volley是一个非常流行的Android开源框架，自己平时也经常使用它，但自己对于它的内部的实现过程并没有进行太多的深究。所以为了以后能更通透的使用它，了解它的实现是一个非常重要的过程。自己有了一点研究，做个笔记同时与大家一起分享。期间自己也画了一张图，希望能更好的帮助我们理解其中的步骤与原理。如下：

Volley框架图

开始看可能会一脸懵逼，我们先结合源码一步一步来，现在让我们一起进入Volley的源码世界，来解读大神们的编程思想。

newRequestQueue

如果使用过Volley的都知道，不管是进行网络请求还是什么别的图片加载，首先都要创建好一个RequestQueue

public static final RequestQueue volleyQueue = Volley.newRequestQueue(App.mContext);

而RequestQueue的创建自然离不开Volley中的静态方法newRequestQueue，从上面的图片也能知道首先进入点是newRequestQueue,好了现在我们来看下该方法中到底做了什么：

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;
    }    

从上面的源码中我们能发现有一个stack，它代表着网络请求通信HttpClient与HttpUrlConnection，但我们一般都是默认设置为null。因为它会默认帮我们进判断选择更适合的。当手机版本大于等于9时会使用HurlStack它里面使用HttpUrlConnection进行实现通信的，而小于9时则创建HttpClientStack，它里面自然是HttpClient的实现。通过BasicNetwork构造成Network来进行传递使用；在这之后构建了RequestQueue，其中帮我们构造了一个缓存new DiskBasedCache(cacheDir)，默认为5MB,缓存目录为volley。所以我们能在data/data/应用包名/volley下找到缓存。最后调用其start方法，并返回RequestQueue。这就是我们前面第一段代码的内部实现。下面我们进入RequestQueue中的start方法看下它到底做了什么呢？

RequestQueue

在RequestQueue中通过this调用自身来默认帮我们调用了下面的构造函数

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

cache后续在NetworkDispatcher中会帮我们进行response.cacheEntry的缓存，netWork是前面的根据版本所封装的通信，threadPoolSize线程池大小默认为4，delivery,是ExecutorDelivery作用在主线程,在最后对请求响应的分发。

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();
        }
    }

在start方法中我们发现它实现了两个dispatch,一个是CacheDispatcher缓存派遣,另一个是networkDispatcher进行网络派遣，其实他们都是Thread，所以都调用了他们的start方法。其中networkDispatcher默认构建了4个，相当于包含4个线程的线程池。现在我们先不去看他们内部的run方法到底实现了什么，我们还是接着看RequestQueue中我们频繁使用的add方法。

add

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;
        }
    }

我们结合代码与前面的图，首先会为当前的request设置RequestQueue,并且根据情况同步是否是当前正在进行的请求，加入到mCurrentRequests中，看11行代码，之后对当前request进行判断是否需要缓存（默认实现是true，如果不需要可调用request.setShouldCache()进行设置）,如果不需要则直接加入到前面的mNetworkQueue中，它会在CacheDispatcher与NetworkDispatcher中做相应的处理，然后返回request。如果需要缓存，看16行代码，则对mWaitingRequests中是否包含cacheKey进行相应的处理。其中cacheKey为请求的url。最后再加入到缓存队列mCacheQueue中。

finish

细心的人会发现当对应cacheKey的value不为空时，创建了LinkedList即Queue,只是将request加入到了Queue中，只是更新了mWaitingRequests中相应的value但并没有加入到mCacheQueue中。其实不然，因为后续会调用finish方法,我们来看下源码:

<T> void finish(Request<T> request) {
        // Remove from the set of requests currently being processed.
        synchronized (mCurrentRequests) {
            mCurrentRequests.remove(request);
        }
        synchronized (mFinishedListeners) {
          for (RequestFinishedListener<T> listener : mFinishedListeners) {
            listener.onRequestFinished(request);
          }
        }
        if (request.shouldCache()) {
            synchronized (mWaitingRequests) {
                String cacheKey = request.getCacheKey();
                Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
                if (waitingRequests != null) {
                    if (VolleyLog.DEBUG) {
                        VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
                                waitingRequests.size(), cacheKey);
                    }
                    // Process all queued up requests. They won't be considered as in flight, but
                    // that's not a problem as the cache has been primed by 'request'.
                    mCacheQueue.addAll(waitingRequests);
                }
            }
        }
    }

