Gson serialize a list of polymorphic objects

There’s a simple solution: Gson’s RuntimeTypeAdapterFactory (from com.google.code.gson:gson-extras:$gsonVersion). You don’t have to write any serializer, this class does all work for you. Try this with your code:

    ObixBaseObj lobbyObj = new ObixBaseObj();
    lobbyObj.setIs("obix:Lobby");

    ObixOp batchOp = new ObixOp();
    batchOp.setName("batch");
    batchOp.setIn("obix:BatchIn");
    batchOp.setOut("obix:BatchOut");

    lobbyObj.addChild(batchOp);

    RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
                    RuntimeTypeAdapterFactory
                   .of(ObixBaseObj.class)
                   .registerSubtype(ObixBaseObj.class)
                   .registerSubtype(ObixOp.class);


    Gson gson2=new GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create();
    Gson gson = new Gson();
    System.out.println(gson.toJson(lobbyObj));
    System.out.println("---------------------");
    System.out.println(gson2.toJson(lobbyObj));

}

Output:

{"obix":"obj","is":"obix:Lobby","children":[{"obix":"op","name":"batch","children":[]}]}
---------------------
{
  "type": "ObixBaseObj",
  "obix": "obj",
  "is": "obix:Lobby",
  "children": [
    {
      "type": "ObixOp",
      "in": "obix:BatchIn",
      "out": "obix:BatchOut",
      "obix": "op",
      "name": "batch",
      "children": []
    }
  ]
}

EDIT: Better working example.

You said that there are about 25 classes that inherits from ObixBaseObj.

We start writing a new class, GsonUtils

public class GsonUtils {

    private static final GsonBuilder gsonBuilder = new GsonBuilder()
            .setPrettyPrinting();

    public static void registerType(
            RuntimeTypeAdapterFactory<?> adapter) {
        gsonBuilder.registerTypeAdapterFactory(adapter);
    }

    public static Gson getGson() {
        return gsonBuilder.create();
    }

Every time we need a Gson object, instead of calling new Gson(), we will call

GsonUtils.getGson()

We add this code to ObixBaseObj:

public class ObixBaseObj {
    protected String obix;
    private String display;
    private String displayName;
    private String name;
    private String is;
    private ArrayList<ObixBaseObj> children = new ArrayList<ObixBaseObj>();
    // new code
    private static final RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
            RuntimeTypeAdapterFactory.of(ObixBaseObj.class);

    private static final HashSet<Class<?>> registeredClasses= new HashSet<Class<?>>();

    static {
        GsonUtils.registerType(adapter);
    }

    private synchronized void registerClass() {
        if (!registeredClasses.contains(this.getClass())) {
            registeredClasses.add(this.getClass());
            adapter.registerSubtype(this.getClass());
        }
    }
    public ObixBaseObj() {
        registerClass();
        obix = "obj";
    }

Why? because every time this class or a children class of ObixBaseObj is instantiated,
the class it’s gonna be registered in the RuntimeTypeAdapter

In the child classes, only a minimal change is needed:

public class ObixOp extends ObixBaseObj {
    private String in;
    private String out;

    public ObixOp() {
        super();
        obix = "op";
    }

    public ObixOp(String in, String out) {
        super();
        obix = "op";
        this.in = in;
        this.out = out;
    }

Working example:

public static void main(String[] args) {

        ObixBaseObj lobbyObj = new ObixBaseObj();
        lobbyObj.setIs("obix:Lobby");

        ObixOp batchOp = new ObixOp();
        batchOp.setName("batch");
        batchOp.setIn("obix:BatchIn");
        batchOp.setOut("obix:BatchOut");

        lobbyObj.addChild(batchOp);



        Gson gson = GsonUtils.getGson();
        System.out.println(gson.toJson(lobbyObj));

    }

Output:

{
  "type": "ObixBaseObj",
  "obix": "obj",
  "is": "obix:Lobby",
  "children": [
    {
      "type": "ObixOp",
      "in": "obix:BatchIn",
      "out": "obix:BatchOut",
      "obix": "op",
      "name": "batch",
      "children": []
    }
  ]
}

I hope it helps.