android volley封装及源码解析

1.简单使用volley

Volley.newRequestQueue(this).add(new StringRequest(Request.Method.GET, "http://api.wumeijie.net/list", new Response.Listener<String>() {    @Override    public void onResponse(String response) {        //TODO 处理响应数据    }}, new Response.ErrorListener() {    @Override    public void onErrorResponse(VolleyError error) {        //TODO 处理请求失败情况    }}));

2.封装VolleyManager

完整代码：https://github.com/snailycy/volley_manager

注意，volley里面的请求队列建议使用单例，因为每次实例化ReqeustQueue并start()时，会创建1个缓存线程和4个网络请求线程，多次调用start()会创建多余的被interrupt的线程，造成资源浪费

import com.android.volley.AuthFailureError;import com.android.volley.DefaultRetryPolicy;import com.android.volley.Request;import com.android.volley.RequestQueue;import com.android.volley.Response;import com.android.volley.toolbox.JsonObjectRequest;import com.android.volley.toolbox.StringRequest;import com.android.volley.toolbox.Volley;import org.json.JSONObject;import java.util.HashMap;import java.util.Map;public class VolleyManager {    private static RequestQueue requestQueue;    //超时时间 30s    private final static int TIME_OUT = 30000;    private static RequestQueue getRequestQueue() {        if (requestQueue == null) {            synchronized (VolleyManager.class) {                if (requestQueue == null) {                    //使用全局context对象                    requestQueue = Volley.newRequestQueue(MyApplication.getContext());                }            }        }        return requestQueue;    }    private static <T> void addRequest(RequestQueue requestQueue, Request<T> request) {        request.setShouldCache(true);        request.setRetryPolicy(new DefaultRetryPolicy(TIME_OUT,                DefaultRetryPolicy.DEFAULT_MAX_RETRIES,                DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));        requestQueue.add(request);    }    public static void sendJsonObjectRequest(int method, String url,                                             JSONObject params,                                             final Response.Listener<JSONObject> listener,                                             final Response.ErrorListener errorListener) {        try {            JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(method,                    url, params, listener, errorListener) {                @Override                public Map<String, String> getHeaders() throws AuthFailureError {                    Map<String, String> headers = new HashMap<>();                    headers.put("accept-encoding", "utf-8");                    return headers;                }            };            addRequest(getRequestQueue(), jsonObjectRequest);        } catch (Exception e) {            e.printStackTrace();        }    }    public static void sendStringRequest(int method, String url,                                         final Map<String, String> params,                                         final Response.Listener<String> listener,                                         final Response.ErrorListener errorListener) {        try {            StringRequest stringRequest = new StringRequest(method, url, listener, errorListener) {                @Override                protected Map<String, String> getParams() throws AuthFailureError {                    return params;                }            };            addRequest(getRequestQueue(), stringRequest);        } catch (Exception e) {            e.printStackTrace();        }    }}

封装完了，使用起来也非常简单

//使用StringRequestHashMap<String, String> params = new HashMap<>();params.put("params1", "xixi");VolleyManager.sendStringRequest(Request.Method.GET,        "http://api.wumeijie.net/list", params,        new Response.Listener<String>() {            @Override            public void onResponse(String response) {                //TODO 处理响应数据            }        }, new Response.ErrorListener() {            @Override            public void onErrorResponse(VolleyError error) {                //TODO 处理请求失败情况            }        });

或者

//使用JsonObjectRequestJSONObject params = new JSONObject();try {    params.put("params1", "xixi");} catch (JSONException e) {    e.printStackTrace();}VolleyManager.sendJsonObjectRequest(Request.Method.GET,        "http://api.wumeijie.net/list", params,        new Response.Listener<JSONObject>() {            @Override            public void onResponse(JSONObject response) {                //TODO 处理响应数据            }        }, new Response.ErrorListener() {            @Override            public void onErrorResponse(VolleyError error) {                //TODO 处理请求失败情况            }        });

3.源码分析

3.1 Volley在创建RequestQueue时，会先创建一个HttpStack对象，该对象在系统版本>=9时，是由实现类HurlStack创建，具体是用HttpURLConnection来实现网络请求，在系统版本<9时，是由实现类HttpClientStack创建，使用的是HttpClient来实现网络请求
源码如下：（Volley类中newRequestQueue方法）

