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volley官方文档给出的执行一个请求的步骤是：
RequestQueue mQueue = Volley.newRequestQueue(context);
mQueue.add(request);为什么创建一个mQueue以后，直接放入一个request，就可以执行该request了呢？发起网络请求的代码在哪里呢？这里看下newRequestQueue()做了哪些操作：
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;
}可以看到创建RequestQueue后，调用了start()方法，再看下start()方法：
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;
VolleyCompat.getInstance().setNetworkDispatcherPriorityMax(networkDispatcher);
networkDispatcher.start();
}
}通过源码可以查看到,CacheDispatcher,NetworkDispatcher都是继承了Thread类，这里相当于启动了一个缓存调度线程，四个网络调度线程，这里先不分析缓存，看网络调度线程做了些什么操作：
@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);
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);
//volley扩展:(2015-11-10)当出现错误，如设备无网络，请求服务器出错，并且有缓存时，需要将缓存内容展示出来
VolleyCompat.getInstance().showCacheWhenErrorOccur(mCache,request,mDelivery,volleyError,startTimeMs);
} catch (Exception e) {
e.printStackTrace();
// VolleyLog.e(e, "Unhandled exception %s", e.toString());
// VolleyError volleyError = new VolleyError(e);
// volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
// mDelivery.postError(request, volleyError);
//volley扩展:(2015-11-10)当出现错误，如设备无网络，请求服务器出错，并且有缓存时，需要将缓存内容展示出来
VolleyError volleyError = new VolleyError(e);
VolleyCompat.getInstance().showCacheWhenErrorOccur(mCache,request,mDelivery,volleyError,startTimeMs);
}
}
}很简单，网络调度线程先是从队列mQueue里取出一个request,然后判断该request是否已经取消，如果没有被用户取消，则调用mNetwork的performRequest(request)发起网络请求，这里的mNetWork是一个BasicNetWrok,看下performRequest()的实现，
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
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) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// A HTTP 304 response does not have all header fields. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// 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,
SystemClock.elapsedRealtime() - requestStart);
} 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, SystemClock.elapsedRealtime() - requestStart);
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);
}
}
}
}首先通过调用mHttpStack.performRequest(request,headers) 发起请求，返回的首先是一个org.apache.http.HttpResponse对象,根据statusCode判断，如果是SC_NOT_MODIFIED,并且支持缓存，那么直接加载本地缓存，否则根据httpResponse.getEntity()方法来构建一个NetworkResponse的对象。
现在回到NetWorkDispatcher的run()方法里，当得到一个NetWorkResponse对象后，接着调用request.parseNetWorkResponse(networkResponse)来对结果数据进行解析，解析数据的过程是什么样的呢？查看下request的parseNetWorkResponse()方法。
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);是给抽象方法，再看Request，也是一个抽象类，说明要想发起请求，必须创建一个Request的子类，并且自己实现该方法，由该Requet来决定如何解析这个networkResponse。好，那先看下Volley自己提供的StringRequest中该方法是怎么实现的：
@Override
protected Response<String> parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}对比下另外一个Volley提供的JsonObjectRequest的parseNetWorkResponse实现：
@Override
protected Response<JSONObject> parseNetworkResponse(NetworkResponse response) {
try {
String jsonString = new String(response.data,
HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET));
return Response.success(new JSONObject(jsonString),
HttpHeaderParser.parseCacheHeaders(response));
} catch (UnsupportedEncodingException e) {
return Response.error(new ParseError(e));
} catch (JSONException je) {
return Response.error(new ParseError(je));
}
}其中Response.success()会new 出一个Response对象，该对象中的result变量也是泛型，并且该result的类型也是由新建Request类的子类的时候制定的，这样这个请求就得到了一个Response对象。
接着回到NetWorkDispatcher的run()方法，接着分析下边流程：
mDelivery.postResponse(request, response);这个mDelivery又是什么东西呢？查看NetWorkDispatcher源码，其实是一个ResponseDelivery类型变量，而查看ResponseDelivery发现是一个接口，从这里可以看到,Volley中处处用到了面向接口的编程思想，使得使用者可以自己定义子类型，提供相关服务。那么这个mDelivery是在哪里制定的呢？其实是在RequestQueue的构造方法中指定，然后在start()方法中传递给NetWorkDispatcher的:
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}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;
VolleyCompat.getInstance().setNetworkDispatcherPriorityMax(networkDispatcher);
networkDispatcher.start();
}
}好，接着看ExecutorDelivery的postResponse()方法：
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}调用了mResponsePoster的execute()方法，接着看下mResponsePoster：
/** Used for posting responses, typically to the main thread. */
private final Executor mResponsePoster;
/**
* Creates a new response delivery interface.
