Transmitting Network Data Using Volley - Setting Up a 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 choice of

AndroidHttpClient

or

HttpURLConnection

.

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

AndroidHttpClient

or

HttpURLConnection

:

Use

AndroidHttpClient

for
apps targeting Android API levels lower than API Level 9 (Gingerbread). Prior to Gingerbread,

HttpURLConnection

was
unreliable. For more discussion of this topic, see Android's HTTP Clients.
Use

HttpURLConnection

for
apps targeting Android API Level 9 (Gingerbread) and higher.

To create an app that runs on all versions of Android, you can check the version of Android the device is running and choose the appropriate HTTP client, for example:

HttpStack stack;
...
// If the device is running a version >= Gingerbread...
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
// ...use HttpURLConnection for stack.
} else {
// ...use AndroidHttpClient for stack.
}
Network network = new BasicNetwork(stack);

This snippet shows you the steps involved in setting up a

RequestQueue

:

RequestQueue mRequestQueue;

// 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.
mRequestQueue = new RequestQueue(cache, network);

// Start the queue
mRequestQueue.start();

String url ="http://www.myurl.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.
mRequestQueue.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:

private static MySingleton mInstance;
private RequestQueue mRequestQueue;
private ImageLoader mImageLoader;
private static Context mCtx;

private MySingleton(Context context) {
mCtx = context;
mRequestQueue = getRequestQueue();

mImageLoader = new ImageLoader(mRequestQueue,
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 (mInstance == null) {
mInstance = new MySingleton(context);
}
return mInstance;
}

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

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

public ImageLoader getImageLoader() {
return mImageLoader;
}
}

Here are some examples of performing

RequestQueue

operations using the singleton class:

// 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);

NEXT: MAKING A STANDARD REQUEST