Java EE (Java Enterprise Edition) General Discussion

I'm gonna write an utility that can convert text flat files (both delimiter and fix-width inputs) to XML. I'm wondering what open source or 3rd part library you would use to do this instead of re-inventing the wheel from scratch. I browsed this forum for a while, but couldn't find any answer. I also have to build another utility to convert XML back to text file, which I already figured that we can easily use XSLT. I was thinking if there is an API out there that can do the opposite. Any comments or suggestion would be really appreciated.