A month or two ago I ran into an issue where I was marshalling Objects to json using Jersey with JAXB annotations on my pojos but only the Super Class attributes were being marshalled. I have uploaded some code to github to demonstrate this:
https://github.com/nebnagrom/jaxbInheritance
In this contrived example we have a Super Class
House which has a member variable which is a
String and an
@XmlRootElement annotation. There are three child Classes of
House:
- BlueHouse which has an additional Integer
- RedHouse which has an additional String
- GreenHouse which has an additional Boolean
We then also have a
HousingEstate<T extends House> which wraps a typed collection of houses. In this example it has no behavior but in the situation I had we had behavior on this collection as well.
The issue occured once I started trying to return
HousingEstate objects from my RESTful resource. When you invoked the resource you only got back member variables from the
House class, i.e. something like this:
{"houses":[{"houseName":"first blue"},{"houseName":"2 blue"}]}
When what I wanted was more like:
{"houses":[{"houseName":"first blue", "blueAttribute":1},
{"houseName":"2 blue", "blueAttribute":2}]}
After a bit of digging I found that I needed to add another annotation on the House class -
@XMLSeeAlso like this:
@XmlSeeAlso({ GreenHouse.class })
After I added that I started getting the results I wanted.
I also tried this without generics or just using the Super Class but this did not work either. Interestingly I found that:
- Using no generics meant that I got House member variables only, even when I annotated BlueHouse with annotation and added BlueHouses to the collection
- Using the parent class as the Generic gave me Marshalling exceptions, probably to be expected though.
I haven't really tried to dig into what is going on here, I assume that there is something with generics and type erasure that makes it impossible for JAXB / Jackson to know that there is a child class that it should be looking at for the marshalling.
Stack trace of error you get with the House based version over the fold:
