Mapping a fixed-size list to an POJO? (@JsonFormat(shape=JsonFormat.Shape.ARRAY))

[indyteo]

Hi,

I have a vague memory that it is possible to map items of a list to POJO properties. But I can't seem to find anything about it, so I begin to wonder whether I hallucinated that or not...

To be clear, our use-case is a 3D object (bounding box) expressed as a list of 3 elements (each element is an "interval", we have a custom deserializer to turn min..max syntax to our IntegerInterval object). Currently, we deserialize it as a list, then validate its size to be 3, and create our bounding box object from the 3 elements.

Serialized form looks like this (YAML): [ 0..2, -5..-1, 3..4 ] (we wanted to have a fixed-size list instead of a more verbose "x: ..., y: ..., z: ..." style)
Our current code works, but I wonder if instead of having a List<IntegerInterval>, we couldn't have something like:

@JsonExplodeList // idk
class BoundingBoxParams {
    // Instead of:
    //public List<IntegerInterval> xyz;

    // Fields "exploded" automatically:
    @JsonIndex(0)
    public IntegerInterval x;

    @JsonIndex(1)
    public IntegerInterval y;

    @JsonIndex(2)
    public IntegerInterval z;

    public BoundingBox create() {
        // Instead of having to validate list size and extract elements,
        // it would be much more direct code with x, y, z here...
    }
}

Here, @JsonIndex would give the index of the element in the list, and @JsonExplodeList mark the POJO to have this kind of handling.

If such feature does exist, then I'd be happy to find out I'm not crazy. Otherwise, that's really not a big deal, our current code works perfectly, but my colleagues are gonna call me crazy for telling them it was possible hahaha 😄

Thx

[pjfanning]

AI suggests that there is no Jackson feature like this and you would be best served writing a custom deserializer (and serializer).

One thing that it did suggest that might work would look like:

public class MyObject {
    private final IntegerInterval field1;
    private final IntegerInterval field2;
    private final IntegerInterval field3;

    @JsonCreator
    public MyObject(@JsonProperty("data") IntegerInterval[] data) {
        this.field1 = data[0];
        this.field2 = data[1];
        this.field3 = data[2];
    }
    // getters...
}

This example would need work and would only work for deserialization but you might be able to extend it to serialize as you like as well.

[cowtowncoder]

This is why AI doesn't yet replace library authors. :-D

The thing @indyteo is looking for is, I think, @JsonFormat(shape=JsonFormat.Shape.ARRAY) like so:

@JsonFormat(shape=JsonFormat.Shape.ARRAY)
// Possibly also: @JsonPropertyOrder(alphabetic=true)
class POJO {
   // or: @JsonProperty(index=0)
   public int a;
}

and for more examples see f.ex https://github.com/FasterXML/jackson-databind/blob/3.x/src/test/java/tools/jackson/databind/struct/POJOAsArrayTest.java#L64

NOTE: works particularly well for Records since they define property order by declaration order.

[indyteo]

Yes yes yes yes yes! That's the thing I remembered, I am not crazy, yay!
Thank you very much :)

[cowtowncoder]

Great! Figured it sounded exactly like the thing.

[indyteo]

I tested to improve our code with this, it works however extra elements are silently ignored despite having "fail on unknown properties" enabled. Is there something that can be done to make Jackson throw when deserializing an array of greater length than the number of properties of the POJO? I tested with a record, if that matters:

@JsonFormat(shape = JsonFormat.Shape.ARRAY)
public record BoundingBoxParams(IntegerInterval x, IntegerInterval y, IntegerInterval z) {
    public BoundingBox create() {
        // ...
    }
}

Deserializing [ 1, 2 ] throws for "missing z", but deserializing [ 1, 2, 3, 4 ] does not throw despite 4 being an "unknown" property here

[cowtowncoder]

@indyteo Hmmmh. I think this is worth reporting a bug: FAIL_ON_UNKNOWN_PROPERTIES should, I think, catch that.
I can't think of another way to force failure for this case.