Volley 源码分析

Volley 源码分析

图片分析

要说源码分析，我们得先看一下官方的配图：

从这张图中我们可以了解到 volley 工作流程：

1.请求加入优先队列

2.从缓存调度器中查看是否存在该请求，如果有（没有进入第三步）直接缓存中读取并解析数据，最后分发到 UI 线程（主线程）。

3.从网络中获取数据（如果设置可以缓存，则写入缓存）并解析数据，最后分发到 UI 线程（主线程）。

从图中，我们还可以看到 volley 的工作其实就是三个线程之间的数据传递 主线程 缓存线程 网络线程。

代码分析

既然是源码分析，当然是得从源码开始啦（怎么下载源码我就不说了！不会的自行谷歌）！那我们从哪段源码开始嘞？我们就从我们使用 volley 框架的第一句代码开始。

Volley.newRequestQueue(context) 是我们使用 volley 框架的第一句代码！这句代码是用创建个 RequestQueue (请求队列。对，这个就是 volley 工作流程中的第一步的铺垫。这样请求才能有容器装啊！)。那好，我们现在就来看一下 newRequestQueue(context) 这个静态方法：

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; } 

从代码中可以看出，我们使用的是 newRequestQueue(Context context, HttpStack stack) 这个函数的重载方法！我简要对这个方法说明下：在系统版本大于等于 9 的时候，我们创建 HurlStack(就是 HttpUrlConnection),在小于 9 的时候我们创建 HttpClientStack(就是 HttpClient) 至于为啥要要这么做。那就是 HttpUrlConnection 的性能要比HttpClient 的好。当然这里我还可以使用另外第三发的 HTTP 库 。比如说 okhttp 。这里我们只要调用 newRequestQueue(Context context, HttpStack stack) 这个方法就行了！当然 要对 okhttp 做简单的封装了！前提是要继承 HttpStack 这个接口啊！

接下来 我们直接看这句代码：RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); 这就是我们要创建的请求队列啦！看 RequestQueue 这个类：

我们先看构造函数（直接看最复杂的！哈哈）

public RequestQueue(Cache cache, Network network, int threadPoolSize,         ResponseDelivery delivery) {     mCache = cache;     mNetwork = network;     mDispatchers = new NetworkDispatcher[threadPoolSize];     mDelivery = delivery; }

可以看到，需要传入 缓存 ，网络执行器，线程池大小，返回结果分发器。这四个参数！接着 我们再来看一下 queue.start() 这句代码的含义！还是一样先看代码：

 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  (mNetworkQueue, mNetwork,     mCache, mDelivery);   mDispatchers[i] = networkDispatcher;   networkDispatcher.start();  } } 

先来解释下这这段代码：先停止当前正在运行的所有的线程！接着 初始化 CacheDispatcher(缓存调度器) 并启动他！接着就是启动 NetworkDispatcher(网络调度器) 这有多个。他的个数全靠 threadPoolSize 这个变量控制（默认大小是4）。这样一来。请求队列就是初始化好了！就等待任务加入了啦！！那我们就趁热打铁，直接看加入任务队列的源码：

 public Request   {   request.setRequestQueue(this);  synchronized (mCurrentRequests) {   mCurrentRequests.add(request);  }  request.setSequence(getSequenceNumber());  request.addMarker("add-to-queue");  if (!request.shouldCache()) {   mNetworkQueue.add(request);   return request;  }  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 {    mWaitingRequests.put(cacheKey, null);    mCacheQueue.add(request);   }   return request;  } } 

依旧是来解读这段代码，在解读这该段代码的时候，我们先来弄清楚其中三个变量的含义：

//当前 RequestQueue 中所有的请求队列 private final Set<Request> mCurrentRequests = new HashSet<Request>(); //缓存队列 private final PriorityBlockingQueue<Request> mCacheQueue =     new PriorityBlockingQueue<Request>(); //网络队列 正在进入的 private final PriorityBlockingQueue<Request> mNetworkQueue =     new PriorityBlockingQueue<Request>();     

含义如注释！那么我们接着看 add(Request request) 这个方法！首先还是先把任务加入 当前队列。之后request.shouldCache() 判断该任务需要被缓存。不需要的话 直接进入 网络队列！否则的话就加入缓存队列！！

分析完了怎么加入队列之后，我们要来分析下两外两个类了 CacheDispatcher 和 NetworkDispatcher 这里先说下这两个类的共同点：那就是都继承了 Thread 类！也就说他们都是线程类！可以被执行。这也就解释了 RequestQueue 类中的start() 方法中 mCacheDispatcher.start() 和 networkDispatcher.start() 这两句代码！

