CODEWITHSUNDEEP
javajsongson
Ben
Ben

Reputation: 6741

Using different classes to map JSON data with GSON

My application is receiving JSON messages from a WebSocket connection.

There are different types of answers, which are formatted like that:

{
    "type": "snapshot",
    "product_id": "BTC-EUR",
    "bids": [["1", "2"]],
    "asks": [["2", "3"]]
}

or 

{
    "type": "l2update",
    "product_id": "BTC-EUR",
    "changes": [
        ["buy", "1", "3"],
        ["sell", "3", "1"],
        ["sell", "2", "2"],
        ["sell", "4", "0"]
    ]
}

... for example (see full API here).

Depending on the "type", I would like GSON to map a different class (e.g. Snapshot.class and l2update.class).

I have message handlers that subscribe to the WebSocket connection and I want the message to be processed by the relevant handler. For instance:

My problem is to dispatch the messages depending on their type.

I was thinking to convert them to the appropriate class and then call the relevant handler using a factory. I'm currently stuck at the first step, converting the JSON in Error.class or Snapshot.class depending on the "type".

How can I do that?

Upvotes: 2

Views: 1284

Answers (2)

pirho
pirho

Reputation: 12205

For Gson you could use com.google.gson.typeadapters.RuntimeTypeAdapterFactory.

Assuming you have - for example - following classes:

public class BaseResponse {
    private String type, product_id;
    // rest of the common fields
}

public class Snapshot extends BaseResponse {    
    // rest of the fields
}

public class L2Update extends BaseResponse {    
    // rest of the fields
}

then you would build following RuntimeTypeAdapterFactory:

RuntimeTypeAdapterFactory<BaseResponse> runtimeTypeAdapterFactory = 
    RuntimeTypeAdapterFactory
        .of(BaseResponse.class, "type") // set the field where to look for value
        .registerSubtype(L2Update.class, "l2update") // values map to 'type' 
        .registerSubtype(Snapshot.class, "snapshot");// value in json

Registering this with Gson will then enable automativcal instantiation of each type of responses:

Gson gson = new GsonBuilder()
        .registerTypeAdapterFactory(runtimeTypeAdapterFactory).create();

and provide BaseResponse for fromJson(..) if using it , like:

gson.fromJson( json , BaseResponse.class);

NOTE: that Gson omits de- & serializing the type field. However it needs to be set in Json. Just as it is now in responses you get.

Upvotes: 2

stdunbar
stdunbar

Reputation: 17435

You may want to consider using a library that requires a bit less of a solid object model, at least at first. I use JsonPath for this type of thing. You could use it to at least find out the type you're dealing with:

String type = JsonPath.read(yourIncomingJson, "$.type");

and then, based on the string, do a switch statement as @ShafinMahmud suggests.

However, you could use JsonPath for the whole thing too. You could read all of the values using the path notation and know how to parse based on the type.

Adding another library to read a single value may or may not work for you but if you use it to read other values it might end up being worthwhile.

Upvotes: 1

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