非常感谢你能够坚持看到第四篇，同时这也是这个Volley系列教程的最后一篇了。经过前三节的学习，相信你也已经懂得如何运用Volley提供的Request以及自定义Request了，这一节我将从源码的角度带领大家理解Volley的工作流程。

从newRequestQueue()看起

我们都知道，使用Volley最开始要做的就是使用newRequestQueue()获取一个RequestQueue对象，仔细看一下这个方法

  • newRequestQueue()
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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.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }
        Network network = new BasicNetwork(stack);
        
        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);
        }
        queue.start();
        return queue;
    }
在方法内部我们可以看到在api等级大于9的时候，使用HurlStack实例来进行主要的网络请求工作，到这里已经很明显了，Volley底层是使用HttpUrlConnection进行的；而对于小于9的API则创建否则就创建一个HttpClientStack的实例，也就是对于9之前的API使用HttpClient进行网络通讯。最后被包装为一个BasicNetwork对象。
接着根据得到的BasicNetwork对象和一个DiskBasedCache对象（磁盘缓存）来构造一个RequestQueue，并且调用了它的start方法来启动这个线程。
接着看start()
  • start()
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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();
        }
    }
首先先创建CacheDispatcher对象，接着进入for循环这个for循环遍历了mCacheDispatcher，这个mCacheDispatcher其实相当于一个线程池，这个线程池的大小默认是4。然后分别让这里面的线程运行起来（调用了它们的start方法）。这里为什么要有多个线程来处理呢？原因很简单，因为我们每一个请求都不一定会马上处理完毕，多个线程进行同时处理的话效率会提高。 所以最终这里会有5个线程，4个是网络线程NetworkDispatcher，1个是缓存线程CacheDispatcher。
得到了RequestQueue之后，我们只需要构建出相应的Request，然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了，那就先来看一下add()吧！
add()方法
  • add()
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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;
        }
    }
可以看到，在第13行的时候会判断当前的请求是否可以缓存，如果不能缓存则在第14行直接将这条请求加入网络请求队列，可以缓存的话则在第36行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。 
那就先来看看NetworkDispatcher的run()吧!
  • run()
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@Override
    public 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);
            }
        }
    }
第4行设置了这些线程的优先级，这个优先级比较低，目的是为了尽量减少对UI线程的影响保证流畅度。  

接着第12行，调用mQueue的take方法取出队列头的一个请求进行处理，这个mQueue就是我们在上面add方法中添加进去的一个请求。  

直接看到第34行，如果请求没有被取消，也就是正常的情况下，我们会调用mNetwork的performRequest方法进行请求的处理。不知道你还记的这个mNetwork不，它其实就是我们上面提到的那个由HttpUrlConnection层层包装的网络请求对象。  

如果请求得到了结果，我们会看到55行调用了mDelivery的postResponose方法来回传我们的请求结果。
先来看performRequest()
因为Network是一个接口，这里具体的实现是BasicNetwork，所以我们可以看到其中重写的performRequest()如下：  
  • performRequest()
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@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());
                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#sec10.3.5
                    entry.responseHeaders.putAll(responseHeaders);
                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
                            entry.responseHeaders, true,
                            SystemClock.elapsedRealtime() - requestStart);
                }
                
                // Handle moved resources
                if (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 (Exception e) {
                ···
            } 
        }
    }
这段代码中，先10和11行代码将cache的属性设置给header，接着第12行调用mHttpStack对象的performRequest方法并传入请求对象和头部来进行请求，得到一个HttpResponse对象。  

接着将HttpResponse对象中的状态码取出，如果值为HttpStatus.SC_NOT_MODIFIED（也就是304），则表示请求得到的Response没有变化，直接显示缓存内容。  

第45行表示请求成功并且获取到请求内容，将内容取出并作为一个NetworkResponse对象的属性并返回给NetworkDispatcher。  

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，这就是为什么我们在自定义Request的时候必须要重写parseNetworkResponse()这个方法的原因了。
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。
接着是postResponse()
  • postResponse()
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@Override
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
        request.markDelivered();
        request.addMarker("post-response");
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    }
这里看到第5行调用了mResponsePoster的execute方法并传入了一个ResponseDeliveryRunnable对象，再看mResponsePoster的定义：  
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public ExecutorDelivery(final Handler handler) {
       // Make an Executor that just wraps the handler.
       mResponsePoster = new Executor() {
           @Override
           public void execute(Runnable command) {
               handler.post(command);
           }
       };
   }
也就是我们在这里把ResponseDeliveryRunnable对象通过Handler的post方法发送出去了。这里为什么要发送到MainLooper中？因为RequestQueue是在子线程中执行的，回调到的代码也是在子线程中的，如果在回调中修改UI，就会报错。再者，为什么要使用post方法？原因也很简单，因为我们在消息发出之后再进行回调，post方法允许我们传入一个Runnable的实现类，post成功会自动执行它的run方法，这个时候在run方法中进行结果的判断并且