Android HTTP框架Volley详解

From:http://www.androidhive.info/2014/05/android-working-with-volley-library-1/

Android volley is a networking library was introduced to make networking calls much easier, faster without writing tons of code. By default all the volley network calls works asynchronously, so we don’t have to worry about using asynctask anymore.

Volley comes with lot of features. Some of them are

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

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Before getting into this tutorial, I suggested you to view the below presentation by Ficus Kirkpatrick at Google I/O to get an overview of volley.

1. Creating New Project

1. In Android Studio, create a new project by navigating to File ⇒ New Project and fill all the required details. When it prompts to select a default activity, select Blank Activity and proceed.

2. Create two packages named app and utils to keep the project organized.

3. Open build.gradle and add volley support by adding
compile ‘com.mcxiaoke.volley:library-aar:1.0.0’ under dependencies section.

dependencies {
    compile fileTree(dir: 'libs' , include: [ '*.jar' ])
    compile 'com.android.support:appcompat-v7:22.2.0'
    compile 'com.android.volley:volley:1.0.0'
}

1.1 Creating Volley Singleton Class

The best way to maintain volley core objects and request queue is, making them global by creating a singleton class which extends Application object.

4. Under utils package, create a class named LruBitmapCache.java and paste the below code. This class is required to handle image cache.

package info.androidhive.volleyexamples.volley.utils;

import com.android.volley.toolbox.ImageLoader.ImageCache;

import android.graphics.Bitmap;
import android.support.v4.util.LruCache;

public class LruBitmapCache extends LruCache implements
        ImageCache {
    public static int getDefaultLruCacheSize() {
        final int maxMemory = ( int ) (Runtime.getRuntime().maxMemory() / 1024 );
        final int cacheSize = maxMemory / 8 ;

        return cacheSize;
    }

    public LruBitmapCache() {
        this (getDefaultLruCacheSize());
    }

    public LruBitmapCache( int sizeInKiloBytes) {
        super (sizeInKiloBytes);
    }

    @Override
    protected int sizeOf(String key, Bitmap value) {
        return value.getRowBytes() * value.getHeight() / 1024 ;
    }

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

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

5. Under app package, create a class name AppController.java and extend the class from Application and add the following code.

package info.androidhive.volleyexamples.app;

import info.androidhive.volleyexamples.volley.utils.LruBitmapCache;
import android.app.Application;
import android.text.TextUtils;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.ImageLoader;
import com.android.volley.toolbox.Volley;

public class AppController extends Application {

    public static final String TAG = AppController. class
            .getSimpleName();

    private RequestQueue mRequestQueue;
    private ImageLoader mImageLoader;

    private static AppController mInstance;

    @Override
    public void onCreate() {
        super .onCreate();
        mInstance = this ;
    }

    public static synchronized AppController getInstance() {
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null ) {
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    public ImageLoader getImageLoader() {
        getRequestQueue();
        if (mImageLoader == null ) {
            mImageLoader = new ImageLoader( this .mRequestQueue,
                    new LruBitmapCache());
        }
        return this .mImageLoader;
    }

    public void addToRequestQueue(Request req, String tag) {
        // set the default tag if tag is empty
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

    public void addToRequestQueue(Request req) {
        req.setTag(TAG);
        getRequestQueue().add(req);
    }

    public void cancelPendingRequests(Object tag) {
        if (mRequestQueue != null ) {
            mRequestQueue.cancelAll(tag);
        }
    }
}

6. Now open AndroidManifest.xml and add this singleton class in tag using android:name property to execute the class automatically whenever app launches. Also add INTERNET permission as we are going to make network calls.

xml version = "1.0" encoding = "utf-8" ?>
< manifest xmlns:android = "http://schemas.android.com/apk/res/android"
    package = "info.androidhive.volleyexamples"
    android:versionCode = "1"
    android:versionName = "1.0" >

    < uses-sdk
        android:minSdkVersion = "8"
        android:targetSdkVersion = "19" />

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

    < application
        android:name = "info.androidhive.volleyexamples.app.AppController"
        android:allowBackup = "true"
        android:icon = "@drawable/ic_launcher"
        android:label = "@string/app_name"
        android:theme = "@style/AppTheme" >

    application >

manifest >

Now the primary setup required for volley is done. Let’s move on to individual options those volley provides to make the http requests.

2. Making JSON request

Volley provides an easy to make json requests. If you are expecting json object in the response, you should use JsonObjectRequest class or if the response is json array, JsonArrayRequest class should be used.

2.1 Making json object request
Following code will make a json object request where the json response will start with object notation ‘{‘

// Tag used to cancel the request
String tag_json_obj = "json_obj_req" ;

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

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

        JsonObjectRequest jsonObjReq = new JsonObjectRequest(Method.GET,
                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());
                        // hide the progress dialog
                        pDialog.hide();
                    }
                });

// Adding request to request queue
AppController.getInstance().addToRequestQueue(jsonObjReq, tag_json_obj);

2.2 Making json array request
Following will make json array request where the json response starts with array notation ‘[‘

// Tag used to cancel the request
String tag_json_arry = "json_array_req" ;

String url = "http://api.androidhive.info/volley/person_array.json" ;

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

JsonArrayRequest req = new JsonArrayRequest(url,
                new Response.Listener() {
                    @Override
                    public void onResponse(JSONArray 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();
                    }
                });

// Adding request to request queue
AppController.getInstance().addToRequestQueue(req, tag_json_arry);

3. Making String request

StringRequest class will be used to fetch any kind of string data. The response can be json, xml, html or plain text.

