Write java class with nested Map to custom json format using Gson

Question

What Java data structure can be used to serialize to the following Json which have a set of child objects?

Example:

{
    "John": {
        "Type": "Person",
        "age": 30,
        "Sean": {
            "Type": "child",
            "age": 3
        },
        "Julian": {
            "Type": "child",
            "age": 4
        }
    },
    "Paul": {
        "Type": "Person",
        "age": 64,
        "Stella": {
            "Type": "child",
            "age": 10
        },
        "James": {
            "Type": "child",
            "age": 12
        }
    }
}

Writing John and Paul can be done by: Map<String,Person> but i cannot figure out how to nest the Child without having the 'children' property.

Example:

{
    "John": {
        "Type": "Person",
        "age": 30,
        "children": {
            "Sean": {
                "Type": "child",
                "age": 3
            },
            "Julian": {
                "Type": "child",
                "age": 4
            }
        }
    }
}

I am not sure it is relevant, but Gson is being used to create the Json file

I have tried registering a Gson TypeAdapter that override the write for the children Map, I wasn't able to get it to work as expected and it seems like a very cumbersome solution. — Erez Ben Harush
– Erez Ben Harush, Commented Jun 9, 2020 at 16:00

hradecek · Accepted Answer · 2020-06-09 20:46:38Z

1

I'm not sure wheher this is possible with GSON though it's possible with Jackson.

With GSON you can try custom JsonSerializer, which might look something like this:

private static class PersonTypeSerializer implements JsonSerializer<Person> {

    @Override
    public JsonElement serialize(Person person, Type type, JsonSerializationContext jsonSerializationContext) {
        JsonObject personJson = personToJson(person);
        for (Map.Entry<String, Person> child : person.getChildren().entrySet()) {
            personJson.add(child.getKey(), personToJson(child.getValue()));
        }
        return personJson;
    }

    private static JsonObject personToJson(Person person) {
        JsonObject personJson = new JsonObject();
        personJson.addProperty("Type", person.getType());
        personJson.addProperty("age", person.getAge());

        return personJson;
    }
}

and register to GSON instance like this:

Gson gson = new GsonBuilder().registerTypeAdapter(Person.class, new PersonTypeSerializer())
                             .create();

Note that code assumes that both type "Person" and "child" are represented by the same Person class. It should be pretty easy to modify this if not.

edited Jun 9, 2020 at 20:46

answered Jun 9, 2020 at 16:00

hradecek

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2 Comments

Erez Ben Harush Over a year ago

I am looking to serialize some data structure to the Example Json, the question is how to represent the Json example with a Java class

hradecek Over a year ago

Ah, right! Sorry for misunderstanding. I've edited the original answer. The idea behind stays the same. Again I'm not sure whether this can be done by single datastructre using GSON.

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