Volley原理分析之网络请求层

前言

13年google就推出volley了，作为一个喜欢使用这个网络请求框架的娃，也是时候研究研究下该框架的原理了。

初始化

初始化volley，大家都知道会调用Volley.newRequestQueue()，那我们就沿着源码追溯下去。

     /**     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.     * You may set a maximum size of the disk cache in bytes.     *     * @param context A {@link Context} to use for creating the cache dir.     * @param stack An {@link HttpStack} to use for the network, or null for default.     * @param maxDiskCacheBytes the maximum size of the disk cache, in bytes. Use -1 for default size.     * @return A started {@link RequestQueue} instance.     */    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) {                //HurlStack其实是封装了HttpURLConnection的类                stack = new HurlStack();            } else {                // Prior to Gingerbread, HttpUrlConnection was unreliable.                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html                //封装了HttpClient类                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);        }        //注意这里将启动队列        queue.start();        return queue;    }

在上面代码中，关键点如下：
1. 初始化BasicNetwork。这里根据sdk版本选择不同的网络请求类，他的实现正是该框架请求网络所使用的网络请求类，根本还是依赖HttpURLConnection 和 HttpClient
2. 初始化RequestQueue。这是请求分发的队列，构造函数中初始化执行网络请求的线程数为4，而且还初始化ExecutorDelivery，这个是负责处理响应的接口，负责把response传给主线程

    /**     * Starts the dispatchers in this queue.     */    public void start() {        //调用stop()后之前初始化的缓存线程和网络请求线程都会销毁        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.        //初始化是四个线程，注意，这里不是用线程池        for (int i = 0; i < mDispatchers.length; i++) {            NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,                    mCache, mDelivery);            mDispatchers[i] = networkDispatcher;            networkDispatcher.start();        }    }

1. CacheDispatcher与NetworkDispatcher都是继承自线程，这里对总共开启了五个线程，1个缓存线程，4个网络工作线程。
2. 两者都传入mNetworkQueue这个参数，其实例是 PriorityBlockingQueue

 * <p>{@code BlockingQueue} implementations are **thread-safe**.  All * queuing methods achieve their effects atomically using internal * locks or other forms of concurrency control

可见，BlockQueue是线程安全的，这也是不直接使用Queue的原因，而在RequestQueue中，有个currentRequest，这个是直接使用HashSet，作为记录当前的请求，以便进行cancelAll的处理，这里并没有用上线程安全的集合操作类。
到这里，volley就已经准备就绪了

发起请求

通过RequestQueue.addRequest()，我们把自己的请求信息传递进volley中：

    /**     * Adds a Request to the dispatch queue.     * @param request The request to service     * @return The passed-in request     */    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.        //这个序列号是获取的AtomicInteger的自增←_←        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()) {            //添加到这里的时候，就会被工作线程所轮询了咯            //但是默认情况下，shouldCache都是true,也即一般不会直接跳过cache            mNetworkQueue.add(request);            return request;        }        // Insert request into stage if there's already a request with the same cache key in flight.        synchronized (mWaitingRequests) {            //已经发出请求的东东都丢进请求队列，如果多个相同请求，则丢到等待hashMap中去。            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;        }    }

request不是直接丢进networkQueue让工作线程执行，而是先丢进cacheQueue中，让其miss之后再方进networkQueue,再执行网络请求。

执行请求

现在来看一下工作线程，轮询的那部分：

        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.                //主要核心请求，mNetwork就是初始化的netWrok请求方式                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.                //这里对response进行解析，调用自己覆盖的方法，注意                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) {                //注意这里就把请求缓存起来了                    mCache.put(request.getCacheKey(), response.cacheEntry);                    request.addMarker("network-cache-written");                }                // Post the response back.                request.markDelivered();                //将response发送给主线程的handler                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);            }        }

经过网络请求的调用，获得response后，就通过mDelivery.postResponse()将response传递给Request，并调用具体请求实现类的deliverReponse方法。在具体实现类中，回调listener的onResponse方法，成功实现response的回调，精妙之处在于ExecutorDelivery中完成跨线程通信，使工作线程能够切换至UI线程。
分发器原理主要是靠Executor实现，并通过handler post转移

    /**     * Creates a new response delivery interface.     * @param handler {@link Handler} to post responses on     */    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);            }        };    }

总结

整个volley的网络请求其实已经很清晰了，实质就是通过线程轮询任务队列来达到并行操作的效果，在处理线程安全与线程通信方面做到了恰到好处，缓存请求的调度使volley请求的效率提高。
可是给我还留下一些小疑问，就是如果是使用线程池来实现，并发性能是否会更好？这个得等我好好分析先，另外对于NetworkImageView，我也会继续探究下去，欢迎大家一起来交流。