CODEWITHSUNDEEP
javajsongsondeserializationjson-deserialization
lucian.marcuta
lucian.marcuta

Reputation: 1260

GSON: encode map of objects and preserve type when decoding

I have a class like:

public class MyClass {
    private final Map<Property, Object> properties;
}

where Property is an enum.
Let's say that properties contains 2 elements, one whose value is a Double and one whose value is a class instance having only one attribute called ownerName. When I serialise this class I get the following string:

{"properties":{"NAME":{"ownerName":"MyBucket"},"DIVISOR":33.0}}

The problem is that when I tried to obtain a MyClass instance from the string above, the value for NAME property will be a Map instead of an instance of the class having ownerName attribute. I tried to write a custom serializer/deserializer but I was not able to do that only for NAME property. Any ideas?

Upvotes: 1

Views: 338

Answers (1)

Michał Ziober
Michał Ziober

Reputation: 38645

You need to write custom deserialiser for the whole Map. Custom deserialiser could look like below:

class PropertyJsonDeserializer implements JsonDeserializer<Map<Property, Object>>, JsonSerializer<Map<Property, Object>> {

    @Override
    public Map<Property, Object> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        if (!json.isJsonObject()) {
            return Collections.emptyMap();
        }

        JsonObject root = json.getAsJsonObject();
        Map<Property, Object> result = new LinkedHashMap<>();
        root.entrySet().forEach(entry -> {
            Property property = Property.valueOf(entry.getKey());
            switch (property) {
                case DIVISOR:
                    result.put(property, entry.getValue().getAsDouble());
                    break;
                case NAME:
                    Object owner = context.deserialize(entry.getValue(), Owner.class);
                    result.put(property, owner);
            }
        });
        return result;
    }

    @Override
    public JsonElement serialize(Map<Property, Object> src, Type typeOfSrc, JsonSerializationContext context) {
        return context.serialize(src, Map.class);
    }
}

Example usage:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import com.google.gson.annotations.JsonAdapter;

import java.lang.reflect.Type;
import java.math.BigDecimal;
import java.util.Collections;
import java.util.EnumMap;
import java.util.LinkedHashMap;
import java.util.Map;

public class GsonApp {

    public static void main(String[] args) throws Exception {
        Map<Property, Object> properties = new EnumMap<>(Property.class);
        properties.put(Property.DIVISOR, new BigDecimal("33.0"));
        properties.put(Property.NAME, new Owner());

        MyClass myClass = new MyClass(properties);

        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        String json = gson.toJson(myClass);
        System.out.println(json);

        myClass = gson.fromJson(json, MyClass.class);
        System.out.println(myClass);
    }
}

class MyClass {

    @JsonAdapter(PropertyJsonDeserializer.class)
    private final Map<Property, Object> properties;

    public MyClass(Map<Property, Object> properties) {
        this.properties = properties;
    }

    // getters, setters, toString
}

class Owner {
    private String ownerName = "MyBucket";

    // getters, setters, toString
}

enum Property {
    NAME, DIVISOR
}

Above code prints:

{
  "properties": {
    "NAME": {
      "ownerName": "MyBucket"
    },
    "DIVISOR": 33.0
  }
}

MyClass{properties={NAME=Owner{ownerName='MyBucket'}, DIVISOR=33.0}}

Upvotes: 1

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