Volley完全解析——使用、源码
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Feathers's Volley and Demo：https://github.com/xf616510229/VolleyDemo.git
2013年Google I/O大会上推出了一个新的网络通信框架——Volley，它简单易用，适合通信频繁的操作，不适合大数据量的操作 volley本来的意思就是集中射击、集鸣：
Git：git clone https://android.googlesource.com/platform/frameworks/volley ，然后让项目依赖此文件 Gradle： compile 'com.android.volley:volley:1.0.0' 或是在dependencies中搜索并依赖，一般第一个就是
注意：Module app 中使用的是：compile 'eu.the4thfloor.volley:com.android.volley:2015.05.28' 该compile多了一些功能，比如
ImageRequest.Transformation
官方文档：https://developer.android.com/training/volley/index.html doc：http://afzaln.com/volley/
RequestQueue queue = Volley.newRequestQueue(this);
StringRequest stringRequest = new StringRequest("http://www.baidu.com",
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
// TODO 请求成功回调
Log.d("TAG", response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// TODO 请求失败回调
Log.e("TAG", error.getMessage(), error);
}
});
// 将请求添加到请求队列，会自动开始请求
queue.add(stringRequest);设置请求方法
StringRequest stringRequest = new StringRequest(Method.POST, url, listener, errorListener);
子类JsonRequest/JsonArrayRequest
JsonObjectRequest jsonObjectRequest = new JsonObjectRequest("http://m.weather.com.cn/data/101010100.html", null,
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
Log.d("TAG", response.toString());
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("TAG", error.getMessage(), error);
}
});
mQueue.add(jsonObjectRequest); ImageRequest imageRequest = new ImageRequest(
"http://developer.android.com/images/home/aw_dac.png",
new Response.Listener<Bitmap>() {
@Override
public void onResponse(Bitmap response) {
// 请求成功设置图片
imageView.setImageBitmap(response);
}
}, 0, 0, Config.RGB_565, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// 请求失败设置默认图片
imageView.setImageResource(R.drawable.default_image);
} ImageLoader imageLoader = new ImageLoader(mQueue, new ImageCache() {
@Override
public void putBitmap(String url, Bitmap bitmap) {
// 缓存图片到本地
}
@Override
public Bitmap getBitmap(String url) {
// 读取缓存的图片
return null;
}
});
ImageListener listener = ImageLoader.getImageListener(imageView,
R.drawable.default_image, R.drawable.failed_image);
imageLoader.get("http://img.my.csdn.net/uploads/201404/13/1397393290_5765.jpeg", listener);
/*
imageLoader.get("http://img.my.csdn.net/uploads/201404/13/1397393290_5765.jpeg",
listener, 200, 200);
*/<com.android.volley.toolbox.NetworkImageView
android:id="@+id/niv"
android:layout_width="100dp"
android:layout_height="100dp"
android:layout_above="@id/iv"
android:layout_centerHorizontal="true"/>niv = (NetworkImageView) findViewById(R.id.niv);
niv.setDefaultImageResId(R.drawable.no_image);
niv.setErrorImageResId(R.drawable.image_failed);
niv.setImageUrl("http://avatar.csdn.net/1/7/7/1_xf616510229.jpg",
new ImageLoader(mQueue, new BitmapCache()));请看类GsonRequest
请看类XMLRequest
Request added to queue in priority order
Request dequeued by CacheDispatcher
cache hit: request read from cache and parsed
cache miss:cache dequeued by NetworkDispatcher(round-robin)
HTTP、HTTP transaction,response parse,cache write(if applicable)、HTTP
Parsed response delivered on main thread.
解析源码，首先要从他的方法上着手
创建一个RequestQueue
/* [Volley.java] */
public static RequestQueue newRequestQueue(Context context, int maxDiskCacheBytes) {
return newRequestQueue(context, null, maxDiskCacheBytes);
}
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
return newRequestQueue(context, stack, -1);
}
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
// 1. 生成缓存文件File对象
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
// 2. 根据packageName与版本号生成一个userAgent
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
// 3. 创建httpStack
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
stack = new HurlStack(); // 创建HurlStack，内部为HttpUrlConnection
} else { // 创建HttpClientStack，内部是由HttpClient实现
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
// 4. 创建一个Network，将httpStack传入
Network network = new BasicNetwork(stack);
// 5. 创建requestQueue
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);
}
// 6.