那么我们先来看一下 CacheDispatcher 这个类！我们直接该类最核心的方法 run() 方法！

 public void run() {  if (DEBUG) VolleyLog.v("start new dispatcher");  Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);  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;   }  } }   

这段代码中有句代码非常抢眼，没错！那就是 while(true) 这句话了！有的童鞋可能已经想到了：这是个死循环(这不废话！)，另外肯定是在不同的从某个队列中取/存数据。没错，就是这样啊！其实很简单！我们接着细说：首先从缓存队列中去除队列，接着判断该请求是否已经取消 if (request.isCanceled()) 如果已经取消的话，就是不走下面的代码！继续从头循环！反之，从缓存中读取数据，如果没有的话就把该队列加入网络请求队列。如果有的但是缓存已经过期的话 也是加入网络请求队列（ if (entry == null) 和 if (entry.isExpired()) 这两个 if 下的语句就是处理上面两个功能的）。如果以上两个条件都不满足的话！就直接 request.parseNetworkResponse(new NetworkResponse(entry.data, entry.responseHeaders)) 解析缓存中的数据进行回调了！

下面我们看 NetworkDispatcher 类的代码，同样的我们直接看 核心代码:

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.    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));   }  } }   

同样的是，先从网络请求队列中取出任务，接着在判断是否要取消，如果要则跳过下面的代码，重新取任务！调用 NetworkResponse networkResponse = mNetwork.performRequest(request); 这句代码，获取 网络返回结果！接着，代码和 CacheDispatcher 中差不多！唯一的区别就是：如果当前的请求需要加入缓存，则加入缓存！细心的同学可能发现了，CacheDispatcher 和 NetworkDispatcher 这两个类中有句相同的代码 Response<?> response = request.parseNetworkResponse(networkResponse); 就是这句！核心的就是parseNetworkResponse(networkResponse) 这个函数的实现我在上一篇 Volley 的使用以及自定义Request 中已经说过了！是有我们实现的！

这里的我们还得在注意一个类：那就是 BasicNetwork ！其实这个类没有什么可以细说的！他就是请求网络接着返回结果！我这里也把核心代码上一下：

  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().data, responseHeaders, true);    }    responseContents = entityToBytes(httpResponse.getEntity());    // if the request is slow, log it.    long requestLifetime = SystemClock.elapsedRealtime() - requestStart;    logSlowRequests(requestLifetime, request, responseContents, statusLine);    if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {     throw new IOException();    }    return new NetworkResponse(statusCode, responseContents, responseHeaders, false);   } 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);     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);    }   }  } } 

这个类 我就真的不细讲啦！！

上面已经说了，在解析玩数据之后其实就分发数据了！让用户能够在 UI线程中调用了！现在我们就来看一下这个分发类：ExecutorDelivery。 我们还是先看这个类的构造函数：

 public  (final Handler handler) {  // Make an Executor that just wraps the handler.  mResponsePoster = new Executor() {   @Override   public void execute(Runnable command) {    handler.post(command);   }  }; } 

从代码中我们可以看到，我们需要传一个Handller进入，此时我们在回想一下这个类在 RequestQueue 类中是怎么初始化的？

 public RequestQueue(Cache cache, Network network, int threadPoolSize) {     this(cache, network, threadPoolSize,             new ExecutorDelivery(new Handler(Looper.getMainLooper()))); }

没错！大家可以看到，是传入了一个主线中的handler！

当在 CacheDispatcher 和 NetworkDispatcher 这两个类中调用了 mDelivery.postResponse(request, response); 这个方法的时，我们来看一下 ExecutorDelivery 这个类都做了什么！

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

我们直接执行了了个Runnable-->ResponseDeliveryRunnable。那他又做了什么呢？

 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();  } } 

我们看最直接的这句代码 mRequest.deliverResponse(mResponse.result); 我们调用了 Request的中的一个方法（这个方法，依然是要我们自己实现！）。接着，我们在看 ExecutorDelivery 的构造函数时，我们就会豁然开朗！数据终于到了 UI线程了！

至此 volley 框架的整体流程分析完毕！！！！

还在说几句：volley 框架的架构设计非常优美！扩展性极高！这个大概得益于 volley 面向接口的设计方案吧！面向接口的架构设计 也是我不断努力的方向！！！！