Parsing Json Feeds with google Gson

Question

I would like to know how to parse a JSON feed by items (eg. url / title / description for each item). I have had a look to the doc / api but, it didn't help me.

This is what I got so far

import com.google.gson.Gson;
import com.google.gson.JsonObject;

public class ImportSources extends Job {
    public void doJob() throws IOException {
        String json = stringOfUrl("http://feed.test/all.json");
        JsonObject jobj = new Gson().fromJson(json, JsonObject.class);
        Logger.info(jobj.get("responseData").toString());
    }
    public static String stringOfUrl(String addr) throws IOException {
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        URL url = new URL(addr);
        IOUtils.copy(url.openStream(), output);
        return output.toString();
    }
}   

Solution 1

Depends on the actual JSON format. You can in fact just create a custom Javabean class which matches the JSON format. Any fields in JSON can be mapped as String, Integer, Boolean, etc Javabean properties. Any arrays can be mapped as List properties. Any objects can be mapped as another nested Javabean property. It greatly eases further processing in Java.

Without a JSON string example from your side, it's only guessing how it would look like, so I can't give a basic example here. But I've posted similar answers before here, you may find it useful:

• Converting JSON to Java
• Generate Java class from JSON?

Gson has also an User Guide, you may find it useful as well.

Solution 2

Gson 1.4 has a new API JsonStreamParser that lets you parse multiple JSON objects one by one from a stream.

Solution 3

You can create corresponding java classes for the json objects. The integer, string values can be mapped as is. Json can be parsed like this-

 Gson gson = new GsonBuilder().create();
 Response r = gson.fromJson(jsonString, Response.class);

Here is an example- http://rowsandcolumns.blogspot.com/2013/02/url-encode-http-get-solr-request-and.html

Comments

[Roch]

Well I was using ROME for RSS feeds, and the framework I'm using contains gson, Thats why I'm trying to use it, but I'm open to any other library.

[Roch]

Thanks for your answer, I have tried to apply the examples to the following feed: stocktwits.com/streams/all.json . I didn't manage to get any other values than null. Do I need to create two groups : stream and tweets?

[StaxMan]

Ok. Like I said, I think GSON only does full document binding, but I am not 100%. For an alternative, Jackson (jackson.codehaus.org) does allow incremental binding, as well as multiple other operating modes. The way that works is that you need to get a JsonParser for the main thing, iterate over main JSON array or object, and call parser.readValueAs() for actual binding. User mailing list can help with details, if documentation does not have enough pointers.