Gson - attempting to convert json string to custom object

Question

Here is my Json returned from the server

{"ErrorCode":1005,"Message":"Username does not exist"}

Here is my class for an error

public class ErrorModel {
public int ErrorCode;
public String Message;
}

and here is my conversion code.

public static ErrorModel GetError(String json) {

    Gson gson = new Gson();

    try
    {
        ErrorModel err = gson.fromJson(json, ErrorModel.class);

        return err;
    }
    catch(JsonSyntaxException ex)
    {
        return null;
    }
}

It is always throwing a JsonSyntaxException. Any ideas what could be my problem here?

EDIT : As requested, here is further elaboration.

My backend is an ASP.NET MVC 2 application acting as a rest API. The backend isn't the problem here, as my actions (and even server errors) all return Json (using the built in JsonResult ). Here's a sample.

[HttpPost]
public JsonResult Authenticate(AuthenticateRequest request)
{
    var authResult = mobileService.Authenticate(request.Username, request.Password, request.AdminPassword);

    switch (authResult.Result)
    {
         //logic omitted for clarity
         default:
            return ExceptionResult(ErrorCode.InvalidCredentials, "Invalid username/password");
            break;
    }

    var user = authResult.User;

    string token = SessionHelper.GenerateToken(user.UserId, user.Username);

    var result = new AuthenticateResult()
    {
        Token = token
    };

    return Json(result, JsonRequestBehavior.DenyGet);
}

The basic logic is to auth the user cretentials and either return an ExceptionModel as json or an AuthenticationResult as json.

Here is my server side Exception Model

public class ExceptionModel
{
    public int ErrorCode { get; set; }
    public string Message { get; set; }

    public ExceptionModel() : this(null)
    {

    }

    public ExceptionModel(Exception exception)
    {
        ErrorCode = 500;
        Message = "An unknown error ocurred";

        if (exception != null)
        {
            if (exception is HttpException)
                ErrorCode = ((HttpException)exception).ErrorCode;

            Message = exception.Message;
        }
    }

    public ExceptionModel(int errorCode, string message)
    {
        ErrorCode = errorCode;
        Message = message;
    }
}

When the above authentication is called with invalid credentials, the error result is returned as expected. The Json returned is the Json above in the question.

On the android side, I first build an object with my key-value pairs.

public static HashMap<String, String> GetAuthenticationModel(String username, String password, String adminPassword, String abbr)
{
    HashMap<String, String> request = new HashMap<String, String>();
    request.put("SiteAbbreviation", abbr);
    request.put("Username", username);
    request.put("Password", password);
    request.put("AdminPassword", adminPassword);

    return request;
}

Then, I send off an http post and return as a string whatever is sent back.

public static String Post(ServiceAction action, Map<String, String> values) throws IOException {
    String serviceUrl = GetServiceUrl(action);

    URL url = new URL(serviceUrl);

    URLConnection connection = url.openConnection();
    connection.setDoInput(true);
    connection.setDoOutput(true);
    connection.setUseCaches(false);
    connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

    String data = GetPairsAsString(values);

    DataOutputStream output = new DataOutputStream(connection.getOutputStream());
    output.writeBytes(data);
    output.flush();
    output.close();

    DataInputStream input = new DataInputStream(connection.getInputStream());

    String line;
    String result = "";
    while (null != ((line = input.readLine())))
    {
        result += line;
    }
    input.close ();

    return result;
}

private static String GetServiceUrl(ServiceAction action)
{
    return "http://192.168.1.5:33333" + action.toString();
}

private static String GetPairsAsString(Map<String, String> values){

    String result = "";
    Iterator<Entry<String, String>> iter = values.entrySet().iterator();

    while(iter.hasNext()){
        Map.Entry<String, String> pairs = (Map.Entry<String, String>)iter.next();

        result += "&" + pairs.getKey() + "=" + pairs.getValue();
    }

    //remove the first &
    return result.substring(1);
}

Then I take that result and pass it into my parser to see if it is an error

public static ErrorModel GetError(String json) {

    Gson gson = new Gson();

    try
    {
        ErrorModel err = gson.fromJson(json, ErrorModel.class);

        return err;
    }
    catch(JsonSyntaxException ex)
    {
        return null;
    }
}

But, JsonSyntaxException is always thrown.

Answer 1

Might help to know more about the exception, but the same code sample works fine here. I suspect there's a piece of code you omitted that's causing the problem (perhaps the creation/retrieval of the JSON string). Here's a code sample that worked fine for me on Java 1.6 and Gson 1.6:

import com.google.gson.Gson;

public class ErrorModel {
  public int ErrorCode;
  public String Message;
  public static void main(String[] args) {
    String json = "{\"ErrorCode\":1005,\"Message\":\"Username does not exist\"}";
    Gson gson = new Gson();
    ErrorModel err = gson.fromJson(json, ErrorModel.class);
    System.out.println(err.ErrorCode);
    System.out.println(err.Message);
  }
}