queue.start();
// 7. 返回queue
return queue;
}我们从源码上可以看到，newRequestQueue最终都是调用三个参数的构造方法完成的，七种的三个参数分别为：
context
HttpStack
maxDiskCacheBytes 磁盘缓存最大值，默认是-1,如果小于0，代表没有缓存最大限制
相信你们看完之后最疑惑的地方就是在于HttpStack了，让我们看一下HttpStack:
HttpStack
/*[HttpStack.java]*/
public interface HttpStack {
/**
* <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;
}
HttpStack是一个接口，只有一个抽象方法:performRequest，需要三个参数
request
additionalHeaders，附加的请求头,请求头信息大都是键值对，所以此处使用Map集合
根据方法的名字，以及返回值Response，可以初步判断，该类是用于执行请求的，为了证实猜想，我们看看他的两个子类： HurlStack HttpClientStack
/*[HutlStack]*/
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
String url = request.getUrl();
HashMap<String, String> map = new HashMap<String, String>();
map.putAll(request.getHeaders());
// 添加额外的头
map.putAll(additionalHeaders);
// 重写URL
if (mUrlRewriter != null) {
String rewritten = mUrlRewriter.rewriteUrl(url);
if (rewritten == null) {
throw new IOException("URL blocked by rewriter: " + url);
}
url = rewritten;
}
// 使用URLConnection进行网络请求
URL parsedUrl = new URL(url);
HttpURLConnection connection = openConnection(parsedUrl, request);
// 将请求头添加到connection中
for (String headerName : map.keySet()) {
connection.addRequestProperty(headerName, map.get(headerName));
}
// 为请求设置必要的参数，比如给connection设置请求方式（GET POST），具体清查看源码
setConnectionParametersForRequest(connection, request);
// 初始化HttpResponse，注意，这里的HttpResponse是HttpClient中的，所以需要必要的转换
// Initialize HttpResponse with data from the HttpURLConnection.
ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
int responseCode = connection.getResponseCode();
if (responseCode == -1) {
// -1 is returned by getResponseCode() if the response code could not be retrieved.
// Signal to the caller that something was wrong with the connection.
throw new IOException("Could not retrieve response code from HttpUrlConnection.");
}
StatusLine responseStatus = new BasicStatusLine(protocolVersion,
connection.getResponseCode(), connection.getResponseMessage());
BasicHttpResponse response = new BasicHttpResponse(responseStatus);
response.setEntity(entityFromConnection(connection));
for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
if (header.getKey() != null) {
Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
response.addHeader(h);
}
}
return response;
}
/**
* 内部类 用于重写URL的接口
* An interface for transforming URLs before use.
*/
public interface UrlRewriter {
/**
* 返回重写后的url
* Returns a URL to use instead of the provided one, or null to indicate
* this URL should not be used at all.
*/
public String rewriteUrl(String originalUrl);
}/*[HttpClientStack.java]*/
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);
addHeaders(httpRequest, additionalHeaders);
addHeaders(httpRequest, request.getHeaders());
onPrepareRequest(httpRequest);
HttpParams httpParams = httpRequest.getParams();
int timeoutMs = request.getTimeoutMs();
// TODO: Reevaluate this connection timeout based on more wide-scale
// data collection and possibly different for wifi vs. 3G.
HttpConnectionParams.setConnectionTimeout(httpParams, 5000);
HttpConnectionParams.setSoTimeout(httpParams, timeoutMs);
return mClient.execute(httpRequest);
}HttpClientStack就相对简单了，对请求作出了一些相应的处理，然后执行获取了response对象
HttpStack就是这样，它的功能主要还是让执行request，获取response
然后，第四步骤中，又使用这个stack对象生成了一个Network实例，让我们看一下Network
Network
/*[Network.java]*/
public interface Network {
/**
* Performs the specified request.
* @param request Request to process
* @return A {@link NetworkResponse} with data and caching metadata; will never be null
* @throws VolleyError on errors
*/
public NetworkResponse performRequest(Request<?> request) throws VolleyError;
}又是一个接口，我们看到，Network中也只有一个方法，也叫performRequest，执行请求， 返回的同样也是response对象，但是是NetworkResponse对象,其中包含了相应码，响应头，响应体等信息
BasicNetwork：
@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());
// 这里
httpResponse = mHttpStack.performRequest(request, headers);
....
} catch (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
....
}
...
