There's no built-in way to do this. You'll have to write your own JsonSerializer. Something like

class ModelSerializer extends JsonSerializer<List<Model>> {

    @Override
    public void serialize(List<Model> value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException {
        jgen.writeStartArray();
        for (Model model : value) {
            jgen.writeStartObject();
            jgen.writeObjectField("model", model);
            jgen.writeEndObject();    
        }
        jgen.writeEndArray();
    }

}

and then annotate the models field so that it uses it

@JsonSerialize(using = ModelSerializer.class)
private List<Model> models;

This would serialize as

{
    "status": "success",
    "models": [
        {
            "model": {
                "id": 1,
                "color": "red"
            }
        },
        {
            "model": {
                "id": 2,
                "color": "green"
            }
        }
    ]
}

If you're both serializing and deserializing this, you'll need a custom deserializer as well.

Found a SerializationFeature WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED by which we can obtain the above mentioned scenario in JSON

Easy way to do it without annotations is to use Gson library

Simple as that:

Gson gson = new Gson();
String json = gson.toJson(listaDePontos);

To easily serialize objects to json in Java, I suggest using the GSON library.

Gson gson = new Gson();
gson.toJson(events);

You can then turn this back to a Java object with

List<Event> events = gson.fromJson(string, Event.class);

    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.8.5</version>
    </dependency>

Use GSON library for that. Here is the sample code

List<String> foo = new ArrayList<String>();
foo.add("A");
foo.add("B");
foo.add("C");

String json = new Gson().toJson(foo );

Here is the maven dependency for Gson

<dependencies>
    <!--  Gson: Java to Json conversion -->
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.2.2</version>
        <scope>compile</scope>
    </dependency>
</dependencies>

Or you can directly download jar from here and put it in your class path

http://code.google.com/p/google-gson/downloads/detail?name=gson-1.0.jar&can=4&q=

To send Json to client you can use spring or in simple servlet add this code

response.getWriter().write(json);

Jackson depends on public accessors to determine fields to serialize.

Your list is the only field that isn't publicly exposed, so that's why it's hidden in the output

You don't realy need a class FriendArray for this. You can just searialize a list to JSON. Also it's easier to load the existing friend list to a list, add the new friend to the list and serialize the new list, instead of appending a string.
This way you won't have to worry about the correct JSON format or string manipulation. You just add an object to a list, and serialize the list.

Something like this should work (in java, sorry I don't know enough kotlin to implement this):

public void addFriendAndSerializeToFile(Friend friend) {
    // load existing friend list from the file
    Json json = new Json();
    // here the first parameter is the List (or Collection) type and the second parameter is the type of the objects that are stored in the list
    List<Friend> friendList = json.fromJson(List.class, Friend.class, friendsListFileHandle);
    
    // add the new friend to the deserialized list
    friendList.add(friend);

    // serialize the whole new list to the file
    String serializedFriendListWithNewFriendAdded = json.prettyPrint(friendList);
    
    // write to the file handle
    fileHandle.writeString(serializedFriendListWithNewFriendAdded, false);
}

Consider defining the cascade type for your collection.

When you persist your Candidate objects the operation is not cascaded by default and thus you need to define it yourself unless you persist your CandidateField objects directly.