// Tag used to cancel the request
String  tag_string_req = "string_req" ;

String url = "http://api.androidhive.info/volley/string_response.html" ;

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

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

                    @Override
                    public void onResponse(String 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();
                    }
                });

// Adding request to request queue
AppController.getInstance().addToRequestQueue(strReq, tag_string_req);

4. Adding post parameters

It is obvious that sometimes we need to submit request parameters while hitting the url. To do that we have to override getParams() method which should return list of parameters to be send in a key value format.

If you observe below example, I am submitting name, email and password as request parameters.

// Tag used to cancel the request
String tag_json_obj = "json_obj_req" ;

String url = "http://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
            protected Map getParams() {
                Map params = new HashMap();
                params.put( "name" , "Androidhive" );
                params.put( "email" , "[email protected]" );
                params.put( "password" , "password123" );

                return params;
            }

        };

// Adding request to request queue
AppController.getInstance().addToRequestQueue(jsonObjReq, tag_json_obj);

5. Adding request headers

Just like adding request parameters, to send request headers, we have to override getHeaders(). In below example I am sending Content-Type and apiKey in request headers.

// Tag used to cancel the request
String tag_json_obj = "json_obj_req" ;

String url = "http://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();
                    }
                }) {

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

        };

// Adding request to request queue
AppController.getInstance().addToRequestQueue(jsonObjReq, tag_json_obj);

6. Making Image request

Volley introduced custom image view element called NetworkImageView to display the images from an URL. Previously downloading images and maintaining caches is a tough job. Now using volley this can be done with very few lines of code.

6.1 Loading image in NetworkImageView
Following will load an image from an URL into NetworkImageView.

ImageLoader imageLoader = AppController.getInstance().getImageLoader();

// If you are using NetworkImageView
imgNetWorkView.setImageUrl(Const.URL_IMAGE, imageLoader);

6.2 Loading image in ImageView
If you want to load image into ImageView instead of NetworkImageView, you can do that too as mentioned below. Here we will have success and error callbacks, you have to take appropriate action depending on the need. Below in onResponse() method using response.getBitmap() I am loading bitmap into an ImageView.

ImageLoader imageLoader = AppController.getInstance().getImageLoader();

// If you are using normal ImageView
imageLoader.get(Const.URL_IMAGE, new ImageListener() {

    @Override
    public void onErrorResponse(VolleyError error) {
        Log.e(TAG, "Image Load Error: " + error.getMessage());
    }

    @Override
    public void onResponse(ImageContainer response, boolean arg1) {
        if (response.getBitmap() != null ) {
            // load image into imageview
            imageView.setImageBitmap(response.getBitmap());
        }
    }
});

6.3 Defining placeholder image and error image
Here is another way of displaying image into ImageView with the option of placeholder for loader and error. The loader placeholder will be displayed until the image gets downloaded. If the image fails to download, the error placeholder will be displayed.

// Loading image with placeholder and error image
imageLoader.get(Const.URL_IMAGE, ImageLoader.getImageListener(
                imageView, R.drawable.ico_loading, R.drawable.ico_error));

7. Handling the volley Cache

Volley comes with powerful cache mechanism to maintain request cache. This saves lot of internet bandwidth and reduces user waiting time. Following are few example of using volley cache methods.

7.1 Loading request from cache
Like below you can check for a cached response of an URL before making a network call.

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 {
    // Cached response doesn't exists. Make network call here
}

7.2 Invalidate cache
Invalidate means we are invalidating the cached data instead of deleting it. Volley will still uses the cached object until the new data received from server. Once it receives the response from the server it will override the older cached response.

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

7.3 Turning off cache
If you want disable the cache for a particular url, you can use setShouldCache() method as below.

// String request
StringRequest stringReq = new StringRequest(....);

// disable cache
stringReq.setShouldCache( false );

7.4 Deleting cache for particular URL
Use remove() to delete cache of an URL.

AppController.getInstance().getRequestQueue().getCache().remove(url);

7.5 Deleting all the cache
Followoing will delete the cache for all the URLs.

AppController.getInstance().getRequestQueue().getCache().clear(url);

8. Cancelling requests

If you notice addToRequestQueue(request, tag) method, it accepts two parameters. One is request object and other is request tag. This tag will be used to identify the request while cancelling it. If the tag is same for multiple requests, all the requests will be cancelled. cancellAll() method is used to cancel any request.

8.1 Cancel single request
Following will cancel all the request with the tag named “feed_request”

String tag_json_arry = "json_req" ;
ApplicationController.getInstance().getRequestQueue().cancelAll( "feed_request" );

8.2 Cancel all requests
If you don’t pass any tag to cancelAll() method, it will cancel the request in request queue.

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

9. Request prioritization

If you are making multiple request at the same time, you can prioritize the requests those you want be executed first. The priory can be Normal, Low, Immediate and High.

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;
            }

        };

Missing! Making XML request

As of now volley doesn’t provided any native classes to make XML requests, but this can be achieved by building a custom xml wrapper class by utilizing volley’s customization capabilities. The part of writing xml parser using volley will be covered in upcoming tutorial.

I have given a sample project covering the scenarios explained in this tutorial. Download it and let’s discuss the queries if you have any in the comments section