Android JSON Parsing with Gson Tutorial

Apart from XML, JSON is a very common format used in API responses. Its simplicity has helped to gain quite the adoption in favor of the more verbose XML. Additionally, JSON can easily be combined with REST producing clear and easy to use APIs. Android includes support for JSON in its SDK as someone can find in the JSON package summary. However, using those classes, a developer has to deal with low level JSON parsing, which in my opinion is tedious and boring. For this reason, in this tutorial, I am going to show you how to perform automatic JSON parsing.

For this purpose we are going to use the Google Gson library. From the official site:

Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.
There are a few open-source projects that can convert Java objects to JSON. However, most of them require that you place Java annotations in your classes something that you can not do if you do not have access to the source-code. Most also do not fully support the use of Java Generics. Gson considers both of these as very important design goals.

Excellent, exactly what we need. Before delving into code, you might want to take a look at the Gson User Guide and bookmark the Gson API Javadocs. Let's get started by downloading Gson, with the current version being 1.6. We need the gson-1.6.jar from the distribution.

Add the Gson JAR to your project's classpath.

To illustrate how to use Gson for JSON parsing we are going to parse a JSON response from the Twitter API. Check the Twitter API Documentation for more info. We are going to use the Search API method for performing ad-hoc searches.

For example, for searching Twitter about Robin and retrieving the results in JSON format, here is the corresponding URL:

http://search.twitter.com/search.json?q=robin

This will give a one line JSON response containing all the relevant info. This one liner is quite hard to read, so a JSON editor would be quite handy. I use the Eclipse Json Editor plugin and works really well. Here is how the response looks formatted in my Eclipse IDE:

 

As you can see, we have a number of results and after that we have some other fields, such as “max_id”, “since_id”, “query” etc.

Thus, our main model object, named “SearchResponse” will be as follows:

01	package com.robin.android.json.model;
02

03	import java.util.List;
04

05	import com.google.gson.annotations.SerializedName;
06

07	public class SearchResponse {
08

09	public List<Result> results;
10

11	@SerializedName("max_id")
12	public long maxId;

13
14	@SerializedName("since_id")

15	public int sinceId;
16

17	@SerializedName("refresh_url")
18	public String refreshUrl;

19
20	@SerializedName("next_page")

21	public String nextPage;
22

23	@SerializedName("results_per_page")
24	public int resultsPerPage;

25
26	public int page;

27
28	@SerializedName("completed_in")

29	public double completedIn;
30

31	@SerializedName("since_id_str")
32	public String sinceIdStr;

33
34	@SerializedName("max_id_str")

35	public String maxIdStr;
36

37	public String query;
38

39

}

We provide the various public fields (getter/setters with private fields can also be used) and in those case that the field name does not match the JSON response, we annotate with the SerializedName annotation.

Note that we also have a list of results, with the corresponding model class being:

01	package com.robin.android.json.model;
02

03	import com.google.gson.annotations.SerializedName;
04

05	public class Result {
06

07	@SerializedName("from_user_id_str")
08	public String fromUserIdStr;

09
10	@SerializedName("profile_image_url")

11	public String profileImageUrl;
12

13	@SerializedName("created_at")
14	public String createdAt;

15
16	@SerializedName("from_user")

17	public String fromUser;
18

19	@SerializedName("id_str")
20	public String idStr;

21
22	public Metadata metadata;

23
24	@SerializedName("to_user_id")

25	public String toUserId;
26

27	public String text;
28

29	public long id;
30

31	@SerializedName("from_user_id")
32	public String from_user_id;

33
34	@SerializedName("iso_language_code")

35	public String isoLanguageCode;
36

37	@SerializedName("to_user_id_str")
38	public String toUserIdStr;

39
40	public String source;

41
42	}

Finally, we have one more class named “Metadata”:

01	package com.robin.android.json.model;
02

03	import com.google.gson.annotations.SerializedName;
04

05	public class Metadata {
06

07	@SerializedName("result_type")
08	public String resultType;

09
10	}

Let's now see how all these get wired using Gson. Here is our Activity:

01	package com.robin.android.json;
02

03	import java.io.IOException;
04	import java.io.InputStream;

05	import java.io.InputStreamReader;
06	import java.io.Reader;

07	import java.util.List;
08

09	import org.apache.http.HttpEntity;
10	import org.apache.http.HttpResponse;

11	import org.apache.http.HttpStatus;
12	import org.apache.http.client.methods.HttpGet;

13	import org.apache.http.impl.client.DefaultHttpClient;
14

15	import android.app.Activity;
16	import android.os.Bundle;

17	import android.util.Log;
18	import android.widget.Toast;

19
20	import com.google.gson.Gson;

21	import com.robin.android.json.model.Result;
22	import com.robin.android.json.model.SearchResponse;

23
24	public class JsonParsingActivity extends Activity {

25
26	String url = "http://search.twitter.com/search.json?q=robin";

27
28	@Override

29	public void onCreate(Bundle savedInstanceState) {
30

31	super.onCreate(savedInstanceState);
32	setContentView(R.layout.main);

33
34	InputStream source = retrieveStream(url);

35
36	Gson gson = new Gson();

37
38	Reader reader = new InputStreamReader(source);

39
40	SearchResponse response = gson.fromJson(reader, SearchResponse.class);

41
42	Toast.makeText(this, response.query, Toast.LENGTH_SHORT).show();

43
44	List<Result> results = response.results;

45
46	for (Result result : results) {

47	Toast.makeText(this, result.fromUser, Toast.LENGTH_SHORT).show();
48	}

49
50	}

51
52	private InputStream retrieveStream(String url) {

53
54	DefaultHttpClient client = new DefaultHttpClient();

55
56	HttpGet getRequest = new HttpGet(url);

57
58	try {

59
60	HttpResponse getResponse = client.execute(getRequest);

61	final int statusCode = getResponse.getStatusLine().getStatusCode();
62

63	if (statusCode != HttpStatus.SC_OK) {
64	Log.w(getClass().getSimpleName(),

65	"Error " + statusCode + " for URL " + url);
66	return null;

67	}
68

69	HttpEntity getResponseEntity = getResponse.getEntity();
70	return getResponseEntity.getContent();

71
72	}

73	catch (IOException e) {
74	getRequest.abort();

75	Log.w(getClass().getSimpleName(), "Error for URL " + url, e);
76	}

77
78	return null;

79
80	}

81
82	}

First, we perform an HTTP GET request and retrieve the resource as a stream. We create a Gson instance and use it to perform the JSON parsing and retrieve our model object with all its fields populated.