Group list of objects in a list attribute during deserialization using Jackson

Question

I have a JSON Structure looking like this:

[
    {
        "id": 0,
        "name": "Foo"
    },
    {
        "id": 1,
        "name": "Bar"
    }
]

and a corresponding Java Object for Data binding:

public class Thing {
    public int id;
    public String name;
}

I know how I could deserialize the JSON list into a list of Thing.

Now here comes the tricky part: What I want to do is deserializing the JSON into a class looking like the following snippet by only doing changes to this class:

public class Things {
    private List<Thing> things;

    public void setThings(List<Thing> things) {
        this.things = things;
    }

    public List<Thing> getThings() {
        return this.things;
    }
}

This is because the JSON deserialization is build in deep in our application by using an ObjectMapper like this:

private static <T> T parseJson(Object source, Class<T> t) {

    TypeReference<T> ref = new TypeReference<T>() {
    };
    TypeFactory tf = TypeFactory.defaultInstance();

    //[...]

    obj = mapper.readValue((String) source, tf.constructType(ref));

    //[...]

    return obj;
}

Are there any annotations with which I can achieve what I want or do I have to make changes to the mapper-code?

Much thanks in advance, McFarlane

Answer 1

The whole point of TypeReference, as described in this link, is to use the generic type argument to retrieve type information.

Internally it does the following

Type superClass = getClass().getGenericSuperclass();
...
_type = ((ParameterizedType) superClass).getActualTypeArguments()[0];

where getActualTypeArguments()[0] will give you the actual type argument. In this case, that will be the type variable T, regardless of what you pass in for the Class<T> t parameter of your method.

The proper usage is

TypeReference<List<Thing>> ref = new TypeReference<List<Thing>>() {};
...
List<Thing> thingsList = ...;
Things things = new Things();
things.setThings(thingsList);

In other words, no, you'll need to change your mapper code to achieve what you want.

As far as I know, you won't be able to map a root JSON array as a property of a class. The alternatives is the TypeReference example above or some other ones found here.