Android Volley核心源码解析

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闲聊Volley

Volley下载

Volley.jar及源码下载：http://download.csdn.net/detail/qq_17250009/9458711

Volley gitHub地址：https://github.com/mcxiaoke/android-volley

Volley特点

Google I/O 2013上发布了Volley！Volley是Android平台上的网络通信库，能使网络通信更快，更简单，更健壮！

名字由来：a burst or emission of many things or a large amount at once



1、特别适合数据量小，通信频繁的网络操作。

2、扩展性强。Volley 中大多是基于接口的设计，可配置性强。

3、一定程度符合 Http 规范，包括返回 ResponseCode(2xx、3xx、4xx、5xx）的处 理,请求头的处理,缓存机制的支持等。并支持重试及优先级定义。

4、默认 Android2.3 及以上基于 HttpURLConnection,2.3 以下基于 HttpClient 实现。

5、提供简便的图片加载工具。总之Volley就是很牛逼啦！

Volley执行流程图

英语好的看洋文，不行看国语。

Volley中的一些概念简介

Volley：Volley 对外暴露的 API，通过 newRequestQueue(…) 函数新建并启动一个请求队列RequestQueue。

Request：表示一个请求的抽象类。StringRequest、JsonRequest、ImageRequest 都是它的子类，表示某种类型的请求。

RequestQueue：表示请求队列，里面包含一个CacheDispatcher(用于处理走缓存请求的调度线程)、NetworkDispatcher数组(用于处理走网络请求的调度线程，默认长度为4)，一个ResponseDelivery(返回结果分发接口)，通过 start() 函数启动时会启动CacheDispatcher和NetworkDispatcher。

CacheDispatcher：一个线程，用于调度处理走缓存的请求。启动后会不断从缓存请求队列中取请求处理，队列为空则等待，请求处理结束则将结果传递给ResponseDelivery去执行后续处理。当结果未缓存过、缓存失效或缓存需要刷新的情况下，该请求都需要重新进入NetworkDispatcher去调度处理。

NetworkDispatcher：一个线程，用于调度处理走网络的请求。启动后会不断从网络请求队列中取请求处理，队列为空则等待，请求处理结束则将结果传递给ResponseDelivery去执行后续处理，并判断结果是否要进行缓存。

ResponseDelivery：返回结果分发接口，目前只有基于ExecutorDelivery的在入参 handler 对应线程内进行分发。

HttpStack：处理 Http 请求，返回请求结果。目前 Volley 中有基于 HttpURLConnection 的HurlStack和 基于 Apache HttpClient 的HttpClientStack。

Network：调用HttpStack处理请求，并将结果转换为可被ResponseDelivery处理的NetworkResponse。

Cache：缓存请求结果，Volley 默认使用的是基于 sdcard 的DiskBasedCache。NetworkDispatcher得到请求结果后判断是否需要存储在 Cache，CacheDispatcher会从 Cache 中取缓存结果。

Volley类关系图

其中红色框内组成了Volley的核心。

正式开始Volley之旅

Volley使用示例

/**
* 获取String类型的数据
*/
private void getStringData() {
mQueue = Volley.newRequestQueue(this);
// 接口来自聚合数据
String url = "http://apis.juhe.cn/cook/queryid";
StringRequest stringRequest = new StringRequest(Request.Method.POST, url, new Response.Listener<String>() {
@Override
public void onResponse(String s) {
Log.i(TAG, s);
tv_content.setText(s);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError volleyError) {
Log.i(TAG, volleyError.getMessage());
tv_content.setText(volleyError.getMessage());
}
} ){
@Override
protected Map<String, String> getParams() throws AuthFailureError {
Map<String, String> params = new HashMap<String , String>();
params.put("key","a623b1b9a688bc174b2f92edc4f6008d");
params.put("id","1001");
return params;
}
};

mQueue.add(stringRequest);
}

可以看到，我们首先调用Volley.newRequestQueue(this)获取到一个RequestQueue。那么在这期间，源代码做了什么呢？

public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}

public static RequestQueue newRequestQueue(Context context, HttpStack stack)
{
return newRequestQueue(context, stack, -1);
}

public static RequestQueue newRequestQueue(Context context, int maxDiskCacheBytes) {
return newRequestQueue(context, null, maxDiskCacheBytes);
}

