How to make Jersey/Jackson serialize empty list; single element list as an array

Question

Using Jersey and Jackson to create a REST interface, how do I get List fields to be serialized as a list when there are 0 or 1 elements in them. For example:

Answer 1

I wrote a blog post ages ago about forcing Jersey to serialize single element arrays correctly, not sure if it's out-dated now (its from mid-2010!), but it might be of use.

Note the blog comment from Brill Pappin on the blog demonstrating a different approach which means upgrading the Jettison library that you are using.

In short you can write a custom JaxbContextResolver that looks a little like:

@Provider
@Component
public class JAXBContextResolver implements ContextResolver {

    private JAXBContext context;

    public JAXBContextResolver() throws Exception {
        MappedBuilder builder = JSONConfiguration.mapped();
        builder.arrays("invite");
        builder.rootUnwrapping(true);
        this.context = new JSONJAXBContext(builder.build(), Payload.class);
    }

    public JAXBContext getContext(Class objectType) {
        return (Payload.class.equals(objectType)) ? context : null;
    }
}

For clarity, my payload class looked a little like

@XmlRootElement(name = "response")
@XmlAccessorType(XmlAccessType.FIELD)
public class Payload {

    @XmlElement(name = "invite")
    List invites;

    ... etc.

Regarding stopping Jackson serializing bean properties as null, see my previous answer here, about using annotations to change that behaviour.