Jackson Nested Object Deserialization into property

Question

Suppose I have a json object that looks like:

  {
    id: 1,
    name: "john doe"
    spouse: 2
  }

and the class I want it to deserialize it to:

class Person{

  private Long id;
  private String name;

  private Person spouse;

  //getters/setters
}

Is there any way to tell jackson to expand the spouse: 2 property into a new Person POJO with id=2 when deserializing the JSON?

I have run into this issue as a result of deserializing JSON into persistent entities and would like to be able to easily persist the relationships between these entities.

Answer 1

Aside from a full deserializer, there is a simpler way: define a POJO with a single int-arg constructor like so:

class Person {
   int id;
   public Person(int id) {
     this.id = id;
   }
}

This actually works, as Jackson will try to find limited number of special constructors (single-arg public constructors that that take String, int, long, double or boolean).

You can optionally also denote these with @JsonCreator -- and if constructor is not public, you must do it, to make it discoverable. But for public ctors this is not needed.

Answer 2

It is impossible of course for Jackson to infer a fully populated Person object representing the spouse from the number 2. You would likely need to register a custom deserializer that checks if the input is an integer, and if so, looks up the spouse from wherever it is stored. I have not done this kind of thing for classes that contain references to themselves (e.g. your Person contains a Person) so I can only give you rough guidance.

I believe this may only work with Jackson version 1.9 or later. Basically, you can register a module with the object mapper that tells Jackson to use a custom deserializer.

SimpleModule module = new SimpleModule("PeopleModule", new Version(1, 1, 0, null);
module.addDeserializer(Person.class, new JacksonPersonDeserializer());

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);

Alternately on the Person class itself, you can do something like:

class Person {

  @JsonDeserialize(using=JacksonPersonDeserializer.class)
  Person spouse;
}

This works before 1.9 but pollutes your object. Either way, you will need to write a custom deserializer.