volley是一个非常流行的Android开源框架，自己平时也经常使用它，但自己对于它的内部的实现过程并没有进行太多的深究。所以为了以后能更通透的使用它，了解它的实现是一个非常重要的过程。自己有了一点研究，做个笔记同时与大家一起分享。期间自己也画了一张图，希望能更好的帮助我们理解其中的步骤与原理。如下：
开始看可能会一脸懵逼，我们先结合源码一步一步来，现在让我们一起进入Volley的源码世界，来解读大神们的编程思想。
newRequestQueue
如果使用过Volley的都知道，不管是进行网络请求还是什么别的图片加载，首先都要创建好一个RequestQueue
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| public static final RequestQueue volleyQueue = Volley.newRequestQueue(App.mContext);
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而RequestQueue的创建自然离不开Volley中的静态方法newRequestQueue，从上面的图片也能知道首先进入点是newRequestQueue,好了现在我们来看下该方法中到底做了什么：
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| 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; }
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从上面的源码中我们能发现有一个stack，它代表着网络请求通信HttpClient与HttpUrlConnection，但我们一般都是默认设置为null。因为它会默认帮我们进判断选择更适合的。当手机版本大于等于9时会使用HurlStack它里面使用HttpUrlConnection进行实现通信的，而小于9时则创建HttpClientStack，它里面自然是HttpClient的实现。通过BasicNetwork构造成Network来进行传递使用；在这之后构建了RequestQueue，其中帮我们构造了一个缓存new DiskBasedCache(cacheDir)，默认为5MB,缓存目录为volley。所以我们能在data/data/应用包名/volley下找到缓存。最后调用其start方法，并返回RequestQueue。这就是我们前面第一段代码的内部实现。下面我们进入RequestQueue中的start方法看下它到底做了什么呢？
RequestQueue
在RequestQueue中通过this调用自身来默认帮我们调用了下面的构造函数
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| public RequestQueue(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) { mCache = cache; mNetwork = network; mDispatchers = new NetworkDispatcher[threadPoolSize]; mDelivery = delivery; }
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cache后续在NetworkDispatcher中会帮我们进行response.cacheEntry的缓存，netWork是前面的根据版本所封装的通信，threadPoolSize线程池大小默认为4，delivery,是ExecutorDelivery作用在主线程,在最后对请求响应的分发。
start
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| 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(); } }
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在start方法中我们发现它实现了两个dispatch,一个是CacheDispatcher缓存派遣,另一个是networkDispatcher进行网络派遣，其实他们都是Thread，所以都调用了他们的start方法。其中networkDispatcher默认构建了4个，相当于包含4个线程的线程池。现在我们先不去看他们内部的run方法到底实现了什么，我们还是接着看RequestQueue中我们频繁使用的add方法。
add
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| 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; } }
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我们结合代码与前面的图，首先会为当前的request设置RequestQueue,并且根据情况同步是否是当前正在进行的请求，加入到mCurrentRequests中，看11行代码，之后对当前request进行判断是否需要缓存（默认实现是true，如果不需要可调用request.setShouldCache()进行设置）,如果不需要则直接加入到前面的mNetworkQueue中，它会在CacheDispatcher与NetworkDispatcher中做相应的处理，然后返回request。如果需要缓存，看16行代码，则对mWaitingRequests中是否包含cacheKey进行相应的处理。其中cacheKey为请求的url。最后再加入到缓存队列mCacheQueue中。
finish
细心的人会发现当对应cacheKey的value不为空时，创建了LinkedList即Queue,只是将request加入到了Queue中，只是更新了mWaitingRequests中相应的value但并没有加入到mCacheQueue中。其实不然，因为后续会调用finish方法,我们来看下源码:
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| <T> void finish(Request<T> request) { // Remove from the set of requests currently being processed. synchronized (mCurrentRequests) { mCurrentRequests.remove(request); } synchronized (mFinishedListeners) { for (RequestFinishedListener<T> listener : mFinishedListeners) { listener.onRequestFinished(request); } } if (request.shouldCache()) { synchronized (mWaitingRequests) { String cacheKey = request.getCacheKey(); Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey); if (waitingRequests != null) { if (VolleyLog.DEBUG) { VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.", waitingRequests.size(), cacheKey); } // Process all queued up requests. They won't be considered as in flight, but // that's not a problem as the cache has been primed by 'request'. mCacheQueue.addAll(waitingRequests); } } } }