看14、22行代码，正如上面我所说，会将Queue中的request全部加入到mCacheQueue中。
好了RequestQueue的主要源码差不多就这些，下面我们进入CacheDispatcher的源码分析,看它究竟如何工作的呢？

CacheDispatcher

前面提到了它与NetworkDispatcher本质都是Thread，那么我们自然是看run方法

run

@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;
            }
        }
    }

看起来很多，我们结合图来挑主要的看。首先创建了一个无限循环一直在监视着request的变化。从缓存队列mCacheQueue中获取request,如果该请求是cancle了，调用request.finish()清除相应数据并进行下一个请求的操作，否则从前面提到的mCache中获取Cache.Entry。如果不存在或者已经过期，将请求加入到网络队列中mNetWorkQueue，进行后续的网络请求。如果存在（41行代码）则进行request.parseNetworkResponse()解析出response,不同的request对应不同的解析方法。例如StringRequest与JsonObjectRequest有各自的解析实现。再看45行，发现不管entry是否需要更新的，都会进一步对response进行mDelivery.postResponse(request, response)递送，不同的是需要更新的话重新设置request的entry与加入到mNetworkQueue中，也就相当与重新进行网络请求一遍。那么再回到递送的阶段，前面已经提到在创建RequestQueue是实现了ExecutorDelivery,mDelivery.postResponse就是其中的方法。我们来看一下

ExecutorDelivery

在这里创建了一个Executor，对后面进行递送,作用在主线程

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);
            }
        };
    }

postResponse

@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));
    }

这个方法就简单了就是调用execute进行执行，在进入ResponseDeliveryRunnable的run看它如何执行

ResponseDeliveryRunnable

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();
            }
       }

主要是第9行代码，对于不同的响应做不同的递送，deliverResponse与deliverError内部分别调用的就是我们非常熟悉的Listener中的onResponse与onErrorResponse方法，进而返回到我们对网络请求结果的处理函数。

这就是整个的缓存派遣，简而言之，存在请求响应的缓存数据就不进行网络请求，直接调用缓存中的数据进行分发递送。反之执行网络请求。
下面我来看下NetworkDispatcher是如何处理的

NetworkDispatcher

run

@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);
 
                // 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);
            }
        }
    }

在这里也创建了一个无限循环的while,同样也是先获取request，不过是从mQueue中，即前面多次出现的mNetWorkQueue,通过看代码发现一些实现跟CacheDispatcher中的类似。也正如图片中所展示的一样，（23行）如何请求取消了，直接finish；否则进行网络请求，调用(31行)mNetwork.performRequest(request)，这里的mNetWork即为前面RequestQueue中对不同版本进行选择的stack的封装，分别调用HurlStack与HttpClientStack各自的performRequest方法，该方法中构造请求头与参数分别使用HttpClient或者HttpUrlConnection进行网络请求。我们再来看42行，是不是很熟悉，与CacheDispatcher中的一样进行response进行解析，然后如果需要缓存就加入到缓存中，最后（54行）再调用mDelivery.postResponse(request, response)进行递送。至于后面的剩余的步骤与CacheDispatcher中的一模一样，这里就不多累赘了。

好了，Volley的源码解析先就到这里了，我们再回过去看那张图是不是感觉很清晰了呢?

总结

我们来对使用Volley网络请求做个总结

• 通过newRequestQueue初始化与构造RequestQueue
• 调用RequestQueue中的add方法添加request到请求队列中
• 缓存派遣，先进行CacheDispatcher,判断缓存中是否存在，有则解析response，再直接postResponse递送，否则进行后续的网络请求
• 网络派遣，NetworkDispatcher中进行相应的request请求，解析response如设置了缓存就将结果保存到cache中，再进行最后的postResponse递送。

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