Parsing JSON from URL

GSON has a builder that takes a Reader object: fromJson(Reader json, Class classOfT).

This means you can create a Reader from a URL and then pass it to Gson to consume the stream and do the deserialisation.

Only three lines of relevant code.

import java.io.InputStreamReader;
import java.net.URL;
import java.util.Map;

import com.google.gson.Gson;

public class GsonFetchNetworkJson {

    public static void main(String[] ignored) throws Exception {

        URL url = new URL("https://httpbin.org/get?color=red&shape=oval");
        InputStreamReader reader = new InputStreamReader(url.openStream());
        MyDto dto = new Gson().fromJson(reader, MyDto.class);

        // using the deserialized object
        System.out.println(dto.headers);
        System.out.println(dto.args);
        System.out.println(dto.origin);
        System.out.println(dto.url);
    }
    
    private class MyDto {
        Map<String, String> headers;
        Map<String, String> args;
        String origin;
        String url;
    }
}

^{If you happen to get a 403 error code with an endpoint which otherwise works fine (e.g. with curl or other clients) then a possible cause could be that the endpoint expects a User-Agent header and by default Java URLConnection is not setting it. An easy fix is to add at the top of the file e.g. System.setProperty("http.agent", "Netscape 1.0");.}

1. First you need to download the URL (as text):

   private static String readUrl(String urlString) throws Exception {
       BufferedReader reader = null;
       try {
           URL url = new URL(urlString);
           reader = new BufferedReader(new InputStreamReader(url.openStream()));
           StringBuffer buffer = new StringBuffer();
           int read;
           char[] chars = new char[1024];
           while ((read = reader.read(chars)) != -1)
               buffer.append(chars, 0, read); 
   
           return buffer.toString();
       } finally {
           if (reader != null)
               reader.close();
       }
   }
   

2. Then you need to parse it (and here you have some options).

   • GSON (full example):

     static class Item {
         String title;
         String link;
         String description;
     }
     
     static class Page {
         String title;
         String link;
         String description;
         String language;
         List<Item> items;
     }
     
     public static void main(String[] args) throws Exception {
     
         String json = readUrl("http://www.javascriptkit.com/"
                               + "dhtmltutors/javascriptkit.json");
     
         Gson gson = new Gson();        
         Page page = gson.fromJson(json, Page.class);
     
         System.out.println(page.title);
         for (Item item : page.items)
             System.out.println("    " + item.title);
     }
     

     Outputs:

     javascriptkit.com
         Document Text Resizer
         JavaScript Reference- Keyboard/ Mouse Buttons Events
         Dynamically loading an external JavaScript or CSS file
     

   • Try the java API from json.org:

     try {
         JSONObject json = new JSONObject(readUrl("..."));
     
         String title = (String) json.get("title");
         ...
     
     } catch (JSONException e) {
         e.printStackTrace();
     }