    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);        String userAgent = "volley/0";        try {            String packageName = context.getPackageName();            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);            userAgent = packageName + "/" + info.versionCode;        } catch (NameNotFoundException e) {        }        if (stack == null) {            if (Build.VERSION.SDK_INT >= 9) {                stack = new HurlStack();            } else {                // Prior to Gingerbread, HttpUrlConnection was unreliable.                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));            }        }        Network network = new BasicNetwork(stack);        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);        queue.start();        return queue;    }

3.2 创建RequestQueue，主要是创建了1个缓存线程和4个网络请求线程，这5个线程共用1个请求队列，共用1个缓存对象，共用1个ResponseDelivery
源码如下：（请求队列RequestQueue类中start方法）

public void start() {    stop();  // Make sure any currently running dispatchers are stopped.    // Create the cache dispatcher and start it.    mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);    mCacheDispatcher.start();    // Create network dispatchers (and corresponding threads) up to the pool size.    for (int i = 0; i < mDispatchers.length; i++) {        NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,                mCache, mDelivery);        mDispatchers[i] = networkDispatcher;        networkDispatcher.start();    }}

3.3 缓存线程run方法中，主要是一个死循环，不断的从缓存队列中取出请求对象，如果该请求对象缓存丢失或者不需要缓存或者需要刷新缓存数据，则加入到请求队列中，否则，直接解析缓存后通过ResponseDelivery对象中的handler post到主线程中执行响应回调接口
源码如下：（缓存线程CacheDispatcher类中run方法）

@Overridepublic void run() {    if (DEBUG) VolleyLog.v("start new dispatcher");    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);    // Make a blocking call to initialize the cache.    mCache.initialize();    while (true) {        try {            // Get a request from the cache triage queue, blocking until            // at least one is available.            final Request<?> request = mCacheQueue.take();            request.addMarker("cache-queue-take");            // If the request has been canceled, don't bother dispatching it.            if (request.isCanceled()) {                request.finish("cache-discard-canceled");                continue;            }            // Attempt to retrieve this item from cache.            Cache.Entry entry = mCache.get(request.getCacheKey());            if (entry == null) {                request.addMarker("cache-miss");                // Cache miss; send off to the network dispatcher.                mNetworkQueue.put(request);                continue;            }            // If it is completely expired, just send it to the network.            if (entry.isExpired()) {                request.addMarker("cache-hit-expired");                request.setCacheEntry(entry);                mNetworkQueue.put(request);                continue;            }            // We have a cache hit; parse its data for delivery back to the request.            request.addMarker("cache-hit");            Response<?> response = request.parseNetworkResponse(                    new NetworkResponse(entry.data, entry.responseHeaders));            request.addMarker("cache-hit-parsed");            if (!entry.refreshNeeded()) {                // Completely unexpired cache hit. Just deliver the response.                mDelivery.postResponse(request, response);            } else {                // Soft-expired cache hit. We can deliver the cached response,                // but we need to also send the request to the network for                // refreshing.                request.addMarker("cache-hit-refresh-needed");                request.setCacheEntry(entry);                // Mark the response as intermediate.                response.intermediate = true;                // Post the intermediate response back to the user and have                // the delivery then forward the request along to the network.                mDelivery.postResponse(request, response, new Runnable() {                    @Override                    public void run() {                        try {                            mNetworkQueue.put(request);                        } catch (InterruptedException e) {                            // Not much we can do about this.                        }                    }                });            }        } catch (InterruptedException e) {            // We may have been interrupted because it was time to quit.            if (mQuit) {                return;            }            continue;        }    }}

3.4 网络线程run方法中，主要是一个死循环，不断的从请求队列中取出请求对象，然后根据系统版本使用HttpStack对象来进行网络请求（包括设置请求参数，请求头，请求方法等信息），最终返回一个HttpResponse对象，接着就是解析响应数据，处理缓存情况，最后通过ResponseDelivery对象中的handler post到主线程中执行响应回调接口
源码如下：（网络请求线程NetWorkDispatcher类中run方法）