* @param handler {@link Handler} to post responses on
*/
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);
}
};
}可以看到看到handler，想必熟悉handler机制的童鞋们就特别想知道这个handler是哪个线程的handler了吧？会不会是UI Thread的呢？接着看
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}可以看到，确实是UI 线程的，这样我们就可以将ResponseDeliveryRunnable放到UI线程里去执行啦。好了，那看下让UI线程执行什么操作呢？
@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();
}
}其中这个mRequest,就是我们这次网络请求发送的Request,Response就是我们这次请求返回的结果封装类，再次提醒，该Response里的result变量的类型，就是我们在新建Request子类时候指定的类型。好的，万里长城差最后一步了，再看下Request的deliverResponse方法()
/**
* Subclasses must implement this to perform delivery of the parsed
* response to their listeners. The given response is guaranteed to
* be non-null; responses that fail to parse are not delivered.
* @param response The parsed response returned by
* {@link #parseNetworkResponse(NetworkResponse)}
*/
abstract protected void deliverResponse(T response);也是一个抽象方法，那么我们还是看下JsonRequest类的该方法是如何实现的：
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}mListener是Response的一个内部接口：
public class Response<T> {
/** Callback interface for delivering parsed responses. */
public interface Listener<T> {
/** Called when a response is received. */
public void onResponse(T response);
}那mListener是在哪里赋值的呢？
public JsonRequest(int method, String url, String requestBody, Listener<T> listener,
ErrorListener errorListener) {
super(method, url, errorListener);
mListener = listener;
mRequestBody = requestBody;
}也就是在创建JsonRequest的时候，就要指定这个Response.Listener,这里也是面相接口编程，因为不知道将来谁会对这个Request请求的结果数据感兴趣，那么就在设计框架的时候，调用listener的onResponse()方法，这样以后谁对结果数据敢兴趣，那就实现接口，并且在创建相应Request的时候，传递进来就可以了。
有一个细节值得注意的就是，在Volley原生Request类的构造函数里，是只有Response.ErrorListener,并没有Response.Listener的：
/**
* Creates a new request with the given method (one of the values from {@link Method}),
* URL, and error listener. Note that the normal response listener is not provided here as
* delivery of responses is provided by subclasses, who have a better idea of how to deliver
* an already-parsed response.
*/
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
setRetryPolicy(new DefaultRetryPolicy());
mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}也就是说请求如果发生错误信息了，那么必须要有能通知到的listener来处理，但是请求成功后，可以任何事情也不做。
正查数据请求过来的流程分析清楚后，最后再看下请求出错后的处理逻辑：
在ExecutorDelivery的run()方法里：
// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}而在Request的deliverError()方法中：
/**
* Delivers error message to the ErrorListener that the Request was
* initialized with.
*
* @param error Error details
*/
public void deliverError(VolleyError error) {
if (mErrorListener != null) {
mErrorListener.onErrorResponse(error);
}
}也印证了上面的请求出错必须有处理的猜测。
至此，Volley一个请求的执行流程就分析完成了，总结下：
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原文地址：http://blog.csdn.net/github_26939093/article/details/51087536