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看14、22行代码，正如上面我所说，会将Queue中的request全部加入到mCacheQueue中。
好了RequestQueue的主要源码差不多就这些，下面我们进入CacheDispatcher的源码分析,看它究竟如何工作的呢？
CacheDispatcher
前面提到了它与NetworkDispatcher本质都是Thread，那么我们自然是看run方法
run
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| @Override public 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; } } }
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看起来很多，我们结合图来挑主要的看。首先创建了一个无限循环一直在监视着request的变化。从缓存队列mCacheQueue中获取request,如果该请求是cancle了，调用request.finish()清除相应数据并进行下一个请求的操作，否则从前面提到的mCache中获取Cache.Entry。如果不存在或者已经过期，将请求加入到网络队列中mNetWorkQueue，进行后续的网络请求。如果存在（41行代码）则进行request.parseNetworkResponse()解析出response,不同的request对应不同的解析方法。例如StringRequest与JsonObjectRequest有各自的解析实现。再看45行，发现不管entry是否需要更新的，都会进一步对response进行mDelivery.postResponse(request, response)递送，不同的是需要更新的话重新设置request的entry与加入到mNetworkQueue中，也就相当与重新进行网络请求一遍。那么再回到递送的阶段，前面已经提到在创建RequestQueue是实现了ExecutorDelivery,mDelivery.postResponse就是其中的方法。我们来看一下
ExecutorDelivery
在这里创建了一个Executor，对后面进行递送,作用在主线程
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| 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); } }; }
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postResponse
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| @Override public void postResponse(Request<?> request, Response<?> response) { postResponse(request, response, null); } @Override public void postResponse(Request<?> request, Response<?> response, Runnable runnable) { request.markDelivered(); request.addMarker("post-response"); mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable)); }
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这个方法就简单了就是调用execute进行执行，在进入ResponseDeliveryRunnable的run看它如何执行
ResponseDeliveryRunnable
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| public 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(); } }
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主要是第9行代码，对于不同的响应做不同的递送，deliverResponse与deliverError内部分别调用的就是我们非常熟悉的Listener中的onResponse与onErrorResponse方法，进而返回到我们对网络请求结果的处理函数。
这就是整个的缓存派遣，简而言之，存在请求响应的缓存数据就不进行网络请求，直接调用缓存中的数据进行分发递送。反之执行网络请求。
下面我来看下NetworkDispatcher是如何处理的
NetworkDispatcher
run
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| @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); } } }
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在这里也创建了一个无限循环的while,同样也是先获取request，不过是从mQueue中，即前面多次出现的mNetWorkQueue,通过看代码发现一些实现跟CacheDispatcher中的类似。也正如图片中所展示的一样，（23行）如何请求取消了，直接finish；否则进行网络请求，调用(31行)mNetwork.performRequest(request)，这里的mNetWork即为前面RequestQueue中对不同版本进行选择的stack的封装，分别调用HurlStack与HttpClientStack各自的performRequest方法，该方法中构造请求头与参数分别使用HttpClient或者HttpUrlConnection进行网络请求。我们再来看42行，是不是很熟悉，与CacheDispatcher中的一样进行response进行解析，然后如果需要缓存就加入到缓存中，最后（54行）再调用mDelivery.postResponse(request, response)进行递送。至于后面的剩余的步骤与CacheDispatcher中的一模一样，这里就不多累赘了。
好了，Volley的源码解析先就到这里了，我们再回过去看那张图是不是感觉很清晰了呢?
总结
我们来对使用Volley网络请求做个总结
- 通过
newRequestQueue初始化与构造RequestQueue
- 调用
RequestQueue中的add方法添加request到请求队列中
- 缓存派遣，先进行
CacheDispatcher,判断缓存中是否存在，有则解析response，再直接postResponse递送，否则进行后续的网络请求
- 网络派遣，
NetworkDispatcher中进行相应的request请求，解析response如设置了缓存就将结果保存到cache中，再进行最后的postResponse递送。
