Jackson: deserialize JSON extract deep attribute into parent class

Question

I have some trouble wording my title, so if my question should be re-worded, I'd be happy to repost this question for clarification. :)

Problem: I have this JSON structure

{
    "name": "Bob",
    "attributes": {
        "evaluation": {
            "stats": [
                {
                    "testDate": "2020-02-04",
                    "score": 50
                },
                {
                    "testDate": "2020-04-01",
                    "score": 90
                },
                {
                    "testDate": "2020-05-10",
                    "score": 85
                }
            ],
            "survey": {...}
        },
        "interests": {...},
        "personality": [...],
        "someRandomUnknownField": {...}
    }
}

attributes is any random number of fields except for evaluation.stats that we want to extract out. I want to be able to deserialize into the following classes:

public class Person {
    String name;
    Map<String, Object> attributes;
    List<Stat> stats;
}

public class Stat {
    LocalDate date;
    int score;
}

When I serialize it back to JSON, I should expect something like this:

{
    "name": "Bob",
    "attributes" : {
        "evaluation": {
            "survey": {...}
        },
        "interests" : {...},
        "personality": {...},
        "someRandomUnknownField": {...}
    },
    "stats": [
        {
            "testDate": "2020-02-04",
            "score": 50
        },
        {
            "testDate": "2020-04-01",
            "score": 90
        },
        {
            "testDate": "2020-05-10",
            "score": 85
        }
    ]
}

I could technically map the whole Person class to its own custom deserializer, but I want to leverage the built-in Jackson deserializers and annotations as much as possible. It's also imperative that stats is extracted (i.e., stats shouldn't also exist under attributes). I'm having trouble finding a simple and maintainable serialization/deserialization scheme. Any help would be appreciate!

Answer 1

I'm not sure if this meets your criterion for a simple and maintainable serialization/deserialization scheme, but you can manipulate the JSON tree to transform your starting JSON into the structure you need:

Assuming I start with a string containing your initial JSON:

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new JavaTimeModule());
JsonNode root = mapper.readTree(inputJson);

// copy the "stats" node to the root of the JSON:
ArrayNode statsNode = (ArrayNode) root.path("attributes").path("evaluation").path("stats");
((ObjectNode) root).set("stats", statsNode);

// delete the original "stats" node:
ObjectNode evalNode = (ObjectNode) root.path("attributes").path("evaluation");
evalNode.remove("stats");

This now gives you the JSON you need to deserialize to your Person class:

Person person = mapper.treeToValue(root, Person.class);

When you serialize the Person object you get the following JSON output:

{
  "name" : "Bob",
  "attributes" : {
    "evaluation" : {
      "survey" : { }
    },
    "interests" : { },
    "personality" : [ ],
    "someRandomUnknownField" : { }
  },
  "stats" : [ {
    "score" : 50,
    "testDate" : "2020-02-04"
  }, {
    "score" : 90,
    "testDate" : "2020-04-01"
  }, {
    "score" : 85,
    "testDate" : "2020-05-10"
  } ]
}

Just to note, to get this to work, you need the java.time module:

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jsr310</artifactId>
    <version>2.11.3</version>
</dependency>

And you saw how this was registered in the above code:

mapper.registerModule(new JavaTimeModule());

I also annotated the LocalDate field in the Stat class, as follows:

@JsonProperty("testDate")
@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
private LocalDate date;

Very minor note: In your starting JSON (in the question) you showed this:

"personality": [...],

But in your expected final JSON you had this:

"personality": {...},

I assumed this was probably a typo, and it should be an array, not an object, in both cases.