Volley设计

Dispatch Thread不断从RequestQueue中取出请求，根据是否已缓存调用Cache或Network这两类数据获取接口之一，从内存缓存或是服务器取得请求的数据，然后交由ResponseDelivery去做结果分发及回调处理。

Volley中的类简介

Volley：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)，一个ResponseDelivery(返回结果分发接口)，通过 start() 函数启动时会启动CacheDispatcher和NetworkDispatchers。

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。

类图

Volley

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

调用了newRequestQueue()的方法重载，并给第二个参数传入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 {  stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));  }  }  Network network = new BasicNetwork(stack);  RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);  queue.start();  return queue;
}  

如果stack是等于null的，则去创建一个HttpStack对象，这里会判断如果手机系统版本号是大于9的，则创建一个HurlStack的实例，否则就创建一个HttpClientStack的实例。

实际上HurlStack的内部就是使用HttpURLConnection进行网络通讯的，而HttpClientStack的内部则是使用HttpClient进行网络通讯的。

得到了 HttpStack,然后通过它构造一个代表网络（Network）的具体实现BasicNetwork。

接着构造一个代表缓存（Cache）的基于 Disk 的具体实现DiskBasedCache。

最后将网络（Network）对象和缓存（Cache）对象传入构建一个 RequestQueue，启动这个 RequestQueue，并返回。

Volley 会将请求头中的 User-Agent 字段设置为 App 的 ${packageName}/${versionCode}，如果异常则使用 "volley/0"

Request

我们通过构建一个Request类的非抽象子类(StringRequest、JsonRequest、ImageRequest 或自定义)对象，并将其加入到·RequestQueue·中来完成一次网络请求操作。

Volley 支持 8 种 Http 请求方式 GET, POST, PUT, DELETE, HEAD, OPTIONS, TRACE, PATCH

因为是抽象类，子类必须重写的两个方法。

abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
abstract protected void deliverResponse(T response);

以下两个方法也经常会被重写

public byte[] getBody();
protected Map<String, String> getParams();

RequestQueue

RequestQueue 中维护了两个基于优先级的 Request 队列，缓存请求队列和网络请求队列。

private final PriorityBlockingQueue<Request<?>> mCacheQueue = new PriorityBlockingQueue<Request<?>>();
private final PriorityBlockingQueue<Request<?>> mNetworkQueue = new PriorityBlockingQueue<Request<?>>();

维护了一个正在进行中，尚未完成的请求集合。

private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

维护了一个等待请求的集合，如果一个请求正在被处理并且可以被缓存，后续的相同 url 的请求，将进入此等待队列。

private final Map<String, Queue<Request<?>>> mWaitingRequests = new HashMap<String, Queue<Request<?>>>();

在Volley类中启动了RequestQueue，queue.start();

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();  }

先是创建了一个CacheDispatcher的实例，然后调用了它的start()方法，接着在一个for循环里去创建NetworkDispatcher的实例，并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的，而默认情况下for循环会执行四次，也就是说当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中CacheDispatcher是缓存线程，NetworkDispatcher是网络请求线程。

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;  }
}  

程序会判断当前的请求是否可以缓存，如果不能缓存则直接将这条请求加入网络请求队列，可以缓存的话则将这条请求加入缓存队列。在默认情况下，每条请求都是可以缓存的，当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。

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。

public class CacheDispatcher extends Thread {  ……  @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();  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.  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;  }  }  }
}  

首先可以看到一个while(true)循环，说明缓存线程始终是在运行的，接着会尝试从缓存当中取出响应结果，如何为空的话则把这条请求加入到网络请求队列中，如果不为空的话再判断该缓存是否已过期，如果已经过期了则同样把这条请求加入到网络请求队列中，否则就认为不需要重发网络请求，直接使用缓存中的数据即可。之后会调用Request的parseNetworkResponse()方法来对数据进行解析，再往后就是将解析出来的数据进行回调了.

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。

public class NetworkDispatcher extends Thread {  ……  @Override  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;  }  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) {  parseAndDeliverNetworkError(request, volleyError);  } catch (Exception e) {  VolleyLog.e(e, "Unhandled exception %s", e.toString());  mDelivery.postError(request, new VolleyError(e));  }  }  }
}  

看到了类似的while(true)循环，说明网络请求线程也是在不断运行的。访问网络的时候会调用Network的performRequest()方法来去发送网络请求，而Network是一个接口，这里具体的实现是BasicNetwork。

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 = new HashMap<String, String>();  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) {  return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,  request.getCacheEntry() == null ? null : request.getCacheEntry().data,  responseHeaders, true);  }  // 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);  } catch (Exception e) {  ……  }  }  }
}  

调用了HttpStack的performRequest()方法，这里的HttpStack就是在一开始调用newRequestQueue()方法是创建的实例，之后会将服务器返回的数据组装成一个NetworkResponse对象进行返回。

在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据，以及将数据写入到缓存，这个方法的实现是交给Request的子类来完成的，因为不同种类的Request解析的方式也肯定不同。其中parseNetworkResponse()这个方法就是必须要重写的。

在解析完了NetworkResponse中的数据之后，又会调用ExecutorDelivery的postResponse()方法来回调解析出的数据。

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

ResponseDeliveryRunnable

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;  }  // 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();  }  }
}  

ByteArrayPool

缓存接口，代表了一个可以获取请求结果，存储请求结果的缓存。

byte[] 的回收池，用于 byte[] 的回收再利用，减少了内存的分配和回收。 主要通过一个元素长度从小到大排序的ArrayList作为 byte[] 的缓存，另有一个按使用时间先后排序的ArrayList属性用于缓存满时清理元素。

public synchronized void returnBuf(byte[] buf)

将用过的 byte[] 回收，根据 byte[] 长度按照从小到大的排序将 byte[] 插入到缓存中合适位置。

public synchronized byte[] getBuf(int len)

获取长度不小于 len 的 byte[]，遍历缓存，找出第一个长度大于传入参数len的 byte[]，并返回；如果最终没有合适的 byte[]，new 一个返回。

private synchronized void trim()

当缓存的 byte 超过预先设置的大小时，按照先进先出的顺序删除最早的 byte[]。

我是天王盖地虎的分割线