Java processing JSON

Question

I am getting different type of JSON response out of HTTP request API. There are might be couple of JSON format option coming back from API. For example it might be valid response with expected data but in some cases it might be internal server error detailed message.

At the moment I am using Gson to convert incoming string into the object, but since sometimes it comes as different format Gson not able to convert it as different template class is provided.

NOTE: Error does not mean an exception. For example JSON body contain just information that authentication is failed for example, but call was made successfully and JSON body is VALID. HTTP is actually always successful and will be 200. Problem is that sometimes authentication might fail and it will return different JSON.

String response = restTemplate.getForObject(request, String.class);
ObjectResponse objResponse = gson.fromJson(response, ObjectResponse.class);

Could you please suggest better way of doing it so that I can handle different types of responses? Or maybe you know completely different way of doing it.

Thanks!

Answer 1

If you can't predict the structure of the response, map it to a tree of simple Java maps, arrays, and strings. The Jackson library supports this with 'readTree' methods. Once you look at the tree and decide what it is, you can then ask the library to map a tree to an object of a class.

Answer 2

One option is to make a class representing the JSON data, and deserialize into that. This way, if the data does not match that structure, you will get an exception.

When you try and create your object and it fails, catch the exception and try and decode it as an error - you can then deal with that case as you wish (and the potential case where it is neither the object you expect or a valid error).

Answer 3

Check HTTP Response Codes. If you receive a status code that isn't OK(200) then you shouldn't try to parse for a successful response. For instance you may check the code and handle response like this (the object types are not actual Java types, but are given to provide an example):

MyHttpResponse response = MyHttpHelper.execute(...);
int status = response.getMyStatusCode();
String responseData = response.getStringBody();
switch(status) {
    case 200: {
        //request is successful, parse valid data
        break;
    }
    default: {
        //request is not valid, parse error data
        break;
    }
}