Mapping JSON field names as MyValue.name in List<MyValue> with Jackson

Question

I have a JSON file:

{
  "alice": {
    age: 23
  },
  "bob": {
    age: 45
  }
}

I want to deserialise it to Collection<Person> with

data class Person(val name: String, val age: Int)

and serialise back to JSON again.

Can Jackson do this using annotations alone? Or do I need a custom JsonSerializer/JsonDeserializer?

Answer 1

Can Jackson do this using annotations alone?

I believe it is not enough to use only annotations. However, there are a few other ways to solve the problem:

1. One way to do it is to implement a custom deserialiser

You may implement StdDeserializer from jackson-databind.

This is an example which works with your code:

const val JSON = """
{
  "alice": {
    "age": 23
  },
  "bob": {
    "age": 45
  }
}
"""

fun main(args: Array<String>) {
    val mapper = ObjectMapper()
    val module = SimpleModule()
    module.addDeserializer(List::class.java, PersonDeserializer(Person::class.java))
    mapper.registerModule(module)

    val readValue = mapper.readValue(JSON, List::class.java)
    println(readValue)
}

data class Person(val name: String, val age: Int)

class PersonDeserializer(vc: Class<*>? = null) : StdDeserializer<List<Person>>(vc) {

    override fun deserialize(p: JsonParser, ctx: DeserializationContext): List<Person> {
        val people = mutableListOf<Person>()

        val node: JsonNode = p.codec.readTree(p)

        if (node is ObjectNode) {
            for (field in node.fields()) {
                val name = field.key
                val age = field.value["age"].asInt()

                people.add(Person(name, age))
            }
        }

        return people
    }
}

The code prints

[Person(name=alice, age=23), Person(name=bob, age=45)]

2. Another way to do so is to use HashMap and intermediate representation and then remap the object.

See the example below:

const val JSON = """
{
  "alice": {
    "age": 23
  },
  "bob": {
    "age": 45
  }
}
"""

fun main(args: Array<String>) {
    val mapper = ObjectMapper()

    val readValue = mapper.readValue(JSON, object : TypeReference<HashMap<String, InterimPerson>>() {})
    println("Interim: $readValue")

    val finalValue = readValue.map { Person(it.key, it.value.age) }.toList()
    println(finalValue)
}

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
data class InterimPerson(@JsonProperty("age") var age: Int)

data class Person(val name: String, val age: Int)

The code yields:

Interim: {bob=InterimPerson(age=45), alice=InterimPerson(age=23)}
[Person(name=bob, age=45), Person(name=alice, age=23)]