使用GSON进行JSON反序列化无法正常工作

Question

I'm currently writing an Android app which uses Google's GSON library for JSON deserialization. I already solved this (or at least a very similar) problem a while ago, but my source files got lost because stupid me made a mistake when setting up my backup process. Anyway, I know there's a solution. ;-)

The JSON I want to serialize comes from a web server sending data in the following form:

{
    "version": "1.1",
    "error": {
        "name": "bla",
        "code": 1234,
        "message": "oops"
    },
    "result": ???
}

Either error or result is always null depending on whether my request could be processed. result can be anything from boolean to a really complex class.

My Java code structure looks like this:

public class Response<T> {

    private String version;
    private T result;
    private Error error;

    public Response() {
    }

    public boolean isError() {
        return error != null;
    }

    public String getVersion() {
        return version;
    }

    public T getResult() {
        return result;
    }
}

public class Error {

    private String name;
    private int code;
    private String message;

    public Error() {
    }

    public String getName() {
        return name;
    }

    public int getCode() {
        return code;
    }

    public String getMessage() {
        return message;
    }
}

When I try to deserialize a Response<Boolean> or even a Response<ArrayList<String>> everything works fine, although I was a bit confused because Response<String[]> didn't work. Even the error field is populated correctly. Anyway, there is one thing which is absolutely not working, it's Response<Device> where Device is a very simple class looking like this:

public class Device {
    public String id;
    public String name;
    public String address;
    public String type;
}

And finally, here is my deserialization code:

private <T> Response<T> sendCommand(/* ... */) {
    Gson gson = new Gson();

    // preparing and sending request etc.

    String responseJson = "..."; // response JSON is valid (already verified)

    Type t = new TypeToken<Response<T>>() { }.getType();
    Response<T> result = gson.fromJson(responseJson, t);
    return result;
}

private void doSomething() {
    Response<Device> response = sendCommand("getSomeDevice");
}

I googled for the last three hours, but I couldn't find any solution. (Maybe it's a basic Java issue, I usually code in C#.) Can anyone help me?

Edit:

I forgot to mention that when I'm debugging my code, it seems GSON deserialized the Device JSON to an object of type LinkedTreeMap . Don't know what that means, but maybe it helps you.

Answer 1

The problem is likely here:

Type t = new TypeToken<Response<T>>() { }.getType();

Due to type erasure , T will not be resolved at runtime. It will need to be up to the caller to pass in a properly reifiable type token:

private <T> Response<T> sendCommand(/* ... */, TypeToken<Response<T>> type) {
    Gson gson = new Gson();

    // preparing and sending request etc.

    String responseJson = "..."; // response JSON is valid (already verified)

    Response<T> result = gson.fromJson(responseJson, type.getType());
    return result;
}

private void doSomething() {
    TypeToken<Response<T>> type = new TypeToken<Response<Device>>() { };
    Response<Device> response = sendCommand("getSomeDevice", type);
}

Using Guava 's version of TypeToken , things can be further abstracted:

private <T> Response<T> sendCommand(/* ... */, TypeToken<T> responseType) {
    Gson gson = new Gson();

    // preparing and sending request etc.

    String responseJson = "..."; // response JSON is valid (already verified)

    Type type = new TypeToken<Response<T>>() { }
            .where(new TypeParameter<T>() { }, responseType)
            .getType();
    Response<T> result = gson.fromJson(responseJson, type);
    return result;
}

private void doSomething() {
    Response<Device> response =
            sendCommand("getSomeDevice", new TypeToken<Device>() { });
}