How to serialize json to Map?

Question

I need to serialize json to Map. My son looks like the following:

{
   items: [{
      "name": "Test1",
      "value": {
         "id": 1,
         "count": 5
      }
   }]
}

and I have following classes:

public class Value {
    public int id;
    public int count;
}
public class ItemManager {
    public Map<String, Value> items;
}

and I was trying to deserialize it like that:

class Main {
    public static void main(String... args) {
        ObjectMapper mapper = new ObjectMapper();
        ItemManager manager = mapper.read(args[0], ItemManager.class);
    }
}

but I get the following exception:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `test.ItemManager` out of START_ARRAY token
 at [Source: (String)"{

I need to put name as a key and value as a value. Can anyone help to do it?

Answer 1

Here is a simple custom Deserializer to do what you want (I’ve used Jackson v2.12.4) :

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.ObjectCodec;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;

import java.io.IOException;

public class ItemManagerDeserializer extends StdDeserializer<ItemManager> {

    public ItemManagerDeserializer() {
        this(null);
    }

    public ItemManagerDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public ItemManager deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
        JsonNode node = jp.getCodec().readTree(jp);
        ObjectCodec mapper = jp.getCodec();

        ItemManager itemManager = new ItemManager();
        for (JsonNode element : node.get("items")) {
            String key = element.get("name").asText();
            ItemManager.Value value = mapper.treeToValue(element.get("value"), ItemManager.Value.class);
            itemManager.getItems().put(key, value);
        }

        return itemManager;
    }
} 

I’ve created a static Value class in the ItemManager class and instantiated the items property in the constructor (new HashMap<>()).

Then, you can register and use this Deserializer like so :

public static void main(String[] args) throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    SimpleModule module = new SimpleModule();
    module.addDeserializer(ItemManager.class, new ItemManagerDeserializer());
    mapper.registerModule(module);

    ItemManager itemManager = mapper.readValue(args[0], ItemManager.class);
    itemManager.getItems().forEach((key, value) -> System.out.println("key: " + key + ", value: " + value));
}