Volley库源码分析

request = mQueue.take();

NetworkResponse networkResponse = mNetwork.performRequest(request);

Response<?> response = request.parseNetworkResponse(networkResponse);

mCache.put(request.getCacheKey(), response.cacheEntry);

@Override
    public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        Request request;
        while (true) {
            try {
                // Take a request from the queue.
                request = mQueue.take();//1.从请求队列中取出一个网络请求，mQueue是BlockingQueue<Request>的实现类
            } 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;
                }

                // Tag the request (if API >= 14)
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
                    TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
                }

                // Perform the network request.
                NetworkResponse networkResponse = mNetwork.performRequest(request);//2.发起网络请求
                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);//3.把这个networkResponse转化为期望的数据类型
                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);//4.将网络响应加入缓存
                    request.addMarker("network-cache-written");
                }

                // Post the response back.
                request.markDelivered();
                mDelivery.postResponse(request, response);
            } catch (VolleyError volleyError) {
                parseAndDeliverNetworkError(request, volleyError);
            } catch (Exception e) {
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
                mDelivery.postError(request, new VolleyError(e));
            }
        }
    }

class Producer implements Runnable {
   private final BlockingQueue queue;
   Producer(BlockingQueue q) { queue = q; }
   public void run() {
     try {
       while (true) { queue.put(produce()); }
     } catch (InterruptedException ex) { ... handle ...}
   }
   Object produce() { ... }
 }

 class Consumer implements Runnable {
   private final BlockingQueue queue;
   Consumer(BlockingQueue q) { queue = q; }
   public void run() {
     try {
       while (true) { consume(queue.take()); }
     } catch (InterruptedException ex) { ... handle ...}
   }
   void consume(Object x) { ... }
 }

 class Setup {
   void main() {
     BlockingQueue q = new SomeQueueImplementation();
     Producer p = new Producer(q);
     Consumer c1 = new Consumer(q);
     Consumer c2 = new Consumer(q);
     new Thread(p).start();
     new Thread(c1).start();
     new Thread(c2).start();
   }
 }}

/** The queue of requests to service. */
    private final BlockingQueue<Request> mQueue;
    /** The network interface for processing requests. */
    private final Network mNetwork;
    /** The cache to write to. */
    private final Cache mCache;
    /** For posting responses and errors. */
    private final ResponseDelivery mDelivery;
    /** Used for telling us to die. */
    private volatile boolean mQuit = false;

    /**
     * Creates a new network dispatcher thread.  You must call {@link #start()}
     * in order to begin processing.
     *
     * @param queue Queue of incoming requests for triage
     * @param network Network interface to use for performing requests
     * @param cache Cache interface to use for writing responses to cache
     * @param delivery Delivery interface to use for posting responses
     */
    public NetworkDispatcher(BlockingQueue<Request> queue,
            Network network, Cache cache,
            ResponseDelivery delivery) {
        mQueue = queue;
        mNetwork = network;
        mCache = cache;
        mDelivery = delivery;
    }

/**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     * @param delivery A ResponseDelivery interface for posting responses and errors
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize,
            ResponseDelivery delivery) {
        mCache = cache;
        mNetwork = network;
        mDispatchers = new NetworkDispatcher[threadPoolSize];
        mDelivery = delivery;
    }
	
	/**
     * Starts the dispatchers in this queue.
     */
    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();
        }
    }

网络请求中有几个转换解析请求获取响应结果的地方：

1.HttpStack接口的performRequest()方法

public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)

2.Network接口的performRequest()方法

public NetworkResponse performRequest(Request<?> request)

3.Request类的parseNetworkResponse()抽象方法

abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

可以很鲜明得看出第一个是对原生Http请求的解析，解析出来是一个Apach的HttpResponse 实例，这个结果就是通过上述两个HttpStack的实现类HttpClientStack和HurlStack执行获取的获取的。

而第二个解析出来是框架自定的NetworkResponse，这是通过Network的实现类BasicNetwork来获取的。

第三个就是第二个得出来NetworkResponse解析成用户期望Response<T> 了，这个Response和Request是对应的，有String型，json型还有Image型的。然后在通过ResponseDelivery把解析好的结果发送到主线程。

