git clone https://android.googlesource.com/platform/frameworks/volley

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

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;
}

//重复的请求将加入这个集合
private final Map<String, Queue<Request>> mWaitingRequests =
 new HashMap<String, Queue<Request>>();
//所有正在处理的请求任务的集合 
private final Set<Request> mCurrentRequests = new HashSet<Request>();
//缓存任务的队列
private final PriorityBlockingQueue<Request> mCacheQueue =
 new PriorityBlockingQueue<Request>();
//网络请求队列
private final PriorityBlockingQueue<Request> mNetworkQueue =
 new PriorityBlockingQueue<Request>();
//默认线程池大小
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
//用于响应数据的存储与获取
private final Cache mCache;
//用于网络请求
private final Network mNetwork;
//用于分发响应数据
private final ResponseDelivery mDelivery;
//网络请求调度
private NetworkDispatcher[] mDispatchers;
//缓存调度
private CacheDispatcher mCacheDispatcher;

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;
}

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();
 }
}

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;
 }
}

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. 取出缓存队列的任务
 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;
 }
 }
}

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. 发送网络请求
 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));
 }
 }
}

public void postResponse(Request<?> request, Response<?> response) {
 postResponse(request, response, null);
}
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
 request.markDelivered();
 request.addMarker("post-response");
 mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}

private final Executor mResponsePoster;

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);
 }
 };