unmarshalling_is_not_working__Help_.html

Unmarshaller u = ...;
// this implementation is a part of the API and convenient for trouble-shooting,
// as it prints out errors to System.out
u.setEventHandler(new javax.xml.bind.helpers.DefaultValidationEventHandler());

u.unmarshal(new File("foo.xml"));

Based on the errors reported by the event handler it was straightforward to fix the problem (some property files that relax some validation rules were not on the classpath).