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
public static final RequestQueue volleyQueue = Volley.newRequestQueue(App.mContext);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：
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
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}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
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();
}
}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
public Request add(Request 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> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList>();
}
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;
}
} 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:
void finish(Request request) {
// Remove from the set of requests currently being processed.
synchronized (mCurrentRequests) {
mCurrentRequests.remove(request);
}
synchronized (mFinishedListeners) {
for (RequestFinishedListener listener : mFinishedListeners) {
listener.onRequestFinished(request);
}
}
if (request.shouldCache()) {
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
Queue> 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);
}
}
}
} 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
@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;
}
}
}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
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);
}
};
}postResponse
@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));
}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
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();
}
}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
@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);
}
}
}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。
如果有所帮助欢迎关注我的下一次解析
更多分享：个人博客
关注
