Marshalling an empty collection to json using jersey - json

Question

I have a strange issue marshalling an empty object collection to json using jersey with the jaxb based json support. My object looks like
...
#XmlWrapper(name = "stuff") #XmlElement(name = "s")
private List<Foo> foos;
...
Marshaling this to json produces the expected results
... stuff: [{ "s": ... }, { "s": ... }] ...
except when the list is empty. I would expect to see
... stuff: [] ...
but I see
... stuff: [null] ...
instead. Any idea what's wrong? The problem seems to be related to the #XmlElementWrapper annotation, removing it I don't get the the stuff property in the output at all.

Answer 1

Are you serializing an empty list, or are you serializing an un-instantiated null object?
ie. I would expect:
private List<Foo> foos; - would serialize to 'stuff: [null]'
and I would also expect:
private List<Foo> foos = new ArrayList<Foo>(); - we serialize to 'stuff: []'
If that isn't the case, you can always direct Jackson (which is the default JSON serializer bundled with Jersey) to omit the writing of bean properties as null value..

Answer 2

I would suggest using POJO mapping based on Jackson. I am not sure why you want that intermediate "s" in there, but POJO would produce (and consume) simpler structure:
"stuff" : [ { ... }, { ... } ]
For that you need no annotations with POJO mapping; JAXB annotations are only needed for XML processing, since XML has no natural mechanism to distinguish arrays from objects (unlike JSON).

Answer 3

I managed to solve JSON array and primitive field "bug" in Jersey json library. Secret ingredient is JSONConfiguration and ContextResolver magic. See my following post it has a full code example, customized ContextResolver and rest Application class might be somewhat fuzzy logic in first look.
How to serialize Java primitives using Jersey REST
json array for zero or single-element Java lists
primitive integer or boolean fields without quotation chars