This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
JDOM has a "pretty" format you can get using Format.getPrettyFormat(). You can then use that in an XMLOutputter instance. The biggest drawback is, however, that it requires you to put the entire XML document into your memory as a org.jdom.Document. Check out JDOM's SAXBuilder for creating a Document from a file or other source.