忽忽，很意外，第二篇大家反映还不错哈，谢谢咯～～对于这篇，要写好信心不大，不过我会尽量的。
1、从哪里开始：
从哪里开始，菜鸟能想到的当然是从最先接触的地方开始啦～～
首先我们看volley.java这个类：
/**
* Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
*
* @param context A {@link Context} to use for creating the cache dir.
* @param stack An {@link HttpStack} to use for the network, or null for default.
* @return A started {@link RequestQueue} instance.
*/
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;
}回忆下之前我们就是通过它拿到我们的请求队列，在对newRequestQueue的描述中我们也得到验证：创建一个工作池（带优先级的阻塞队列）的实例。在这里有两个静态的构造方法，除去我们之前使用过的，另一个多了一个参数HttpStack（HttpStack稍后分析）。而我们之前并没有使用这个带HttpStack的构造方法。因此就有了上面对stack==null的判断：根据不同的版本创建不同的实例。判断的原因注释中也写的很明确： Gingerbread(2.3)之前，HttpUrlConnection不可用。这里就很自然的想到这样一个问题：Gingerbread(2.3)之前和之后的版本在网络请求上有何区别（或者HTTP
Client与HttpURLConnection的区别）？如果那个网页你打不开，可以在这里下载英文原版，或者去看大牛翻译好的。完成HttpStack的创建之后紧接着有创建了一个Network和RequestQueue对象：Network执行网络请求；RequestQueue是A
request dispatch queue with a thread pool of dispatchers。下面我们将围绕着RequestQueue进行展开。对了，还有一个HttpStack接口，只有一个方法，这里简单提一下：public interface HttpStack {
/**
* Performs an HTTP request with the given parameters.
*
* <p>A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise,
* and the Content-Type header is set to request.getPostBodyContentType().</p>
*
* @param request the request to perform
* @param additionalHeaders additional headers to be sent together with
* {@link Request#getHeaders()}
* @return the HTTP response
*/
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError;
}使用给定的参数执行HTTP请求。通过上面的判断我们知道它有两个实现类：HurlStack和HttpClientStack。这样我们就得到下面的一个关系图：
这样他们的关系就相当明了了。
2、请求队列：RequestQueue
直接看关键构造函数：
/**
* 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;
}这里我们传入了一个cache、Network、线程池大小和传递响应的对象。默认的线程池大小是4。创建这个RequestQueue对象之后，我们需要调用它的start()方法，完成对调度器的初始化工作，如根据传入线程池大小创建相应数量的Dispatcher（这里我们暂时只关注网络处理线程），然后启动： /**
* 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();
}
}NetworkDispatcher的构造函数中传入了一个mNetworkQueue对象，这是一个带优先级的阻塞队列：/** The queue of requests that are actually going out to the network. */ private final PriorityBlockingQueue<Request> mNetworkQueue = new PriorityBlockingQueue<Request>();另一个需要关注的就是add方法：
/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
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<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;
}
}通过上面的代码我们可以看出，volley是将我们创建的Request对象直接加到上面的mNetworkQueue中。这样我们很自然的联想到，对request的处理应该是由start中创建的NetworkDispatcher完成的。NetworkDispatcher继承了Thread，run()方法是这样实现的： public 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;
}
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
// 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));
}
}
}首先从请求队列中取出一个请求；然后通过Network开始执行这个request请求，这个时候会拿到请求返回的结果；接着把请求的结果返给request进行处理：Response<?> response = request.parseNetworkResponse(networkResponse);request处理之后会返回一个response，这个response和request最后会传递到ResponseDeliveryRunnable中处理：
/**
* A Runnable used for delivering network responses to a listener on the
* main thread.
*/
@SuppressWarnings("rawtypes")
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")
@Override
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();
}
}
}这里我们可以看到，如果请求的响应是成功的，会去调用request的deliverResponse，这样一个完整的请求响应过程就完成了。细心的你可能已经发现，从RequestQueue的add()方法开始，到NetworkDispatcher的run()方法结束，很多地方调用了request.addMarker("**-**-**");这个方法，通过里面的字符串，我们可以方便的理解整个处理流程：add-to-queue、network-queue-take、network-http-complete、network-parse-complete、network-cache-written、post-response/post-error、intermediate-response，另外还有request.finish("***-***");这个也可以帮助我们理解整个处理流程。
如上图（这个图我想修改下，但是不知道怎么改，都是拖出来的）。
3、请求的细节
从NetworkDispatcher的run()开始（代码上方有贴）。
3.1 第一次取消
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;
}此时状态是刚从队列中取出：network-queue-take。首先，判断此请求是否已经取消，若取消直接调用request的finish()方法。这个时候我们会想到，怎么知道已经取消，或者用户怎么取消请求呢？到request中我们可以找到一个cancel()方法，就是通过此方法来取消请求的。注销 这个方法，我们可以看到在RequestQueue中有调用此方法，它们是： /**
* Cancels all requests in this queue for which the given filter applies.
* @param filter The filtering function to use
*/
public void cancelAll(RequestFilter filter) {
synchronized (mCurrentRequests) {
for (Request<?> request : mCurrentRequests) {
if (filter.apply(request)) {
request.cancel();
}
}
}
}
/**
* Cancels all requests in this queue with the given tag. Tag must be non-null
* and equality is by identity.
*/
public void cancelAll(final Object tag) {
if (tag == null) {
throw new IllegalArgumentException("Cannot cancelAll with a null tag");
}
cancelAll(new RequestFilter() {
@Override
public boolean apply(Request<?> request) {
return request.getTag() == tag;
}
});
}这样，我们就可以知道，取消一个请求可以通过RequestQueue中的cancelAll()方法进行。这个时候取消的话，我们的请求是不会发出去的。当然，取消在任何时候都会发生，下面还会讲到，请求返回后取消的话结果是不会传递给用户的。3.2 发起网络请求
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
通过执行network的performRequest()方法拿到一个请求结果。执行完这个时候，我们就得到了请求的结果。此处是可以自定义的，我们看下默认实现： @Override
public 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().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 (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode = 0;
NetworkResponse networkResponse = null;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(networkResponse);
}
}
}
}关键部分：httpResponse = mHttpStack.performRequest(request, headers);通过这个执行网络请求。具体的细节还是可以自定义的。代码贴的太多就不继续贴了，两个实现大家可以具体去看看，比如网络请求超时时间的设置都是通过：request.getTimeoutMs();得到的。再到Request中你就可以知道设置超时是通过下面代码实现的：/**
* Sets the retry policy for this request.
*/
public void setRetryPolicy(RetryPolicy retryPolicy) {
mRetryPolicy = retryPolicy;
}
3.3 解析请求结果
// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");这个是不是很熟悉，我们在request中实现的parseNetworkResponse()。3.4 最后结果的处理
这就说了，大家可以继续看默认的代码实现。值得提的是，在这里我们会发现，又一次调用了mRequest.isCanceled()决定是否将结果传递给用户。4、总结
Volley.java 这个类的目的很明确，就是获取一个RequestQueue对象。从代码中可以看出，为了达到这个目的，我们至少要创建三个关键对象，Cache、Network、HttpStack。Cache是一个处理响应的缓存；Network是执行网络请求的接口；HttpStack是真正的执行网络请求的接口。Network和HttpStack都是接口，这样volley给我们预留了充足的自定义空间。如果完全自定义，我们可以创建自己的HttpStack和Network实现，这样我们就可以不再使用volley.java这个类了。
这篇文章写了半个月，也算完整的伴随最近的感情变迁。虽然后面写的越来越糟糕，不过现在终于写完了。欢迎各位指教～～