Volley的源码分析

Volley是2013年Google I/O大会上推出了一个新的网络通信框架。因为Volley集成了AsyncHttpClient和Universal-Image-Loader的优点集于了一身，能使网络通信更快，更简单，也可以像Universal-Image-Loader一样轻松加载网络上的图片，所以深受广大开发者的喜爱。虽然volley现在已经过时，已经有其他如okhttp等优秀开源框架可以代替，但是volley的编程思想和源码还是有很多东西值得我们学习。
优点：
1. 适合进行通信频繁的网络操作
2. volley是开源的，可以根据自己的需求进行扩展封装，例如：自定义XmlRequest,GsonRequest可扩展性很强。

缺点：
1. 对于大数据量的网络操作，比如文件下载，volley则表现的不好。
2. volley使用的是使用的是httpclient、HttpURLConnection，但是6.0以后不在支持httpclient

Volley的工作流程图，如下图所示。

RequestQueue.add添加一条网络请求，首先这个request会被添加到到cacheQueue对列当中，如果缓存中有相应的缓存，则读取—>解析—>回调给主线程，如果缓存中没有，则添加到网络请求对列当中，则http request—>paresed—>cache write—>回调给主线程。
这是整个volley请求的思路，我们沿着这条主线进行阅读源码。

• 1.要把http request添加到requestQueue中，就需要先创建requestQueue,下面就从Volley.newRequestQueue(context)开始阅读

 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;    }

首先获取网络缓存路径，context.getCacheDir()获取的路径/data/data/PackageName/cache目录，其次应用的一些信息，第10行开始创建http栈，如果手机系统版本大于9即Android 2.3，则用HurlStack,版本小于9则用HttpClientStack，打大HurlStack在performRequest中可以看到HttpURLConnection connection = openConnection(parsedUrl, request); 而HttpClientStack则用的是HttpClient进行网络通讯的。最后创建RequestQueue网络请求对列，返回queue 。

总结：在创建requestQueue时，当手机系统版本大于9用的是HttpURLConnection ，小于9则是HttpClient，现在基本上没有版本是9的系统的手机了。所以HttpClient可以忽略。
但是为什么现在高版本，或者说主流都是用HttpURLConnection进行网络通讯呢？

1. HttpClient：基于apache,是个重量级的程序而且API数量很多，在Android2.3之前httpClient的bug少，而且实现比较稳定。HttpURLConnection存在bug较多。

2. HttpURLConnection：基于Java的轻量级，在Android2.3以后HttpURLConnection修复完善，存在很少的bug，多用途，api少，简单，具有压缩和缓存机制可以有效的减少网络访问的流量，在提高速度和省电方面起到很大的作用，另外，在Android6.0以后HttpClient库 已经被移除，因而非常适用于Android项目。

   下面看一下queue.start()的源码

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

首先在requestQueue启动的时候，要停止所有的正在执行的线程，CacheDispatcher（是缓存线程）和NetworkDispatcher （网络请求线程）都是继承与Thread , Dispatchers的length默认是DEFAULT_NETWORK_THREAD_POOL_SIZE =4。也就是说当requestQueue启动的时候有5个线程启动，1个缓存线程，4个网络请求线程。

• 2.获取了requestQueue之后就是添加网络请求Request。

下面看一下requestQueue.add(request)的源码：

  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;        }    }

首先将request请求添加到mCurrentRequests当中，mCurrentRequests是保存当前需要处理的所以request。
接下里看如果request.shouldCache()设置不缓存，则直接将request添加到网络请求队列中，然后返回。
然后判断该请求是否有相同的请求正在被处理，如果有则加入mWaitingRequests；如果没有，则加入mWaitingRequests.put(cacheKey, null)和mCacheQueue.add(request)。

• 3.接下来看一下缓存线程的工作原理，查看CacheDispatcher.run()

 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());            }        }    }

首先设置进入while(true)中，request = mCacheQueue.take()中获取request请求对象，
如果request.isCanceled()已经退出则结束，继续执行下一个request，
获取缓存Cache.Entry entry = mCache.get(request.getCacheKey());
如果entry==null,则将该请求添加到网络请求中mNetworkQueue.put(request);
如果entry.isExpired()失效，则将该请求添加到网络请求中mNetworkQueue.put(request);
接下来就是 parseNetworkResponse()方法来对数据进行解析。parseNetworkResponse()必须有子类去实现。

总结：缓存线程中获取request，如果没有取消，则去获取缓存数据，如果缓存不存在或者过期失效，都会加入到网络请求队列当中，如果存在缓存数据在解析数据parseNetworkResponse（response），返回给主线程中deliverResponse(response)。

• 4.看一下NetworkDispatcher.run()方法

 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);            }        }    }

从消息队列中获取request，然后request执行mNetwork.performRequest(request)发送网络请求，而Network是一个接口，这里具体的实现是BasicNetwork，mNetwork.performRequest(request)里面主要是http网络请求的底层封装，版本大于9时用httpUrlClient，小于9用httpClient.
在NetworkDispatcher中收到了NetworkResponse这个返回值后,调用Request.parseNetworkResponse()方法来解析NetworkResponse中的数据，接着是是否需要设置缓存（setShouldCache()），需要则存入缓存。
最后调用mDelivery.postResponse(request, response)

• 看一下mDelivery.postResponse(request, response)的源码

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

ResponseDeliveryRunnable实现了Runnable接口，ResponseDeliveryRunnable的run()方法

        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();            }       }

如果成功则调用mRequest.deliverResponse(result)，stringRequest里面的实现是mListener.onResponse(response)回调给成功接口。
进入mResponsePoster.execute()源码看看

 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);            }        };    }

内部封装的handler.post(runable)完成了数据向主线程的传递。

volley的网络请求思想就是延续开头我们描述的流程走，volley是个优秀的开源框架，具有很好的扩展性，如果想根据自己的需求自定义网络请求格式，只需要继承Request，实现俩个重要的方法就可以了；

1. parseNetworkResponse（NetworkResponse response），发生在子线程中，解析网络请求数据。
   获取网络请求的数据response.data是以字节的形式存在，我们在此根据需求转换成xmlRequest或者GsonRequest…

2. deliverResponse(T response)传递返回
   response是子线程获取的数据（缓存或者网络请求的数据）利用handler消息机制回调到主线程。通过接口返回给我们需要的场景。