Properties inner3 = new Properties();
inner3.put("i1", 1);
inner3.put("i2", 100);

Properties inner2 = new Properties();
inner2.put("aStringProp", "aStringValue");
inner2.put("inner3", inner3);

Properties inner1 = new Properties();
inner1.put("aBoolProp", true);
inner1.put("inner2", inner2);

Properties topLevelProp = new Properties();
topLevelProp.put("count", 1000000);
topLevelProp.put("size", 1);
topLevelProp.put("inner1", inner1);

{
  "inner1": {
    "inner2": {
      "aStringProp": "aStringValue",
      "inner3": {
        "i2": 100,
        "i1": 1
      }
    },
    "aBoolProp": true
  },
  "size": 1,
  "count": 1000000
}

Gson gson = new GsonBuilder().create();
String json = gson.toJson(topLevelProp); //{"inner1":{"inner2":{"aStringProp":"aStringValue","inner3":{"i2":100,"i1":1}},"aBoolProp":true},"size":1,"count":1000000}

//following line throws error: Expected a string but was BEGIN_OBJECT at line 1 column 12 path $.
Properties propObj = gson.fromJson(json, Properties.class);

ObjectMapper mapper = new ObjectMapper();
mapper.configure(MapperFeature.PROPAGATE_TRANSIENT_MARKER, true);
mapper.setVisibility(PropertyAccessor.ALL, Visibility.NONE);
mapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
File file = new File("configs/config1.json");
mapper.writeValue(file, topLevelProp);

Properties jckProp = JsonSerializer.mapper.readValue(json, Properties.class);

ObjectNode jckProp = JsonSerializer.mapper.readValue(json, ObjectNode.class);
System.out.println(jckProp.get("size").asInt());
System.out.println("jckProp: " + jckProp);
System.out.println("jckProp.inner: " + jckProp.get("inner1"));

The Properties class represents a persistent set of properties.
The Properties can be saved to a stream or loaded from a stream.
Each key and its corresponding value in the property list is a string.
...
If the store or save method is called on a "compromised" Properties
object that contains a non-String key or value, the call will fail.

topLevelProp.put("ARRAY", new String[]{ "foo", "bar" });
topLevelProp.put("RAW_LIST", asList("foo", "bar"));

private static final Gson typeAwareGson = new GsonBuilder()
        .registerTypeAdapter(Properties.class, getTypeAwarePropertiesSerializer())
        .registerTypeAdapter(Properties.class, getTypeAwarePropertiesDeserializer())
        .create();

public class Configuration extends Properties {
    public void load(JsonElement json) {
        addJson("", json);
        return;
    }

    public void addJson(String root, JsonElement json) {

        // recursion for objects
        if (json instanceof JsonObject) {
            if (!root.equals("")) root += ".";
            final JsonObject jsonObject = json.getAsJsonObject();
            for ( final Entry<String, JsonElement> e : jsonObject.entrySet() ) {
            addJson(root + e.getKey(), e.getValue());
            }
            return;
        }

        // recursion for arrays
        if (json instanceof JsonArray) {
            final JsonArray jsonArray = json.getAsJsonArray();
            if (!root.equals("")) root += ".";
            int count = 0;
            for(final JsonElement e : jsonArray) {
                addJson(root+count, e);
                count++;
            }
            return;
        }

        // leaves: add property
        this.put(root, json.toString());
    }
}