与Jackson反序列化时，重用父对象的值来构造子对象

Question

I have two classes with a redundant shared attribute:

class Parent {
   int version;
   Child child;
}

class Child {
   int version;
   String name;
}

The version field from Child is just a local copy of the Parent 's own field, so it is represented like this in JSON:

{
    "version": 2,
    "child": {
        "name": "john"
    }
}

I want to deserialize this JSON payload to the corresponding Parent object with parent.child.version == 2 (the value is copied from the parent). How can I achieve this?

I thought about using @JacksonInject to pass the version to the Child , but that requires populating the injected values while deserializing the parent object. I could not find any way to register an injected value in a custom deserializer: the findInjectableValue method of DeserializationContext only lets us look up such a value.

Answer 1

It's not really a Jackson-specific solution but it's probably the simplest one: create a PreChild class whose attributes match the JSON payload, then convert the PreChild to a Child in the constructor of Parent .

class PreChild {
   String name;

   public Child withVersion(int version) {
        return Child(version, name);
   }
}
class Parent {
   int version;
   Child child;

   @JsonCreator
   Parent(
       @JsonProperty("version") int version,
       @JsonProperty("child") PreChild preChild) {
       this.version = version;
       this.child = preChild.withVersion(version);
   }
}