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page.title=Making a Standard Request
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<h2>This lesson teaches you to</h2>
<li><a href="#request-image">Request an Image</a></li>
<li><a href="#request-json">Request JSON</a></li>
<a class="notice-developers-video wide" href="">
<p>Volley: Easy, Fast Networking for Android</p>
This lesson describes how to use the common request types that Volley supports:</p>
<li>{@code StringRequest}. Specify a URL and receive a raw string in response. See
<a href="requestqueue.html">Setting Up a Request Queue</a> for an example.</li>
<li>{@code ImageRequest}. Specify a URL and receive an image in response.</li>
<li>{@code JsonObjectRequest} and {@code JsonArrayRequest} (both subclasses of
{@code JsonRequest}). Specify a URL and get a JSON object or array (respectively) in
<p>If your expected response is one of these types, you probably won't have to implement a
custom request. This lesson describes how to use these standard request types. For
information on how to implement your own custom request, see <a href="requests-custom.html">
Implementing a Custom Request</a>.</p>
<h2 id="request-image">Request an Image</h2>
<p>Volley offers the following classes for requesting images. These classes layer on top
of each other to offer different levels of support for processing images:</p>
<li>{@code ImageRequest}&mdash;a canned request for getting an image at a given URL and
calling back with a decoded bitmap. It also provides convenience features like specifying
a size to resize to. Its main benefit is that Volley's thread scheduling ensures that
expensive image operations (decoding, resizing) automatically happen on a worker thread.</li>
<li>{@code ImageLoader}&mdash;a helper class that handles loading and caching images from
remote URLs. {@code ImageLoader} is a an orchestrator for large numbers of {@code ImageRequest}s,
for example when putting multiple thumbnails in a {@link android.widget.ListView}.
{@code ImageLoader} provides an in-memory cache to sit in front of the normal Volley
cache, which is important to prevent flickering. This makes it possible to achieve a
cache hit without blocking or deferring off the main thread, which is impossible when
using disk I/O. {@code ImageLoader} also does response coalescing, without which almost
every response handler would set a bitmap on a view and cause a layout pass per image.
Coalescing makes it possible to deliver multiple responses simultaneously, which improves
<li>{@code NetworkImageView}&mdash;builds on {@code ImageLoader} and effectively replaces
{@link android.widget.ImageView} for situations where your image is being fetched over
the network via URL. {@code NetworkImageView} also manages canceling pending requests if
the view is detached from the hierarchy.</li>
<h3>Use ImageRequest</h3>
<p>Here is an example of using {@code ImageRequest}. It retrieves the image specified by
the URL and displays it in the app. Note that this snippet interacts with the
{@code RequestQueue} through a singleton class (see <a href="{@docRoot}
training/volley/requestqueue.html#singleton">Setting Up a RequestQueue</a> for more discussion of
this topic):</p>
ImageView mImageView;
String url = "";
mImageView = (ImageView) findViewById(;
// Retrieves an image specified by the URL, displays it in the UI.
ImageRequest request = new ImageRequest(url,
new Response.Listener&lt;Bitmap&gt;() {
public void onResponse(Bitmap bitmap) {
}, 0, 0, null,
new Response.ErrorListener() {
public void onErrorResponse(VolleyError error) {
// Access the RequestQueue through your singleton class.
<h3>Use ImageLoader and NetworkImageView</h3>
<p>You can use {@code ImageLoader} and {@code NetworkImageView} in concert to efficiently
manage the display of multiple images, such as in a {@link android.widget.ListView}. In your
layout XML file, you use {@code NetworkImageView} in much the same way you would use
{@link android.widget.ImageView}, for example:</p>
android:layout_centerHorizontal=&quot;true&quot; /&gt;</pre>
<p>You can use {@code ImageLoader} by itself to display an image, for example:</p>
ImageLoader mImageLoader;
ImageView mImageView;
// The URL for the image that is being loaded.
private static final String IMAGE_URL =
mImageView = (ImageView) findViewById(;
// Get the ImageLoader through your singleton class.
mImageLoader = MySingleton.getInstance(this).getImageLoader();
mImageLoader.get(IMAGE_URL, ImageLoader.getImageListener(mImageView,
R.drawable.def_image, R.drawable.err_image));
<p>However, {@code NetworkImageView} can do this for you if all you're doing is populating
an {@link android.widget.ImageView}. For example:</p>
ImageLoader mImageLoader;
NetworkImageView mNetworkImageView;
private static final String IMAGE_URL =
// Get the NetworkImageView that will display the image.
