Add elements to initialized custom collection

[nlisker]

(Copied from the mailing list)

My class contains an initialized custom collection

class MyClass {

    MyList<Integer> myList = MyList.create();
    //getter
}

where MyList is a j.u.List. When deserializing, I get an exception saying that there is no known deserializer for MyList ("Cannot find a deserializer for non-concrete List type"), which is generally understandable, but I already initialized it, so there's no need for one. Instead, I just want to add the items to the list.
To demonstrate, I can add a private setter inside the class:

private void setMyList(List<Integer> ints) { ints.forEach(MyList::add); }

this works, but I don't think it should be needed. Specifically, Jackson should not create any list. It needs to deserialize the values of the items anyway; then instead of creating a list, add them to the existing one. It's a List, so it should know to call 'add' ('put' for map).

I know there is @JsonMerge, and I thought that that's what I needed:

merging meaning that the current value is first accessed (with a getter or field) and then modified with incoming data. If merging is not used assignment happens without considering current state.
...
For Collection and Arrays, merging is done by appending incoming data into contents of existing Collection/array

but just adding it to the field doesn't work, I still get the same exception. Am I not understanding @JsonMerge or am I doing something wrong?

Thanks!

[nlisker]

From the link you gave.

…

On Sat, Oct 19, 2024 at 5:27 PM Kim, Joo Hyuk ***@***.***> wrote: May I ask where u got I think the information on how the annotation should behave?. Please refer to the actual documentation of JsonMerge <https://www.javadoc.io/doc/com.fasterxml.jackson.core/jackson-annotations/latest/com/fasterxml/jackson/annotation/JsonMerge.html> which tells you how it should behave. — Reply to this email directly, view it on GitHub <#4756 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AI5QOM463XWRWVPPI5KVQEDZ4JT6XAVCNFSM6AAAAABQHK2A2SVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTAOJZGE2DOMY> . You are receiving this because you authored the thread.Message ID: ***@***.*** .com>

[nlisker]

How did you declare MyList?

It's in the code I posted.

Seems like test passes when values field declared like...

@JsonMerge
@JsonProperty
MyList<String> values = create();

... this (try it).

This is a copy-paste of my example. It does not pass.

[JooHyukKim]

No I mean, is it this?

public interface MyList extends List { }

[nlisker]

It seems like BasicDeserializerFactory._mapAbstractCollectionType() is returning null because ContainerDefaultMappings.findCollectionFallback() looks for direct interfaces and not extensions (here we have MyList).

    protected CollectionType _mapAbstractCollectionType(JavaType type, DeserializationConfig config)
    {
        final Class<?> collectionClass = ContainerDefaultMappings.findCollectionFallback(type);
        if (collectionClass != null) {
            return (CollectionType) config.getTypeFactory()
                    .constructSpecializedType(type, collectionClass, true);
        }
        return null;
    }

Is it possible to make ContainerDefaultMappings.findCollectionFallback() also look for input type's interfaces and not just type's direct class, @cowtowncoder ?

More than that, why is it looking for a deserializer at all? It only needs to deserialize the values of the collection (String etc.) and add them to an already existing collection. There's no need to create a new collection, and in fact, it can be detrimental to do so.

I will reiterate that there is a solution with a custom setter as I've shown above. My thoughts were that @JsonMerge would do it for me, and according to its docs it should.

[nlisker]

No I mean, is it this?

public interface MyList extends List { }

Yes, this is it. Copy-paste the whole code snippet and try it. It's the simplest minimal example I could create based on the test you gave.

[JooHyukKim]

More than that, why is it looking for a deserializer at all? It only needs to deserialize the values of the collection (String etc.).

That is what StringCollectionDeserializer does exactly. It simply adds to existing list. Refer to its public Collection<String> deserialize(JsonParser, DeserializationContext, Collection<String>) method.

I will reiterate that there is a solution with a custom setter as I've shown above. My thoughts were that @JsonMerge would do it for me, and according to its docs it should.

I think your solution is more of a work-around. It should probably work without a setter.

[nlisker]

Submitted #4783.

[JooHyukKim]

Thanks!