@Overridepublic void run() {    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);    while (true) {        long startTimeMs = SystemClock.elapsedRealtime();        Request<?> request;        try {            // Take a request from the queue.            request = mQueue.take();        } catch (InterruptedException e) {            // We may have been interrupted because it was time to quit.            if (mQuit) {                return;            }            continue;        }        try {            request.addMarker("network-queue-take");            // If the request was cancelled already, do not perform the            // network request.            if (request.isCanceled()) {                request.finish("network-discard-cancelled");                continue;            }            addTrafficStatsTag(request);            // Perform the network request.            NetworkResponse networkResponse = mNetwork.performRequest(request);            request.addMarker("network-http-complete");            // If the server returned 304 AND we delivered a response already,            // we're done -- don't deliver a second identical response.            if (networkResponse.notModified && request.hasHadResponseDelivered()) {                request.finish("not-modified");                continue;            }            // Parse the response here on the worker thread.            Response<?> response = request.parseNetworkResponse(networkResponse);            request.addMarker("network-parse-complete");            // Write to cache if applicable.            // TODO: Only update cache metadata instead of entire record for 304s.            if (request.shouldCache() && response.cacheEntry != null) {                mCache.put(request.getCacheKey(), response.cacheEntry);                request.addMarker("network-cache-written");            }            // Post the response back.            request.markDelivered();            mDelivery.postResponse(request, response);        } catch (VolleyError volleyError) {            volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);            parseAndDeliverNetworkError(request, volleyError);        } catch (Exception e) {            VolleyLog.e(e, "Unhandled exception %s", e.toString());            VolleyError volleyError = new VolleyError(e);            volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);            mDelivery.postError(request, volleyError);        }    }}

3.5 当步骤3.4中执行

NetworkResponse networkResponse = mNetwork.performRequest(request);

时，会将请求分发给不同版本的网络请求实现类，这里以HurlStack为例，请求最终分发到HurlStack中的performRequest方法执行
源码如下：（HurlStack类中performRequest方法）

@Overridepublic HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)        throws IOException, AuthFailureError {    String url = request.getUrl();    HashMap<String, String> map = new HashMap<String, String>();    map.putAll(request.getHeaders());    map.putAll(additionalHeaders);    if (mUrlRewriter != null) {        String rewritten = mUrlRewriter.rewriteUrl(url);        if (rewritten == null) {            throw new IOException("URL blocked by rewriter: " + url);        }        url = rewritten;    }    URL parsedUrl = new URL(url);    HttpURLConnection connection = openConnection(parsedUrl, request);    for (String headerName : map.keySet()) {        connection.addRequestProperty(headerName, map.get(headerName));    }    setConnectionParametersForRequest(connection, request);    // Initialize HttpResponse with data from the HttpURLConnection.    ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);    int responseCode = connection.getResponseCode();    if (responseCode == -1) {        // -1 is returned by getResponseCode() if the response code could not be retrieved.        // Signal to the caller that something was wrong with the connection.        throw new IOException("Could not retrieve response code from HttpUrlConnection.");    }    StatusLine responseStatus = new BasicStatusLine(protocolVersion,            connection.getResponseCode(), connection.getResponseMessage());    BasicHttpResponse response = new BasicHttpResponse(responseStatus);    response.setEntity(entityFromConnection(connection));    for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {        if (header.getKey() != null) {            Header h = new BasicHeader(header.getKey(), header.getValue().get(0));            response.addHeader(h);        }    }    return response;}

3.6 执行完步骤3.5后，拿到一个响应对象HttpResponse，接着步骤3.4解析响应对象：

Response<?> response = request.parseNetworkResponse(networkResponse);

，执行响应回调：

mDelivery.postResponse(request, response);

这个mDelivery（ExecutorDelivery类型）就是在创建RequestQueue时创建的ResponseDelivery对象，主要负责回调响应接口
注：ExecutorDelivery implements ResponseDelivery
源码如下：（ExecutorDelivery类中postResponse方法）

@Overridepublic void postResponse(Request<?> request, Response<?> response, Runnable runnable) {    request.markDelivered();    request.addMarker("post-response");    mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));}

ExecutorDelivery属性mResponsePoster具体实现有两种方式，这里使用的是handler实现
源码如下：（ExecutorDelivery类的构造方法）

public ExecutorDelivery(final Handler handler) {    // Make an Executor that just wraps the handler.    mResponsePoster = new Executor() {        @Override        public void execute(Runnable command) {            handler.post(command);        }    };}

注：这个构造方法传进来的Handler对象拿的是主线程中的Looper对象

volley源码：https://android.googlesource.com/platform/frameworks/volley