經(jīng)過前三篇文章的學(xué)習(xí)，Volley的用法我們已經(jīng)掌握的差不多了，但是對(duì)于Volley的工作原理，恐怕有很多朋友還不是很清楚。因此，本篇文章中我們就來一起閱讀一下Volley的源碼，將它的工作流程整體地梳理一遍。同時(shí)，這也是Volley系列的最后一篇文章了。

其實(shí)，Volley的官方文檔中本身就附有了一張Volley的工作流程圖，如下圖所示。

多數(shù)朋友突然看到一張這樣的圖，應(yīng)該會(huì)和我一樣，感覺一頭霧水吧？沒錯(cuò)，目前我們對(duì)Volley背后的工作原理還沒有一個(gè)概念性的理解，直接就來看這張圖自然會(huì)有些吃力。不過沒關(guān)系，下面我們就去分析一下Volley的源碼，之后再重新來看這張圖就會(huì)好理解多了。

說起分析源碼，那么應(yīng)該從哪兒開始看起呢？這就要回顧一下Volley的用法了，還記得嗎，使用Volley的第一步，首先要調(diào)用Volley.newRequestQueue(context)方法來獲取一個(gè)RequestQueue對(duì)象，那么我們自然要從這個(gè)方法開始看起了，代碼如下所示：

public static RequestQueue newRequestQueue(Context context) {return newRequestQueue(context, null); }
這個(gè)方法僅僅只有一行代碼，只是調(diào)用了 newRequestQueue()的方法重載，并給第二個(gè)參數(shù)傳入null。那我們看下帶有兩個(gè)參數(shù)的newRequestQueue()方法中的代碼，如下所示：

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 {stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));}}Network network = new BasicNetwork(stack);RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);queue.start();return queue; }
可以看到，這里在第10行判斷如果stack是等于null的，則去創(chuàng)建一個(gè)HttpStack對(duì)象，這里會(huì)判斷如果手機(jī)系統(tǒng)版本號(hào)是大于9的，則創(chuàng)建一個(gè)HurlStack的實(shí)例，否則就創(chuàng)建一個(gè)HttpClientStack的實(shí)例。實(shí)際上 HurlStack的內(nèi)部就是使用HttpURLConnection進(jìn)行網(wǎng)絡(luò)通訊的，而HttpClientStack的內(nèi)部則是使用HttpClient進(jìn)行網(wǎng)絡(luò)通訊的，這里為什么這樣選擇呢？可以參考我之前翻譯的一篇文章Android訪問網(wǎng)絡(luò)，使用HttpURLConnection還是HttpClient？

創(chuàng)建好了HttpStack之后，接下來又創(chuàng)建了一個(gè)Network對(duì)象，它是用于根據(jù)傳入的HttpStack對(duì)象來處理網(wǎng)絡(luò)請(qǐng)求的，緊接著new出一個(gè)RequestQueue對(duì)象，并調(diào)用它的start()方法進(jìn)行啟動(dòng)，然后將RequestQueue返回，這樣newRequestQueue()的方法就執(zhí)行結(jié)束了。

那么RequestQueue的start()方法內(nèi)部到底執(zhí)行了什么東西呢？我們跟進(jìn)去瞧一瞧：

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();} }
這里先是創(chuàng)建了一個(gè)CacheDispatcher的實(shí)例，然后調(diào)用了它的start()方法，接著在一個(gè)for循環(huán)里去創(chuàng)建NetworkDispatcher的實(shí)例，并分別調(diào)用它們的start()方法。這里的CacheDispatcher和NetworkDispatcher都是繼承自Thread的，而默認(rèn)情況下for循環(huán)會(huì)執(zhí)行四次，也就是說當(dāng)調(diào)用了Volley.newRequestQueue(context)之后，就會(huì)有五個(gè)線程一直在后臺(tái)運(yùn)行，不斷等待網(wǎng)絡(luò)請(qǐng)求的到來， 其中 CacheDispatcher是緩存線程，NetworkDispatcher是網(wǎng)絡(luò)請(qǐng)求線程。

得到了RequestQueue之后，我們只需要構(gòu)建出相應(yīng)的Request，然后調(diào)用RequestQueue的add()方法將Request傳入就可以完成網(wǎng)絡(luò)請(qǐng)求操作了，那么不用說，add()方法的內(nèi)部肯定有著非常復(fù)雜的邏輯，我們來一起看一下：

