Using the instantiated collection type for Jackson deserialization instead of initializing a new one?

Question

If I have a class

class DTO {
    final MySet<Types> values = MySetWrapper(EnumSet.of(Types.class));

    public MySet getValues() {
        return values;
    }
}

where MySet extends Set. Jackson complains that

Cannot find a deserializer for non-concrete Collection type MySet

which I understand, but I already instantiated the collection. What I want is for jackson to just call add for each value after it created an instance, something like:

DTO o = new DTO();
MySet<Types> values = o.getValues();
for (Types type : jsonArray) {
    values.add(type );
}

I don't want it to try to create a new collection itself.

@Coder Yes. I have a solution currently that looks like the accepted answer [here](https://stackoverflow.com/questions/36159677/how-to-create-a-custom-deserializer-in-jackson-for-a-generic-type). But even there in the `deserialize` method I need to create the instance *again* myself. Also I don't want ALL of my `MySet` to be instantiated that way. I could have a `MySetWrapper` wrap some other set implementation. Regadless of what's on the right side, the left one is a set and should be treated as a regular set. — Mark, Jul 31 '19 at 14:01
Have you tried using @JsonCreator for your DTO class? With a factory method/ constructor annotated as a creator in Jackson, you can define the object deserialization strategy for DTO.class. Here's a simple example : https://www.logicbig.com/tutorials/misc/jackson/json-creator.html — Arpan Kanthal, Jul 31 '19 at 14:30
@ArpanKanthal the problem is that as much as i know if i do that i would need to manually set all the other fields in my class which are otherwise fine. I want only special treatment for this field, not the whole class. — Mark, Jul 31 '19 at 15:30
@Mark another way i can think of is using a @ JsonSetter annotation to do what you need to . Let me know if that helps — Arpan Kanthal, Jul 31 '19 at 15:57
@ArpanKanthal I managed to make it work with `@JsonProperty`, thanks. — Mark, Jul 31 '19 at 18:52

davidxxx · Answer 1 · 2019-07-31T14:36:16.670

That error message means that the DTO class is configured (by default or explicitly) to deserialize the values part of the JSON input into the DTO values field of DTO :

Cannot find a deserializer for non-concrete Collection type MySet

If you consider that Jackson should not perform the deserialization directly on this field, you could define a constructor to set values and also make sure that Jackson will not perform automatically the deserialization work : to achieve it, remove setter for that field (or add @JsonIgnore on it) and any jackson module that will use reflection to deserialize to fields.

It would give :

final MySet<Types> values = MySetWrapper(EnumSet.of(Types.class));

@JsonCreator
public MyFoo(Set<Types> values) { 
    this.values.addAll(values);
}

Note that I specified in the constructor Set and not MySet (should not be an issue as interface doesn't declare fields), otherwise you would get the same issue since you didn't define a deserializer for MySet.
But if you implement a deserializer for that you could of course do :

public MyFoo(MySet<Types> values) { 
    this.values.addAll(values);
}

"remove setter for that field" it's a final field, there is no setter, only getter. — Mark, Jul 31 '19 at 15:05
Also what about the other fields I have in my class that work fine. If I use this custom constructor would I need to manually set all the other fields, or does jackson know to set after construction everything else? — Mark, Jul 31 '19 at 15:20

Mark · Accepted Answer · 2019-07-31T19:32:19.493

0

Found an answer using @JsonProperty:

@JsonProperty
private void setValues(Set<Types> types) {
    values.addAll(types);
}

Pretty short and simple thankfully.

Edit: seems like you don't even need the annotation.

edited Jul 31 '19 at 19:32

answered Jul 31 '19 at 18:51

Mark

2,167
4
32
64

Using the instantiated collection type for Jackson deserialization instead of initializing a new one?

2 Answers2