How to deserialize and return objects from JSON in Java with Gson without knowing the .class a priori?

Question

I am currently working on a thin-client application, where the communication happens through JSON-serialized message objects. Server serializes the message, sends it through a socket, client receives and deserializes. Answers happen in the same way.

First, let's assume the message class are defined both on server and client.

Problem is that Gson::fromJson function needs a .class/type object for deserializing via introspection (understandably), but in my application, multiple type of objects can be received without knowing the .class in advance.

My idea was to create a message wrapper like this:

class MessageWrapper {
    public class MessageWrapper(Object message, MessageType type) {
        this.message = message;
        this.type = type;
    }

    // getters...

    public enum MesssageType {
        PLACEMENT,
        UPDATE,
        // ...
    }

    private final Object message;
    private final MessageType type;
}

Or even go further by determining type param with introspection. This solution is great for serializing (I repeat, that is not a problem), but while deserializing I would get the message type and loose the message itself, at least if I don't parse it twice. Specializing MessageWrapper through Java "templating" mechanism brings us back to the original problem (I would have multiple classes to choose from).

Another idea, was to send a token to identify the message, before the JSON string, like:

Placement={"foo": 2, "bar": "baz"}

Then read the token to determine the .class type. This could work, but there is still a problem: how would I return the value from my receive function? Of course I could do:

public Object receive(Reader stream) {}

And force the user to do a downcast, but I'd rather avoid it.

EDIT: this is because the client has a reactor-like structure: it runs in a loop and dispatches messages to appropriate handlers.

can you change the structure sent from the server? I mean by using another library and adding type information in the stream. — eugen, Jun 05 '13 at 10:22
I do have type information in the generated JSON, either as a field (first case), or type token (latter case). — Stefano Sanfilippo, Jun 05 '13 at 10:32

Enrichman · Answer 1 · 2013-06-05T10:46:20.163

EDIT:

Ok, now it's clearer.

What about having a Wrapper with a series of defined Objects? How many handlers do you have?

I mean something like:

class HandlerWrapper {
    private final Placement placement;
    private final Update update;
}

then the client will deserialize this and check for the not-null property:

{"placement":{"foo": 2, "bar": "baz"}, "update":null, ...}

I knwo it's not very nice but I cannot think about something else.. : /

OLD

I don't think this is actually a problem, because you can say in your specification what kind of object you have to expect from that specific case.

For example, if you're calling an API to get a User, you will expect a User.class in the Object. The same for an API to get a Book (or whatelse).

(I hope I understood the question)

score 3 · Accepted Answer · answered Jun 05 '13 at 11:28

Using Gson, if you have a reasonable number of possible messages, you could create a class containing all of them (something similar to which @Enrichman wrote, but you don't need to check for nulls...). For example if you have the class

public class Response {
    private Placement placement;
    private Update update;
    //more message types
    //getters & setters
}

Then you can deserialize your response with:

Gson gson = new Gson();
Response response = gson.fromJson(jsonString, Response.class);

And it would deserialize this JSON:

{"placement": {...} }

and this:

{"update": {...} }

and this:

{"placement": {...}, "update": {...} }

Gson will ignore all the fields in the JSON response that don't correspond to any attribute in your class, so you can use a single class to deserialize multiple responses...

Good answers. I'm accepting this one as it outlines the key point that "null fields are not serialized", and upvoting the other as it provided the key intuition. It's a pity that I could not achieve the same result in a simpler way, maybe some CGlib magic could do it. — Stefano Sanfilippo, Jun 06 '13 at 09:18
Btw you can choose to serialize or not null fields (at least you can do this with Jackson: http://stackoverflow.com/questions/11757487/how-to-tell-jackson-to-ignore-a-field-only-if-its-value-is-null ). :) — Enrichman, Jun 07 '13 at 08:17

How to deserialize and return objects from JSON in Java with Gson without knowing the .class a priori?

2 Answers2