public <T> Request<T> add(Request<T> request) {// Tag the request as belonging to this queue and add it to the set of current requests.request.setRequestQueue(this);synchronized (mCurrentRequests) {mCurrentRequests.add(request);}// Process requests in the order they are added.request.setSequence(getSequenceNumber());request.addMarker("add-to-queue");// If the request is uncacheable, skip the cache queue and go straight to the network.if (!request.shouldCache()) {mNetworkQueue.add(request);return request;}// Insert request into stage if there's already a request with the same cache key in flight.synchronized (mWaitingRequests) {String cacheKey = request.getCacheKey();if (mWaitingRequests.containsKey(cacheKey)) {// There is already a request in flight. Queue up.Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);if (stagedRequests == null) {stagedRequests = new LinkedList<Request<?>>();}stagedRequests.add(request);mWaitingRequests.put(cacheKey, stagedRequests);if (VolleyLog.DEBUG) {VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);}} else {// Insert 'null' queue for this cacheKey, indicating there is now a request in// flight.mWaitingRequests.put(cacheKey, null);mCacheQueue.add(request);}return request;} }
可以看到，在第11行的時(shí)候會(huì)判斷當(dāng)前的請(qǐng)求是否可以緩存，如果不能緩存則在第12行直接將這條請(qǐng)求加入網(wǎng)絡(luò)請(qǐng)求隊(duì)列，可以緩存的話則在第33行將這條請(qǐng)求加入緩存隊(duì)列。在默認(rèn)情況下，每條請(qǐng)求都是可以緩存的，當(dāng)然我們也可以調(diào)用Request的setShouldCache(false)方法來改變這一默認(rèn)行為。

OK，那么既然默認(rèn)每條請(qǐng)求都是可以緩存的，自然就被添加到了緩存隊(duì)列中，于是一直在后臺(tái)等待的緩存線程就要開始運(yùn)行起來了，我們看下CacheDispatcher中的run()方法，代碼如下所示：

public class CacheDispatcher extends Thread {……@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() {@Overridepublic 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;}}} }
代碼有點(diǎn)長，我們只挑重點(diǎn)看。首先在11行可以看到一個(gè)while(true)循環(huán)，說明緩存線程始終是在運(yùn)行的，接著在第23行會(huì)嘗試從緩存當(dāng)中取出響應(yīng)結(jié)果，如何為空的話則把這條請(qǐng)求加入到網(wǎng)絡(luò)請(qǐng)求隊(duì)列中，如果不為空的話再判斷該緩存是否已過期，如果已經(jīng)過期了則同樣把這條請(qǐng)求加入到網(wǎng)絡(luò)請(qǐng)求隊(duì)列中，否則就認(rèn)為不需要重發(fā)網(wǎng)絡(luò)請(qǐng)求，直接使用緩存中的數(shù)據(jù)即可。之后會(huì)在第39行調(diào)用Request的 parseNetworkResponse()方法來對(duì)數(shù)據(jù)進(jìn)行解析，再往后就是將解析出來的數(shù)據(jù)進(jìn)行回調(diào)了，這部分代碼我們先跳過，因?yàn)樗倪壿嫼蚇etworkDispatcher后半部分的邏輯是基本相同的，那么我們等下合并在一起看就好了，先來看一下NetworkDispatcher中是怎么處理網(wǎng)絡(luò)請(qǐng)求隊(duì)列的，代碼如下所示：

public class NetworkDispatcher extends Thread {……@Overridepublic void run() {Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);Request<?> request;while (true) {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) {parseAndDeliverNetworkError(request, volleyError);} catch (Exception e) {VolleyLog.e(e, "Unhandled exception %s", e.toString());mDelivery.postError(request, new VolleyError(e));}}} }
同樣地，在第7行我們看到了類似的while(true)循環(huán)，說明網(wǎng)絡(luò)請(qǐng)求線程也是在不斷運(yùn)行的。在第28行的時(shí)候會(huì)調(diào)用Network的performRequest()方法來去發(fā)送網(wǎng)絡(luò)請(qǐng)求，而Network是一個(gè)接口，這里具體的實(shí)現(xiàn)是BasicNetwork，我們來看下它的 performRequest()方法，如下所示：

public class BasicNetwork implements Network {……@Overridepublic NetworkResponse performRequest(Request<?> request) throws VolleyError {long requestStart = SystemClock.elapsedRealtime();while (true) {HttpResponse httpResponse = null;byte[] responseContents = null;Map<String, String> responseHeaders = new HashMap<String, String>();try {// Gather headers.Map<String, String> headers = new HashMap<String, String>();addCacheHeaders(headers, request.getCacheEntry());httpResponse = mHttpStack.performRequest(request, headers);StatusLine statusLine = httpResponse.getStatusLine();int statusCode = statusLine.getStatusCode();responseHeaders = convertHeaders(httpResponse.getAllHeaders());// Handle cache validation.if (statusCode == HttpStatus.SC_NOT_MODIFIED) {return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,request.getCacheEntry() == null ? null : request.getCacheEntry().data,responseHeaders, true);}// Some responses such as 204s do not have content. We must check.if (httpResponse.getEntity() != null) {responseContents = entityToBytes(httpResponse.getEntity());} else {// Add 0 byte response as a way of honestly representing a// no-content request.responseContents = new byte[0];}// if the request is slow, log it.long requestLifetime = SystemClock.elapsedRealtime() - requestStart;logSlowRequests(requestLifetime, request, responseContents, statusLine);if (statusCode < 200 || statusCode > 299) {throw new IOException();}return new NetworkResponse(statusCode, responseContents, responseHeaders, false);} catch (Exception e) {……}}} }

