private RequestQueue mRequestQueue;
if(mRequestQueue == null) {mRequestQueue = Volley.newRequestQueue(this);
}
ImageRequest imageRequest = new ImageRequest("http://g.hiphotos.bdimg.com/imgad/pic/item/6a63f6246b600c3341da97df1d4c510fd9f9a102.jpg",new Response.Listener<Bitmap>() {@Overridepublic void onResponse(Bitmap response) {mImageView.setImageBitmap(response);}
}, 0, 0, Bitmap.Config.RGB_565, new Response.ErrorListener() {@Overridepublic void onErrorResponse(VolleyError error) {Log.d(TAG, "image request error : " + error);}
});mRequestQueue.add(imageRequest);

   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) {}/*如果没有指定处理网络请求的类，则根据sdk版本，选择HttpClient或者HttpUrlConnection */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.htmlstack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));}}Network network = new BasicNetwork(stack);RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);queue.start();return 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();}}

   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.//缓存中不存在的，加入到网络队列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);//失效的情况下加入到网络队列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() {@Overridepublic void run() {try {//缓存中存在，也未失效，但是需要重新获取请求的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);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);}}}

    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#sec10.3.5entry.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);}}}}

  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.//这里是判断请求优先级，优先级高的直接加入到请求队列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.//这里可以保证在短时间内的相同请求，只有一个是加入到缓存队列中，避免同一种请求在队列里面连续插入从而造成阻塞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;}}