从Request到Response就是这样一步步走来的。

/**
     * @title performRequest执行各种Request请求并以NetworkResponse的形式返回结果
     * @param Request
     * @return NetworkResponse
     * @throws VolleyError
     * 定义：{@link Network#performRequest(Request)}
     * 被调：{@link NetworkDispatcher#run()}
     * 
     */
    @Override//NetworkDispatcher的run()方法中调用
    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        long requestStart = SystemClock.elapsedRealtime();//开始请求时间
        while (true) {
            HttpResponse httpResponse = null;//apache的请求结果
            byte[] responseContents = null;//请求的内容
            Map<String, String> responseHeaders = new HashMap<String, String>();//响应结果头部信息
            try {
                // Gather headers.
                Map<String, String> headers = new HashMap<String, String>();//保存缓存数据
                addCacheHeaders(headers, request.getCacheEntry());//先获取缓存数据
                httpResponse = mHttpStack.performRequest(request, headers);//去调用mHttpStack的实现方法执行请求
                StatusLine statusLine = httpResponse.getStatusLine();//获取http状态线
                int statusCode = statusLine.getStatusCode();//获取状态码

                responseHeaders = convertHeaders(httpResponse.getAllHeaders());
                // Handle cache validation.//处理缓存验证
                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {//返回缓存数据
                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
                            request.getCacheEntry().data, responseHeaders, true);
                }

                //把HttpEntity转化为byte[]数据
                responseContents = entityToBytes(httpResponse.getEntity());
                // if the request is slow, log it.//如果请求很慢，就打印出来看一下
                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
                logSlowRequests(requestLifetime, request, responseContents, statusLine);//打印

                //连接正常但是返回无内容，抛出IO异常
                if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {
                    throw new IOException();
                }
                return new NetworkResponse(statusCode, responseContents, responseHeaders, false);
            } catch (SocketTimeoutException e) {//读取超时，重试
                attemptRetryOnException("socket", request, new TimeoutError());
            } catch (ConnectTimeoutException e) {//连接超时，重试
                attemptRetryOnException("connection", request, new TimeoutError());
            } catch (MalformedURLException e) {//Bad URL
                throw new RuntimeException("Bad URL " + request.getUrl(), e);
            } catch (IOException e) {//IO异常
                int statusCode = 0;
                NetworkResponse networkResponse = null;
                if (httpResponse != null) {
                    statusCode = httpResponse.getStatusLine().getStatusCode();
                } else {//如果没有返回httpResponse，就说明没连接
                    throw new NoConnectionError(e);
                }
                VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
                if (responseContents != null) {//返回数据不为空
                    networkResponse = new NetworkResponse(statusCode, responseContents,
                            responseHeaders, false);//创建响应体
                    if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
                            statusCode == HttpStatus.SC_FORBIDDEN) {//认证失败异常，重试
                        attemptRetryOnException("auth",
                                request, new AuthFailureError(networkResponse));
                    } else {//服务器异常
                        // TODO: Only throw ServerError for 5xx status codes.
                        throw new ServerError(networkResponse);//只有状态码为5XX才抛出服务器异常
                    }
                } else {//网络异常
                    throw new NetworkError(networkResponse);
                }
            }
        }
    }

A：先是通过mHttpStack把请求执行并且获取它的响应结果，根据HttpStatus做出各种判断。

B：然后再把httpResponse的Entity转化为ByteArray，并处理各种发生的异常。

C：最后的过程是这样的：通过Volley创建一个RequestQueue请求队列，当这个队列开始运作的时候会启动NetworkDispatcher这个工作线程，而BasicNetwork的performRequest()的方法就在NetworkDispatcher线程run()方法中调用，然后通过mHttpStack的performRequest()方法获取一个networkResponse，在NetworkDispatcher线程把这个networkResponse转化为期望的数据类型，比如Response<String>,Response<Json>,Response<Bitmap>。