Volley源码解析

之前看过郭神的blog，今天也是着按照自己的思路走一遍Volley框架中关于Http请求的源码
首先还是引用一下郭神的图，清楚的阐述了当网络连接请求到来时的工作机制

缓存这个东西几乎是无处不在，无论我们从事的是什么计算机的方向，都对性能优化有着至关重要的作用。
可以看到架构图中，当request到来时，首先判断是否已经缓存过，如果没有直接加入network中，然后是一系列的http的解析等工作，下面会通过源码来详细解析。如果已经缓存过，就从缓存队列中读取并解析。最后把解析结果回调给主线程完成http请求响应工作。

Volley.java

/**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);
    }

从注释中可以清晰的看到，这是创建RequestQueue实例的起点，会调用另一个newRequestQueue的方法，那就跟进去一起看一下。
然而蛋疼的是：

/**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @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.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context, HttpStack stack)
    {
        return newRequestQueue(context, stack, -1);
    }

another method appear。这里可以发现，当我们只是创建RequestQueue的时候，其实就是把第二个方法的HttpStack参数传入null值。
继续跟进！

/**
     * 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) {
                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;
    }

第三个参数的意思是是否使用DiskCache，默认的话我们使用内存缓存，应该是不使用磁盘缓存的吧。
这段代码中的

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));
            }
        }

首先判断传入的httpstack是否为空，如果为null，然后再去判断系统的版本号选择性的创建HurlStack还是HttpClientStack。从注释中看到，当在Gingerbread版本之前的Android系统里面，HttpUrlConnection是不受信任的，不安全的。当对一个可读的InputStream进行close的时候，用于管理多个长连接的连接池可能也会因此失效。
接下来创建一个Network用于根据传入的HttpStack对象处理网络请求

Network network = new BasicNetwork(stack);
跟进去看一下BasicNetwork的构造方法

public class BasicNetwork implements Network {
    protected static final boolean DEBUG = VolleyLog.DEBUG;

    private static int SLOW_REQUEST_THRESHOLD_MS = 3000;

    private static int DEFAULT_POOL_SIZE = 4096;

    protected final HttpStack mHttpStack;

    protected final ByteArrayPool mPool;

    /**
     * @param httpStack HTTP stack to be used
     */
    public BasicNetwork(HttpStack httpStack) {
        // If a pool isn‘t passed in, then build a small default pool that will give us a lot of
        // benefit and not use too much memory.
        this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
    }

    /**
     * @param httpStack HTTP stack to be used
     * @param pool a buffer pool that improves GC performance in copy operations
     */
    public BasicNetwork(HttpStack httpStack, ByteArrayPool pool) {
        mHttpStack = httpStack;
        mPool = pool;
    }

构造方法主要涵盖了两个参数HttpStack、字节缓冲区(用于改善在复制操作时GC的性能)。如果没有传入字节缓冲区的参数，就构造一个默认大小的缓冲区。跳出BasicNetwork的源码，继续来看上面的源码。在创建出Network对象后，在判断我们是否指定了DiskCache之后，new出一个RequestQueue对象并调用start()，最后返回RequestQueue。
跟进RequestQueue中去查看start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();
        }
    }

首先创建了CacheDispatcher的实例，调用start()。随后循环创建NetworkDispatcher并调用start()。CacheDispatcher与NetworkDispatcher都是继承的是线程

public class NetworkDispatcher extends Thread
public class CacheDispatcher extends Thread
所以说当我们从一开始创建RequestQueue实例开始，直到现在，会分别创建NetworkDispatcher与CacheDispatcher线程用于处理网络请求。
回想我们使用Volley框架时的方法，首先构建RequestQueue，然后我们就把request加入到RequestQueue当中

/**
     * 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.
        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;
        }
    }

我们将所有发送过来的请求消息单独构建一个HashSet进行存储，之所以用HashSet因为我们没有必要重复记录相同的request，所以通过HashSet去重。

private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

然后给每个request设置sequenceNumber。随后判断当前请求是否可以缓存，如果不能缓存就加入网络请求队列，如果可以缓存，首先拿到request的cacheKey，判断HashSet中是否已经包括的相同的cacheKey。如果没有的话就把这条请求加入缓存队列。默认情况下，每条请求都是可以缓存的，所以重点关注CacheDispatcher的run()方法：

@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());
            }
        }
    }

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;
            }

阻塞式的从缓存队列中取出request。接着会根据request的cachekey从缓存中取出响应结果，如果为空就将这个请求加入到NetworkDispatcher。如果不为空还要判断缓存是否过期，过期同样的把request加入到NetworkDispatcher。那么NetworkDispaer中如何处理到来的请求：

@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");
                ...

重点看

NetworkResponse networkResponse = mNetwork.performRequest(request);

执行的是BasicNetwork中的performRequest方法，继续跟进

@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();
                ...

我们看到

httpResponse = mHttpStack.performRequest(request, headers);

其实最终的原理就是调用HttpClient或者HttpURLConnection来发送网络请求，Volley框架对其进行了完美的封装。那么发送网络请求之后，自然地就返回NetworkResponse，紧接着在NetworkDispatcher中调用

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

对NetworkResponse中的数据进行解析。随后，回调解析后的数据

// Post the response back.
                request.markDelivered();
                mDelivery.postResponse(request, response);

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

其中，在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象，就可以保证该对象中的run()方法就是在主线程当中运行的了，我们看下run()方法中的代码是什么样的：

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Response成功解析出来的话，调用deliverResponse(result)，通过重写这个方法可以把响应result传入Response.Listener的onResponse()方法中，然后在onResponse()方法中处理响应的result。
例如在StringQuest中使用：

StringRequest stringRequest = new StringRequest("http://www.baidu.com",
                        new Response.Listener<String>() {
                            @Override
                            public void onResponse(String response) {
                                Log.d("TAG", response);
                            }
                        }, new Response.ErrorListener() {
                            @Override
                            public void onErrorResponse(VolleyError error) {
                                Log.e("TAG", error.getMessage(), error);
                            }
                        });

到这里，源码解析基本上也就结束了。
新人试手，如有错误，欢迎指正