Use-un-patr-n-singleton---Request---Android

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;
    }
}

Uso

// 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);

Source: https://developer.android.com/training/volley/requestqueue#java