Volley作为当年Google在2013年的Google I/O上的重点，是一个相当给力的框架。它从设计模式上来说，非常具有扩展性，也比较轻巧。关于Volley的使用，网上介绍的很多了，不再赘述。现在，我将记录我阅读Volley源码的过程，来学习Volley的设计思想和其中的一些小技巧。
值的一提的是，新版的gradle已经支持：
compile 'com.android.volley:volley:1.0.0'
这样导入Volley了。
从最简单的例子看起：
RequestQueue queue; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); queue = Volley.newRequestQueue(this); final StringRequest request = new StringRequest("https://www.baidu.com/", new Response.Listener<String>() { @Override public void onResponse(String response) { ((TextView) findViewById(R.id.test_textview_id)).setText(response); } }, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError error) { } }); findViewById(R.id.test_btn_id).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { queue.add(request); } }); }
可以看到，一个简单的Volley的http请求，只需要三步，建立Request队列--->建立Request请求--->将请求加入Request队列。
今天就只看这三步是怎么运作的。
我们先来看看queue = Volley.newRequestQueue(this);
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; }
从体来说这个方法，做了以下几件事：获取userAgent、创建HttpStack、创建NetWork、创建并启动RequestQueue。。
获取userAgetnt的方法比较简单 userAgent = packageName + "/" + info.versionCode; 一目了然。
接着创建HttpStack和NetWork比较复杂，会单独记录。
最后会创建一个RequestQueue,作为请求队列。并且在初始化时，请求队列会直接启动。
接下来，我们看一下RequestQueue的代码。我们先从构造方法开始阅读：
/** * Creates the worker pool. Processing will not begin until {@link #start()} is called. * * @param cache A Cache to use for persisting responses to disk * @param network A Network interface for performing HTTP requests * @param threadPoolSize Number of network dispatcher threads to create * @param delivery A ResponseDelivery interface for posting responses and errors */ public RequestQueue(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) { mCache = cache; mNetwork = network; mDispatchers = new NetworkDispatcher[threadPoolSize]; mDelivery = delivery; }
在初始化时，RequestQueue对4个成员赋了值。缓存、NetWork、网络分发线程队列和网络响应分发接口。
下面我们看看在初始化Volley初始化时，第一个会调用的方法start()
/** * Starts the dispatchers in this queue. */ 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(); } }
上面的代码比较简洁。首先调用stop();关于stop()：
public void stop() { if (mCacheDispatcher != null) { mCacheDispatcher.quit(); } for (int i = 0; i < mDispatchers.length; i++) { if (mDispatchers[i] != null) { mDispatchers[i].quit(); } } }
它就是通过遍历，结束了线程中的所有任务。
我们可以在NetworkDispatcher.java中找到相关实现
public void quit() { mQuit = true; interrupt(); } public 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; } ...... } }
继续看start方法：
/** * Starts the dispatchers in this queue. */ 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(); } }
stop之后，就会启动CacheDispatcher和NetworkDispatcher。
关于NetworkDispatcher中start()方法的实现，NetworkDispatcher继承了Thread，所以看start()其实是看run方法：
@Override public 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); } } }
代码比较长，我们分段阅读。
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; }
设置线程优先级为标准后台线程，然后进入死循环。虽然这一段代码没有什么实质的实现，但是却很重要，它基本确立了这个线程的工作模式。这个线程在while(true)的死循环中，每次循环有一次停止线程和一次暂停线程的机会，暂停在前。
mQueue是个阻塞型队列。每次循环会在这个队列里取一个请求，如果mQueue为空，取不到请求了，线程就会卡死在这里，这就是暂停的机会，等到mQueue加入请求时，线程又会重新跑起来。通过这样的设计，我们后面发起http请求，只需要往mQueue里加入请求就好了。当调用interrupt方法时，循环就会进入catch分支，然后判断是否真的要取消，所以我们在quit()方法中，看到它的实现是先将mQuit置为true，然后调用interrupt来结束这个线程。
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);
如果请求被取消，则调用请求的finish方法，进入下一轮循环。 然后加入流量标记。我们可以看到addTrafficStatsTag
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH) private void addTrafficStatsTag(Request<?> request) { // Tag the request (if API >= 14) if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) { TrafficStats.setThreadStatsTag(request.getTrafficStatsTag()); } }
我们可以对这个网络请求加上标记，方便调式时知晓它的流量使用情况。
// 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; }
接着便了Network执行它的performRequest方法，方法的具体实现，将在后面阅读。通过这个方法，我们处理了网络请求，取得了响应——NetworkResponse。下面的一个判断语句，注释解释得很清楚。如果我们的请求返回304，并且我们已经分发过这个响应了。我们将不再处理响应，结束这次循环，开始下一轮。同时执行request.finish的方法。如果没有304，那么我们还要考虑如何处理响应。
// 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"); }
这里，进行解析网络响应，获得响应——Response。根据配置，如果需要缓存，则将响应的内容缓存在mCache中。
// Post the response back. request.markDelivered(); mDelivery.postResponse(request, response);
最后，根据请求，分析网络响应。
至此，我们将volley的网络请求及处理流程基本走通了一遍。后面的文章中，将仔细分析一些Volley对于网络请求的一些处理细节。
Done