/** Default on-disk cache directory. */
private static final String DEFAULT_CACHE_DIR = "volley";

public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
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;
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}else{
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}

queue.start();
return queue;
}

本篇博客大部分在解释上面一段代码的执行流程及设计思想。首先创建data/data/packageName/Volley文件夹作为缓存目录。如果stack = null。如果API Level >= 9，采用基于 HttpURLConnection 的 HurlStack，如果小于 9，采用基于 HttpClient 的 HttpClientStack。接着构建了一个基于BasicNetWork的network，然后new DiskBaseCache(cacheDir),紧接着用new DiskBaseCache(cacheDir)和network作为参数构建一个RequestQueue

/** Number of network request dispatcher threads to start. */
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;

public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}

public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}

public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}

在这里初始化了一些相关参数，下面是参数解释说明。

mCache : 基于DiskBasedCache的Cache对象

mNetwork : 基于BasicNetwork的Network对象

mDispatchers : 网络请求线程数组，默认大小为4

mDelivery : 基于ExecutorDelivery的ResponseDelivery对象

最后调用queue.start()就可以使用了。至此，使用Volley相关的参数已经初始化完毕，有没有觉得使用很方便呢？知其然知其所以然，跟进代码RequestQueue#start()

/**
* 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();
}
}

首先stop之前的调用，遍历quit掉五个线程（接下来会详细介绍着五个线程），这个代码很简单并且现在不好理解，直接略过了。回头你会明白的。接下来新建并start一个CacheDispatcher。新建并start四个（mDispatchers 默认大小为4）NetworkDiapatcher，并组成长度为4的数组。1+4就是上文中提到的5个线程。为了方便学习，我们首先查看NetworkDiapatcher类。

NetworkDiapatcher

public class NetworkDispatcher extends Thread{
...
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}

@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
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);
}
}
}

...
}

代码有些长，只要我们细心去查看，还是能发现写蛛丝马迹的。NetworkDispatcher是一个线程，在run方法中是个while(true)死循环，不断从mQueue(RequestQueue.mNetworkQueue)中取出request。然后调用

mNetwork.performRequest(request)

，实际调用的是Network的具体实现

BasicNetWork.performRequest(request)

。下面我们看一下

BasicNetWork.performRequest(request)

BasicNetwork

public class BasicNetwork implements Network {
...
@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);
}

// Handle moved resources
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
String newUrl = responseHeaders.get("Location");
request.setRedirectUrl(newUrl);
}

// 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);
}
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
VolleyLog.e("Request at %s has been redirected to %s", request.getOriginUrl(), request.getUrl());
} else {
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 if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
attemptRetryOnException("redirect",
request, new RedirectError(networkResponse));
} else {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(e);
}
}
}
}
...
}

代码依旧很长，不过不要灰心，我们只看关键的代码就可以了。在

BssicNetWork.performRequest()

中由HttpStack(具体实现是HurlStack或者HttpClientStack).performRequest(request)获取数据，最后

BasicNetWork.performRequest()

跟据各种情况返回不同的NetworkResponse对象。

返回NetworkDispatcher#run()接着看

获取到NetworkResponse后，执行

request.parseNetWorkReqponse

,并返回一个Response<？>对象。这也是为什么自定义的request必须重写parseNetWorkReqponse()方法的原因。最后执行ResponseDelivery（具体实现是ExecutorDelivery）.postResponse(request,response)进行分发。

四个一样的NetWorkDispatcher不断的在后台运行，至此，NetWorkDispatcher分析完毕。接下来我们来看CatchDispatcher类（一定要看懂NetworkDispatcher类再看这个CatchDispatcher类，因为会涉及到。这也是为什么先讲NetworkDispatcher的原因）。

CatchDispatcher

public class CacheDispatcher extends Thread {

private static final boolean DEBUG = VolleyLog.DEBUG;

/** The queue of requests coming in for triage. */
private final BlockingQueue<Request<?>> mCacheQueue;

