一口一口吃掉Volley（四）

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()

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()

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()

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()

@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()

@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()

@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：

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进行回调：

• run()

 @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();
     }
}

可以看到，11行是调用Request的deleverResponse方法将结果回调给Request。举例看一下StringRequest中该方法是如何实现的：

• deliverResponse()

@Override
    protected void deliverResponse(String response) {
        if (mListener != null) {
            mListener.onResponse(response);
        }
    }

直接通过我们构造StringRequest时传进来的Listener的回调方法onResponse来将结果回调给Activity。deleverError也是同样的做法。

---

看完网络线程NetworkDispatcher之后再来看一下缓存线程CacheDispatcher是如何工作的

---

最后来看CacheDispatcher的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();

    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() {
                    @Override
                    public 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());
        }
    }
}

首先在10行可以看到一个while(true)循环，说明缓存线程始终是在运行的，
接着在第33行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。

之后会在第39行调用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，跟上面的回掉思路是完全一样的！

---

至此，我们可以通过通过Volley官方提供的流程图重新回顾一下整个的流程

Volley流程图

其中蓝色部分代表主线程，绿色部分代表缓存线程，橙色部分代表网络线程。我们在主线程中调用RequestQueue的add()方法来添加一条网络请求，这条请求会先被加入到缓存队列当中，如果发现可以找到相应的缓存结果就直接读取缓存并解析，然后回调给主线程。如果在缓存中没有找到结果，则将这条请求加入到网络请求队列中，然后处理发送HTTP请求，解析响应结果，写入缓存，并回调主线程。

希望通过这个系列的文章你能够清晰的掌握和理解Volley，尽管他现在已经不流行了，接下来我会持续为大家讲解比较好的开源框架，TX☺