JAXB Packing Bug

It appears to be a little known fact that Java 1.6 includes a JAXB implementation -- no need for external libraries. However, the version that's currently bundled has an annoying bug related to how it packs lists. Let's say that your XML contains a list of items that you'd like to unmarshal and represent internally as a map. One might thing that all you'd have to do is the following:

And what you'll end up with is an empty map. Here's what happened:

1. Something.getList() is called and returns null.
2. JAXB creates a new ArrayList, and passes it to Something.setList(). At this point there is nothing in the list.
3. JAXB unmarshals all of the items, appending them to the list.

I'm sure you see the problem. Fortunately, there is a solution. The latest version of JAXB (available here) has fixed the problem. It follows the same flow described above, but with one additional step. It calls Something.setList() a second time after populating the list.

Upgrading your version of JAXB is a two step process.

1. Put the new jars on your classpath or install them to your Maven repository.
2. Prior to unmarshalling, call: System.setProperty("javax.xml.bind.JAXBContext", "com.sun.xml.bind.v2.ContextFactory");