這段方法中大多都是一些網(wǎng)絡(luò)請(qǐng)求細(xì)節(jié)方面的東西，我們并不需要太多關(guān)心，需要注意的是在第14行調(diào)用了HttpStack的performRequest()方法，這里的HttpStack就是在一開始調(diào)用newRequestQueue()方法是創(chuàng)建的實(shí)例，默認(rèn)情況下如果系統(tǒng)版本號(hào)大于9就創(chuàng)建的HurlStack對(duì)象，否則創(chuàng)建HttpClientStack對(duì)象。前面已經(jīng)說過，這兩個(gè)對(duì)象的內(nèi)部實(shí)際就是分別使用HttpURLConnection和HttpClient來發(fā)送網(wǎng)絡(luò)請(qǐng)求的，我們就不再跟進(jìn)去閱讀了，之后會(huì)將服務(wù)器返回的數(shù)據(jù)組裝成一個(gè)NetworkResponse對(duì)象進(jìn)行返回。

在NetworkDispatcher中收到了NetworkResponse這個(gè)返回值后又會(huì)調(diào)用Request的parseNetworkResponse()方法來解析NetworkResponse中的數(shù)據(jù)，以及將數(shù)據(jù)寫入到緩存，這個(gè)方法的實(shí)現(xiàn)是交給Request的子類來完成的，因?yàn)椴煌N類的Request解析的方式也肯定不同。還記得我們?cè)谏弦黄恼轮袑W(xué)習(xí)的自定義Request的方式嗎？其中parseNetworkResponse()這個(gè)方法就是必須要重寫的。

在解析完了NetworkResponse中的數(shù)據(jù)之后，又會(huì)調(diào)用ExecutorDelivery的postResponse()方法來回調(diào)解析出的數(shù)據(jù)，代碼如下所示：

public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {request.markDelivered();request.addMarker("post-response");mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable)); }
其中，在mResponsePoster的execute()方法中傳入了一個(gè)ResponseDeliveryRunnable對(duì)象，就可以保證該對(duì)象中的run()方法就是在主線程當(dāng)中運(yùn)行的了，我們看下run()方法中的代碼是什么樣的：

private class ResponseDeliveryRunnable implements Runnable {private final Request mRequest;private final Response mResponse;private final Runnable mRunnable;public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {mRequest = request;mResponse = response;mRunnable = runnable;}@SuppressWarnings("unchecked")@Overridepublic void run() {// If this request has canceled, finish it and don't deliver.if (mRequest.isCanceled()) {mRequest.finish("canceled-at-delivery");return;}// Deliver a normal response or error, depending.if (mResponse.isSuccess()) {mRequest.deliverResponse(mResponse.result);} else {mRequest.deliverError(mResponse.error);}// If this is an intermediate response, add a marker, otherwise we're done// and the request can be finished.if (mResponse.intermediate) {mRequest.addMarker("intermediate-response");} else {mRequest.finish("done");}// If we have been provided a post-delivery runnable, run it.if (mRunnable != null) {mRunnable.run();}} }

代碼雖然不多，但我們并不需要行行閱讀，抓住重點(diǎn)看即可。其中在第22行調(diào)用了Request的deliverResponse()方法，有沒有感覺很熟悉？沒錯(cuò)，這個(gè)就是我們?cè)谧远xRequest時(shí)需要重寫的另外一個(gè)方法，每一條網(wǎng)絡(luò)請(qǐng)求的響應(yīng)都是回調(diào)到這個(gè)方法中，最后我們?cè)僭谶@個(gè)方法中將響應(yīng)的數(shù)據(jù)回調(diào)到Response.Listener的onResponse()方法中就可以了。

好了，到這里我們就把Volley的完整執(zhí)行流程全部梳理了一遍，你是不是已經(jīng)感覺已經(jīng)很清晰了呢？對(duì)了，還記得在文章一開始的那張流程圖嗎，剛才還不能理解，現(xiàn)在我們?cè)賮碇匦驴聪逻@張圖：

其中藍(lán)色部分代表主線程，綠色部分代表緩存線程，橙色部分代表網(wǎng)絡(luò)線程。我們?cè)谥骶€程中調(diào)用RequestQueue的add()方法來添加一條網(wǎng)絡(luò)請(qǐng)求，這條請(qǐng)求會(huì)先被加入到緩存隊(duì)列當(dāng)中，如果發(fā)現(xiàn)可以找到相應(yīng)的緩存結(jié)果就直接讀取緩存并解析，然后回調(diào)給主線程。如果在緩存中沒有找到結(jié)果，則將這條請(qǐng)求加入到網(wǎng)絡(luò)請(qǐng)求隊(duì)列中，然后處理發(fā)送HTTP請(qǐng)求，解析響應(yīng)結(jié)果，寫入緩存，并回調(diào)主線程。

怎么樣，是不是感覺現(xiàn)在理解這張圖已經(jīng)變得輕松簡單了？好了，到此為止我們就把Volley的用法和源碼全部學(xué)習(xí)完了，相信你已經(jīng)對(duì)Volley非常熟悉并可以將它應(yīng)用到實(shí)際項(xiàng)目當(dāng)中了，那么Volley完全解析系列的文章到此結(jié)束，感謝大家有耐心看到最后。

原文URL:http://blog.csdn.net/guolin_blog/article/details/17656437

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