Volley Library in Android

Volley is an HTTP library that makes networking very easy and fast, for Android apps. It was developed by Google and introduced during Google I/O 2013. It was developed because there is an absence in Android SDK, of a networking class capable of working without interfering with the user experience. Although Volley is a part of the Android Open Source Project(AOSP), Google announced in January 2017 that Volley will move to a standalone library. It manages the processing and caching of network requests and it saves developers valuable time from writing the same network call/cache code again and again. Volley is not suitable for large download or streaming operations since Volley holds all responses in memory during parsing. 

Features of Volley:

1. Request queuing and prioritization
2. Effective request cache and memory management
3. Extensibility and customization of the library to our needs
4. Cancelling the requests

Advantages of Using Volley

1. All the tasks that need to be done with Networking in Android, can be done with the help of Volley.
2. Automatic scheduling of network requests.
3. Catching
4. Multiple concurrent network connections.
5. Cancelling request API.
6. Request prioritization.
7. Volley provides debugging and tracing tools.

How to Import Volley and Add Permissions

Before getting started with Volley, one needs to import Volley and add permissions in the Android Project. The steps to do so are as follows:

Create a new project. Open build.gradle(Module: app) and add the following dependency:

dependencies{ 
    //...
    implementation 'com.android.volley:volley:1.0.0'
}

In AndroidManifest.xml add the internet permission:

<uses-permission android:name="android.permission.INTERNET" />

Classes in Volley Library

Volley has two main classes:

1. Request Queue: It is the interest one uses for dispatching requests to the network. One can make a request queue on demand if required, but typically it is created early on, at startup time, and keep it around and use it as a Singleton.
2. Request: All the necessary information for making web API call is stored in it. It is the base for creating network requests(GET, POST).

Types of Request using Volley Library

String Request

Java

String url = "https:// string_url/";
StringRequest
    stringRequest
    = new StringRequest(
        Request.Method.GET,
        url,
        new Response.Listener() {
            @Override
            public void onResponse(String response)
            {
            }
        },
        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error)
            {
            }
        });
requestQueue.add(stringRequest);

JSONObject Request

Java

String url = "https:// json_url/";
JsonObjectRequest
    jsonObjectRequest
    = new JsonObjectRequest(
        Request.Method.GET,
        url,
        null,
        new Response.Listener() {
            @Override
            public void onResponse(JSONObject response)
            {
            }
        },
        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error)
            {
            }
        });
requestQueue.add(jsonObjectRequest);

JSONArray Request

Java

JsonArrayRequest
    jsonArrayRequest
    = new JsonArrayRequest(
        Request.Method.GET,
        url,
        null,
        new Response.Listener() {
            @Override
            public void onResponse(JSONArray response)
            {
            }
        },
        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error)
            {
            }
        });
requestQueue.add(jsonArrayRequest);

Image Request

Java

int max - width = ...;
int max_height = ...;

String URL = "http:// image_url.png";

ImageRequest
    imageRequest
    = new ImageRequest(URL,
                    new Response.Listener() {
                        @Override
                        public void
                        onResponse(Bitmap response)
                        {
                            // Assign the response
                            // to an ImageView
                            ImageView
                                imageView
                                = (ImageView)
                                    findViewById(
                                        R.id.imageView);

                            imageView.setImageBitmap(response);
                        }
                    },
                    max_width, max_height, null);

requestQueue.add(imageRequest);

Adding Post Parameters

Java

String tag_json_obj = "json_obj_req";

String
    url
    = "https:// api.xyz.info/volley/person_object.json";

ProgressDialog pDialog = new ProgressDialog(this);
pDialog.setMessage("Loading...PLease wait");
pDialog.show();

JsonObjectRequest
    jsonObjReq
    = new JsonObjectRequest(
        Method.POST,
        url,
        null,
        new Response.Listener() {

            @Override
            public void onResponse(JSONObject response)
            {
                Log.d(TAG, response.toString());
                pDialog.hide();
            }
        },
        new Response.ErrorListener() {

            @Override
            public void onErrorResponse(VolleyError error)
            {
                VolleyLog.d(TAG, "Error: "
                                    + error.getMessage());
                pDialog.hide();
            }
        }) {

        @Override
        protected Map getParams()
        {
            Map params = new HashMap();
            params.put("name", "Androidhive");
            params.put("email", "abc@androidhive.info");
            params.put("password", "password123");

            return params;
        }

    };

AppController.getInstance()
    .addToRequestQueue(jsonObjReq, tag_json_obj);

Adding Request Headers

Java

String tag_json_obj = "json_obj_req";

String
    url
    = "https:// api.androidhive.info/volley/person_object.json";

ProgressDialog pDialog = new ProgressDialog(this);
pDialog.setMessage("Loading...");
pDialog.show();

JsonObjectRequest
    jsonObjReq
    = new JsonObjectRequest(
        Method.POST,
        url,
        null,
        new Response.Listener() {

            @Override
            public void onResponse(JSONObject response)
            {
                Log.d(TAG, response.toString());
                pDialog.hide();
            }
        },
        new Response.ErrorListener() {

            @Override
            public void onErrorResponse(VolleyError error)
            {
                VolleyLog.d(TAG, "Error: "
                                    + error.getMessage());
                pDialog.hide();
            }
        }) {

        @Override
        public Map getHeaders() throws AuthFailureError
        {
            HashMap headers = new HashMap();
            headers.put("Content-Type", "application/json");
            headers.put("apiKey", "xxxxxxxxxxxxxxx");
            return headers;
        }

    };

AppController.getInstance()
    .addToRequestQueue(jsonObjReq, tag_json_obj);

Handling the Volley Cache

Java

// Loading request from cache
Cache
    cache
    = AppController.getInstance()
        .getRequestQueue()
        .getCache();

Entry entry = cache.get(url);
if (entry != null) {
    try {
        String
            data
            = new String(entry.data, "UTF-8");
        // handle data, like converting it
        // to xml, json, bitmap etc.,
    }
    catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
}
}
else
{
    // If cached response doesn't exists
}

// Invalidate cache
AppController.getInstance()
    .getRequestQueue()
    .getCache()
    .invalidate(url, true);

// Turning off cache
// String request
StringRequest
    stringReq
    = new StringRequest(....);

// disable cache
stringReq.setShouldCache(false);

// Deleting cache for particular cache</strong>
AppController.getInstance()
    .getRequestQueue()
    .getCache()
    .remove(url);

// Deleting all the cache
AppController.getInstance()
    .getRequestQueue()
    .getCache()
    .clear(url);

Cancelling Request

Java

// Cancel single request
String tag_json_arry = "json_req";

ApplicationController.getInstance()
    .getRequestQueue()
    .cancelAll("feed_request");

// Cancel all request
ApplicationController.getInstance()
    .getRequestQueue()
    .cancelAll();

Request Prioritization

Java

private Priority priority = Priority.HIGH;

StringRequest
    strReq
    = new StringRequest(
        Method.GET,
        Const.URL_STRING_REQ,
        new Response
            .Listener() {

                    @Override
                    public void onResponse(String response) {

                        Log.d(TAG, response.toString());
                        msgResponse.setText(response.toString());
                        hideProgressDialog();

                    } },
        new Response
            .ErrorListener() {

                    @Override
                    public void
                    onErrorResponse(VolleyError error) {

                        VolleyLog.d(TAG,
                                    "Error: "
                                        + error.getMessage());
                        hideProgressDialog();
                    } }) {

        @Override
        public Priority getPriority()
        {
            return priority;
        }

    };