}
}Network的performRequest就是调用HttpStack的performRequest方法，根据不同的情况进行不同方式的网络请求，获取response
然后，对queue进行初始化，此时才真正初始化queue
/*[RequestQueue]*/
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}可以看到，RequestQueue共有三个构造器，最终都指向四个参数的构造器 四个参数分别为
Cache缓存类
Network类
threadPoolSize，线程池线程数量
ResponseDelivery ？
先看Cache接口：
/*[Cache.java]*/
public interface Cache {
public Entry get(String key);
public void put(String key, Entry entry);
public void initialize();
public void invalidate(String key, boolean fullExpire);
public void remove(String key);
public void clear();
public static class Entry {
/** The data returned from cache. */
public byte[] data;
/** ETag for cache coherency. */
public String etag;
/** Date of this response as reported by the server. */
public long serverDate;
/** The last modified date for the requested object. */
public long lastModified;
/** TTL for this record. */
public long ttl;
/** Soft TTL for this record. */
public long softTtl;
/** Immutable response headers as received from server; must be non-null. */
public Map<String, String> responseHeaders = Collections.emptyMap();
/** True if the entry is expired. */
public boolean isExpired() {
return this.ttl < System.currentTimeMillis();
}
/** True if a refresh is needed from the original data source. */
public boolean refreshNeeded() {
return this.softTtl < System.currentTimeMillis();
}
}
}Cache接口中提供了存取缓存的方法，和一个保存缓存信息的内部类Entry Network提供了执行request的方法 默认的网络线程池的大小为4：
/** Number of network request dispatcher threads to start. */
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;构造器拿着这个大小，创建了一个NetworkDispatcher数组，这个类我们等用到时去看。
至此，requestQueue已经初始化完成，接着就调用了start()方法
/**
* 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();
}
}我们看到注释是start the dispatchers in the queue,下文中又启动了CacheDispatchers与NetworkDispatcher
我们先看一看创建CacheDispatcher传入的参数的初始化：
/** The cache triage queue. */
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();
/** The queue of requests that are actually going out to the network. */
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();所谓的cacheQueue，就是一个泛型为Request的队列，networkQueue也是，自行配图观看～
注：PriorityBlockingQueue 是Java7推出的含有优先级的队列集合
CacheQueue继承Thread类， 让我们看看CacheDispatcher的构造器：
/*[CacheDispacher.java]*/
/**
* Creates a new cache triage dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param cacheQueue Queue of incoming requests for triage
* @param networkQueue Queue to post requests that require network to
* @param cache Cache interface to use for resolution
* @param delivery Delivery interface to use for posting responses
*/
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}我们从构造器的注释上看得很清楚，这个类会创建一个新的用于对request缓存队列进行缓存处理的线程，调用start()方法启动线程。 让我们看看run()，他究竟做了些什么：
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
// 设置线程优先级为标准优先级，下文将会开启很多线程
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// 初始化缓存类,while 循环，代表该线程不断的从队列中取出请求进行缓存操作
// Make a blocking call to initialize the cache.
mCache.initialize();
while (true) {
try {
// 取出一个request
// 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;
}
// 如果这个request没有缓存，添加标记 cache-miss，并把它交给网络请求队列
// 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;
}
// 如果缓存无效了，同样也交给networkQueue
// 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;
}
// 如果缓存存在，且有效，添加标记cache-hit-parsed，然后使用响应处理类ResponseDelivery进行处理
// 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;
}
}
}我们紧接着看一看NetworkDispatcher：
/**
* Provides a thread for performing network dispatch from a queue of requests.
*
* Requests added to the specified queue are processed from the network via a
* specified {@link Network} interface. Responses are committed to cache, if
* eligible, using a specified {@link Cache} interface. Valid responses and
* errors are posted back to the caller via a {@link ResponseDelivery}.
*/
public class NetworkDispatcher extends Thread {
/** The queue of requests to service. */
private final BlockingQueue<Request<?>> mQueue;
/** The network interface for processing requests. */
private final Network mNetwork;
/** The cache to write to. */
private final Cache mCache;
/** For posting responses and errors. */
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new network dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param queue Queue of incoming requests for triage
* @param network Network interface to use for performing requests
* @param cache Cache interface to use for writing responses to cache
* @param delivery Delivery interface to use for posting responses
*/
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void addTrafficStatsTag(Request<?> request) {
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
}
@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);
// 这里
// 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);
}
}
}
private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
}我们看到： NetworkResponse networkResponse = mNetwork.performRequest(request); 在执行run()时，调用了performRequest(request)获取了response，前面说过，这个方法就是进行网络请求的真正的方法
无论是CacheDispatcher 还是 NetworkDispatcher 最后都执行了mDelivery.postResponse()方法，这个方法用来解析来自缓存或者网络响应，而且是抛给主线程处理: 我们看看这个方法：
/*[ExecutorDelivery]*/
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));
}我们看到这个方法调用了mResponsePoster.execute()，mResponsePoster是一个Runnable 他是ExecutorDelivery的内部类，调用execute()，回去执行run() 如下：
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() {
// 请求被取消，结束请求
// If this request has canceled, finish it and don't deliver.
if (mRequest.isCanceled()) {
mRequest.finish("canceled-at-delivery");
return;
}
// ·请求成功，交给request处理·
// 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();
}
}
}我们看到请求成功的时候，将response.result交给了Request： Request有如下几个实现类： StringRequest ClearCacheRequest ImageRequest 以StringRequest为例：
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}deliverResponse将response交给listener处理，而这个listener就是我们进行回调处理response的Listener
至此整个流程也就结束。
Volley的大体流程：
将请求添加到缓存队列
在缓存线程中处理缓存队列中的请求
如果缓存存在，抛给主线程，进行对response处理
如果缓存不存在，将request抛给网络请求队列，在网络线程中遍历处理request
然后进行写缓存，仍在网络线程中。
最好抛给主线程，对response处理
Demo效果：
