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>() {@Overridepublic void onResponse(String s) {Log.i(TAG, s);tv_content.setText(s);}}, new Response.ErrorListener() {@Overridepublic void onErrorResponse(VolleyError volleyError) {Log.i(TAG, volleyError.getMessage());tv_content.setText(volleyError.getMessage());}} ){@Overrideprotected 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.htmlstack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));}}Network network = new BasicNetwork(stack);RequestQueue queue;if (maxDiskCacheBytes <= -1){// No maximum size specifiedqueue = new RequestQueue(new DiskBasedCache(cacheDir), network);}else{// Disk cache size specifiedqueue = 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;}@Overridepublic 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 {...@Overridepublic 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);}// Handle moved resourcesif (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();}@Overridepublic 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() {@Overridepublic 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() {@Overridepublic 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;}@Overridepublic void postResponse(Request<?> request, Response<?> response) {postResponse(request, response, null);}@Overridepublic void postResponse(Request<?> request, Response<?> response, Runnable runnable) {request.markDelivered();request.addMarker("post-response");mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));}@Overridepublic 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")@Overridepublic 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？