Set up RequestQueue

The previous lesson showed you how to use the convenience method Volley.newRequestQueue to set up a RequestQueue, taking advantage of Volley's default behaviors. This lesson walks you through the explicit steps of creating a RequestQueue, to allow you to supply your own custom behavior.

This lesson also describes the recommended practice of creating a RequestQueue as a singleton, which makes the RequestQueue last the lifetime of your app.

Set up a network and cache

A RequestQueue needs two things to do its job: a network to perform transport of the requests, and a cache to handle caching. There are standard implementations of these available in the Volley toolbox: DiskBasedCache provides a one-file-per-response cache with an in-memory index, and BasicNetwork provides a network transport based on your preferred HTTP client.

BasicNetwork is Volley's default network implementation. A BasicNetwork must be initialized with the HTTP client your app is using to connect to the network. Typically this is an HttpURLConnection.

This snippet shows you the steps involved in setting up a RequestQueue:

// Instantiate the cache
val cache = DiskBasedCache(cacheDir, 1024 * 1024) // 1MB cap

// Set up the network to use HttpURLConnection as the HTTP client.
val network = BasicNetwork(HurlStack())

// Instantiate the RequestQueue with the cache and network. Start the queue.
val requestQueue = RequestQueue(cache, network).apply {
    start()
}

val url = "http://www.example.com"

// Formulate the request and handle the response.
val stringRequest = StringRequest(Request.Method.GET, url,
         Response.Listener<String> { response ->
            // Do something with the response
        },
        Response.ErrorListener { error ->
            // Handle error
            textView.text = "ERROR: %s".format(error.toString())
        })

// Add the request to the RequestQueue.
requestQueue.add(stringRequest)

// ...

RequestQueue requestQueue;

// Instantiate the cache
Cache cache = new DiskBasedCache(getCacheDir(), 1024 * 1024); // 1MB cap

// Set up the network to use HttpURLConnection as the HTTP client.
Network network = new BasicNetwork(new HurlStack());

// Instantiate the RequestQueue with the cache and network.
requestQueue = new RequestQueue(cache, network);

// Start the queue
requestQueue.start();

String url ="http://www.example.com";

// Formulate the request and handle the response.
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
        new Response.Listener<String>() {
    @Override
    public void onResponse(String response) {
        // Do something with the response
    }
},
    new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            // Handle error
    }
});

// Add the request to the RequestQueue.
requestQueue.add(stringRequest);

// ...

If you just need to make a one-time request and don't want to leave the thread pool around, you can create the RequestQueue wherever you need it and call stop() on the RequestQueue once your response or error has come back, using the Volley.newRequestQueue() method described in Sending a Simple Request. But the more common use case is to create the RequestQueue as a singleton to keep it running for the lifetime of your app, as described in the next section.

Use a singleton pattern

If your application makes constant use of the network, it's probably most efficient to set up a single instance of RequestQueue that will last the lifetime of your app. You can achieve this in various ways. The recommended approach is to implement a singleton class that encapsulates RequestQueue and other Volley functionality. Another approach is to subclass Application and set up the RequestQueue in Application.onCreate(). But this approach is discouraged; a static singleton can provide the same functionality in a more modular way.

A key concept is that the RequestQueue must be instantiated with the Application context, not an Activity context. This ensures that the RequestQueue will last for the lifetime of your app, instead of being recreated every time the activity is recreated (for example, when the user rotates the device).

Here is an example of a singleton class that provides RequestQueue and ImageLoader functionality:

class MySingleton constructor(context: Context) {
    companion object {
        @Volatile
        private var INSTANCE: MySingleton? = null
        fun getInstance(context: Context) =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: MySingleton(context).also {
                    INSTANCE = it
                }
            }
    }
    val imageLoader: ImageLoader by lazy {
        ImageLoader(requestQueue,
                object : ImageLoader.ImageCache {
                    private val cache = LruCache<String, Bitmap>(20)
                    override fun getBitmap(url: String): Bitmap {
                        return cache.get(url)
                    }
                    override fun putBitmap(url: String, bitmap: Bitmap) {
                        cache.put(url, bitmap)
                    }
                })
    }
    val requestQueue: RequestQueue by lazy {
        // applicationContext is key, it keeps you from leaking the
        // Activity or BroadcastReceiver if someone passes one in.
        Volley.newRequestQueue(context.applicationContext)
    }
    fun <T> addToRequestQueue(req: Request<T>) {
        requestQueue.add(req)
    }
}

public class MySingleton {
    private static MySingleton instance;
    private RequestQueue requestQueue;
    private ImageLoader imageLoader;
    private static Context ctx;

    private MySingleton(Context context) {
        ctx = context;
        requestQueue = getRequestQueue();

        imageLoader = new ImageLoader(requestQueue,
                new ImageLoader.ImageCache() {
            private final LruCache<String, Bitmap>
                    cache = new LruCache<String, Bitmap>(20);

            @Override
            public Bitmap getBitmap(String url) {
                return cache.get(url);
            }

            @Override
            public void putBitmap(String url, Bitmap bitmap) {
                cache.put(url, bitmap);
            }
        });
    }

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

    public RequestQueue getRequestQueue() {
        if (requestQueue == null) {
            // getApplicationContext() is key, it keeps you from leaking the
            // Activity or BroadcastReceiver if someone passes one in.
            requestQueue = Volley.newRequestQueue(ctx.getApplicationContext());
        }
        return requestQueue;
    }

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

    public ImageLoader getImageLoader() {
        return imageLoader;
    }
}

Here are some examples of performing RequestQueue operations using the singleton class:

// Get a RequestQueue
val queue = MySingleton.getInstance(this.applicationContext).requestQueue

// ...

// Add a request (in this example, called stringRequest) to your RequestQueue.
MySingleton.getInstance(this).addToRequestQueue(stringRequest)

// Get a RequestQueue
RequestQueue queue = MySingleton.getInstance(this.getApplicationContext()).
    getRequestQueue();

// ...

// Add a request (in this example, called stringRequest) to your RequestQueue.
MySingleton.getInstance(this).addToRequestQueue(stringRequest);