/** The queue of requests going out to the network. */
private final BlockingQueue<Request<?>> mNetworkQueue;

/** The cache to read from. */
private final Cache mCache;

/** For posting responses. */
private final ResponseDelivery mDelivery;

/** Used for telling us to die. */
private volatile boolean mQuit = false;

public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}

public void quit() {
mQuit = true;
interrupt();
}

@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();

Request<?> request;
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
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.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}
}

又是这么长，阿西吧！不过没关系，分析完NetWorkDispatcher再来看这个CatchDispatcher就很简单了，因为这两个类有很多相同的地方。

CatchDispatcher也是一个线程。在run()方法中也是一个while(true)死循环。不断从mCacheQueue中取出一个request。然后尝试从mCache中去查找request中key(url)对应的entry。如果entry = null或者过期，则直接插入mNetWorkQueue。在分析NetworkDispatcher的时候提到在run方法中会不断从mQueue(RequestQueue.mNetWorkQueue)中取出request，然后获取数据，解析数据，封装数据，分发。此处流程和Loop不断从MessageQueue中取出Message过程很像，但这里是Loop和MessageQueue交互的升级版，类似于一重循环和二重循环。对Loop、MessageQueue一级循环感兴趣的小伙伴可以看下我的另一篇博客：Handler、Message、MessageQueue、Looper调用过程源码浅析，在此不在赘述。如果entry不为空也没有过期，则认为可以从mCache中直接获取。后续过程和NetWorkDispatcher类似，封装数据，分发。和NetWorkDispatcher不同的一点是还要检查是否需要刷新数据，不需要刷新数据，过程和NetWorkDispatcher一样。需要刷新，则插入mNetWorkQueue。

在获取RequstQueue实例的时候

new ExecutorDelivery(new Hanlder(Looper.getMainLooper()))

。这里的作用就是切换线程，而且是切换到主线程。两个参数的

ExecutorDelivery.postResponse()

方法调用三个参数的

ExecutorDelivery.postResponse()

方法时new 了一个实现了Runnable接口的ResponseDeliveryRunnable对象。看一下ExecutorDelivery类。

ExecutorDelivery

public class ExecutorDelivery implements ResponseDelivery {
/** 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);
}
};
}

/**
* Creates a new response delivery interface, mockable version
* for testing.
* @param executor For running delivery tasks
*/
public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}

@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));
}

@Override
public void postError(Request<?> request, VolleyError error) {
request.addMarker("post-error");
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}

/**
* 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;
}

// 分发核心代码块
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();
}
}
}
}

实例化ExecutorDelivery的时候传入了主线程的looper，这也就意味着分发直接post到主线程，可以直接更新UI。ResponseDeliveryRunnable#run()方法中分发mResponse。这也是为什么自定义的request必须重写deliverResponse（）方法的原因，deliverError（）方法在Request类中已经被重写过，所以不必重写。

至此，RequestQueue#start()及相关的NetWorkDispatcher（网络请求）、CatchDispatcher（缓存）、ExecutorDelivery（分发）类也已经看完，只剩最后一步

RequestQueue.add(reuqest)

了。同志们，胜利的曙光就在眼前！Fighting！

/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
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;
}
}

同志们不要“方”，代码虽长。逻辑还是很简单的。一步步来。可以看到，首先将request插入到mCurrentRequests。如果request不应该被缓存（默认可以缓存，可以调用Request的setShouldCache(false)方法来改变这一默认行为），则直接插入mNetworkQueue。否则判断mWaitingRequests包不包含cacheKey（url）。

如果包含，则将此请求加入mWaitingRequests队列，不再重复请求，在上一个请求返回时直接发送结果，详见RequestQueu#finish()。

如果mWaitingRequests不包含cacheKey（url），则插入mWaitingRequests，然后执行mCacheQueue.add(request)。

上文分析CacheDispatcher时说到,run（）方法也是个死循环，不断从mCacheQueue中取出request，然后走相应的流程。具体可查看CatchDispatcher分析，不再赘述。

至此，Volley源码解析完毕。回头再看Volley执行流程图是不是思路清晰的多。小伙伴们，get到了吗？