Parse JSON Request using Volley and GSON

In previous post,  I showed you how to use Volley to make a simple request in Android application.

This time, I’ll show you the recommended way to make a JSON request using Volley and how to customize Volley request to take advantage of the GSON library.

A - Setting up a RequestQueue

Firstly, create the VolleyHelper class. This will be a singleton class where we store and init all core Volley objects:

{% highlight java %}

public class VolleyHelper { private static VolleyHelper INSTANCE; private RequestQueue requestQueue; private Context context;

private VolleyHelper(Context context){
    this.context = context;
    requestQueue = getRequestQueue();
}

public static synchronized VolleyHelper getInstance(Context context){
    if(INSTANCE == null){
        INSTANCE = new VolleyHelper(context);
    }
    return INSTANCE;
}

public RequestQueue getRequestQueue(){
    if(requestQueue == null){
        requestQueue = Volley.newRequestQueue(context.getApplicationContext());
    }
    return requestQueue;
}

public <T> void addToRequestQueue(Request<T> req) {
    getRequestQueue().add(req);
}

}

{% endhighlight %}

B - Make a JSON Request

Firstly, create the layout for our simple demo with one Button and one TextView to display the result:

{% highlight java %}

<Button android:id="@+id/btnJSONRequest"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="JSON Request"/>

<TextView android:id="@+id/tvResult"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/btnJSONRequest"/>

{% endhighlight %}

In the MainActivity.java, we declare all indeed variables for all the controls and the JSON url:

{% highlight java %}

Button btnJSONRequest; Button btnGSONRequest; TextView tvResult;

String url=“http://icetea09.com/blog-files/demo_json”;

{% endhighlight %}

Make sure that you remember to inject all the variables to the proper controls in the layout:

{% highlight java %}

btnJSONRequest = (Button)findViewById(R.id.btnJSONRequest); btnGSONRequest = (Button)findViewById(R.id.btnGSONRequest); tvResult = (TextView)findViewById(R.id.tvResult);

{% endhighlight %}

Then, the most important step is creating a JsonObjectRequest:

{% highlight java %}

final JsonObjectRequest jsObjRequest = new JsonObjectRequest (Request.Method.GET, url, null, new Response.Listener() {

	@Override
	public void onResponse(JSONObject response) {
		tvResult.setText("Response: " + response.toString());
		String textResult = "";
		try {
			JSONArray arrProducts = response.getJSONArray("products");
			for(int i=0; i<arrProducts.length(); i++){
				JSONObject productItem = (JSONObject)arrProducts.get(i);
				textResult += "Name: " + productItem.getString("name") + "\n";
				textResult += "Description: " + productItem.getString("description") + "\n";
				textResult += "Price: $" + productItem.getString("price") + "\n\n";
			}
			tvResult.setText(textResult);
		} catch (JSONException e) {
			e.printStackTrace();
		}
	}

}, new Response.ErrorListener() {

	@Override
	public void onErrorResponse(VolleyError error) {
		if(error != null) Log.e("MainActivity", error.getMessage());

	}
});

{% endhighlight %}

And add that JsonObjectRequest to RequestQueue in the Button click event:

{% highlight java %}

btnJSONRequest.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { VolleyHelper.getInstance(getApplicationContext()).addToRequestQueue(jsObjRequest); } });

{% endhighlight %}

Run the project and enjoy the result:

C - Custom the Request with GSON

In this section, I'll describe how to implement a custom request types, for types that don't have out-of-the-box Volley support - GSON Request.

As you know that GSON is a very famous and effective library used for JSON converting. Follow this link if you want to learn more about GSON.

If you want to implement a custom request, you have to do 2 steps:

-     Extend the Request class, where represents the type of parsed response the request expects

-     Implement the abstract methods parseNetworkResponse() and deliverResponse()

Now apply all those steps to the GSONRequest class:

{% highlight java %}

public class GsonRequest extends Request { private final Gson gson = new Gson(); private final Class clazz; private final Map<String, String> headers; private final Response.Listener listener;

public GsonRequest(String url, Class<T> clazz, Map<String, String> headers,
                   Response.Listener<T> listener, Response.ErrorListener errorListener) {
    super(Method.GET, url, errorListener);
    this.clazz = clazz;
    this.headers = headers;
    this.listener = listener;
}

@Override
public Map<String, String> getHeaders() throws AuthFailureError {
    return headers != null ? headers : super.getHeaders();
}

@Override
protected Response<T> parseNetworkResponse(NetworkResponse response) {
    try {
        String json = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
        return Response.success(gson.fromJson(json, clazz),
                HttpHeaderParser.parseCacheHeaders(response));
    } catch (UnsupportedEncodingException e) {
        return Response.error(new ParseError(e));
    } catch (JsonSyntaxException e) {
        return Response.error(new ParseError(e));
    }
}

@Override
protected void deliverResponse(T response) {
    listener.onResponse(response);
}

}

{% endhighlight %}

The way to use GSONRequest is very familiar to normal requests.

First, we need to create all the models class that equivalent to the JSON response:

Product.java

{% highlight java %}

public class Product { private String id; private String name; private String description; private String price;

public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getDescription() {
    return description;
}

public void setDescription(String description) {
    this.description = description;
}

public String getPrice() {
    return price;
}

public void setPrice(String price) {
    this.price = price;
}

}

{% endhighlight %}

Products.java

{% highlight java %}

public class Products { private List products;

public List<Product> getProducts() {
    return products;
}

}

{% endhighlight %}

In MainActivity.java, we create a GSONRequest object:

{% highlight java %}

final GsonRequest gsonRequest = new GsonRequest(url, Products.class, null, new Response.Listener() {

@Override
public void onResponse(Products products) {
	String textResult = "";
	for(int i=0; i<products.getProducts().size(); i++){
		Product productItem = products.getProducts().get(i);
		textResult += "Name: " + productItem.getName() + "\n";
		textResult += "Description: " + productItem.getDescription() + "\n";
		textResult += "Price: $" + productItem.getPrice() + "\n\n";
	}
	tvResult.setText(textResult);
}

}, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError volleyError) { if(volleyError != null) Log.e(“MainActivity”, volleyError.getMessage()); } });

{% endhighlight %}

Then, add that GSONRequest object to the RequestQueue:

{% highlight java %}

btnGSONRequest.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { VolleyHelper.getInstance(getApplicationContext()).addToRequestQueue(gsonRequest); } });

{% endhighlight %}

Finally, run the project and enjoy the result:

As you can see, I added one more button called GSON Request.

That’s all for this post, hope you enjoy this! :D