mNetworkImageView = (NetworkImageView) findViewById(;
// Get the ImageLoader through your singleton class.
mImageLoader = MySingleton.getInstance(this).getImageLoader();
// Set the URL of the image that should be loaded into this view, and
// specify the ImageLoader that will be used to make the request.
mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);
<p>The above snippets access the {@code RequestQueue} and the {@code ImageLoader}
through a singleton class, as described in <a href="{@docRoot}training/volley/requestqueue.html#singleton">
Setting Up a RequestQueue</a>. This approach ensures that your app creates single instances of
these classes that last the lifetime of your app. The reason that this is important for
{@code ImageLoader} (the helper class that handles loading and caching images) is that
the main function of the in-memory cache is to allow for flickerless rotation. Using a
singleton pattern allows the bitmap cache to outlive the activity. If instead you create the
{@code ImageLoader} in an activity, the {@code ImageLoader} would be recreated along with
the activity every time the user rotates the device. This would cause flickering.</p>
<h4 id="lru-cache">Example LRU cache</h4>
<p>The Volley toolbox provides a standard cache implementation via the
{@code DiskBasedCache} class. This class caches files directly onto the hard disk in the
specified directory. But to use {@code ImageLoader}, you should provide a custom
in-memory LRU bitmap cache that implements the {@code ImageLoader.ImageCache} interface.
You may want to set up your cache as a singleton; for more discussion of this topic, see
<a href="{@docRoot}training/volley/requestqueue.html#singleton">
Setting Up a RequestQueue</a>.</p>
<p>Here is a sample implementation for an in-memory {@code LruBitmapCache} class.
It extends the {@link} class and implements the
{@code ImageLoader.ImageCache} interface:</p>
import android.util.DisplayMetrics;
public class LruBitmapCache extends LruCache&lt;String, Bitmap&gt;
implements ImageCache {
public LruBitmapCache(int maxSize) {
public LruBitmapCache(Context ctx) {
protected int sizeOf(String key, Bitmap value) {
return value.getRowBytes() * value.getHeight();
public Bitmap getBitmap(String url) {
return get(url);
public void putBitmap(String url, Bitmap bitmap) {
put(url, bitmap);
// Returns a cache size equal to approximately three screens worth of images.
public static int getCacheSize(Context ctx) {
final DisplayMetrics displayMetrics = ctx.getResources().
final int screenWidth = displayMetrics.widthPixels;
final int screenHeight = displayMetrics.heightPixels;
// 4 bytes per pixel
final int screenBytes = screenWidth * screenHeight * 4;
return screenBytes * 3;
<p>Here is an example of how to instantiate an {@code ImageLoader} to use this
RequestQueue mRequestQueue; // assume this exists.
ImageLoader mImageLoader = new ImageLoader(mRequestQueue, new LruBitmapCache(
<h2 id="request-json">Request JSON</h2>
<p>Volley provides the following classes for JSON requests:</p>
<li>{@code JsonArrayRequest}&mdash;A request for retrieving a
{@link org.json.JSONArray}
response body at a given URL.</li>
<li>{@code JsonObjectRequest}&mdash;A request for retrieving a
{@link org.json.JSONObject}
response body at a given URL, allowing for an optional
{@link org.json.JSONObject}
to be passed in as part of the request body.</li>
<p>Both classes are based on the common base class {@code JsonRequest}. You use them
following the same basic pattern you use for other types of requests. For example, this
snippet fetches a JSON feed and displays it as text in the UI:</p>
TextView mTxtDisplay;
ImageView mImageView;
mTxtDisplay = (TextView) findViewById(;
String url = "http://my-json-feed";
JsonObjectRequest jsObjRequest = new JsonObjectRequest
(Request.Method.GET, url, null, new Response.Listener&lt;JSONObject&gt;() {
public void onResponse(JSONObject response) {
mTxtDisplay.setText("Response: " + response.toString());
}, new Response.ErrorListener() {
public void onErrorResponse(VolleyError error) {
// TODO Auto-generated method stub
// Access the RequestQueue through your singleton class.
For an example of implementing a custom JSON request based on
<a href="">Gson</a>, see the next lesson,
<a href="request-custom.html">Implementing a Custom Request</a>.