Volley源码分析（1）----Volley 队列

Android网络框架很多，但是基于Google自己的volley，无疑是优秀的一款。

网络框架，无外乎解决一下几个问题，队列，缓存，图片异步加载，统一的网络请求和处理等。

一.Volley 队列 启动

Volley的队列，首先我们看队列的启动：com.android.volley.toolbox.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.
     * @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) {
        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 = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

我们依次分析这个方法。这个方法是队列启动的代码。

开始就是获取缓存文件夹，以及userAgent 一些信息。

Build.VERSION.SDK_INT >= 9

会使用HTTPURLConnection作为网络请求，而老的版本就使用httpclient来处理。

关于这2者的区别，很多地方有介绍。HttpUrlConnection是HttpClient轻量级版本。

应该说性能更好，并且足够android平台使用了。

当然，也可以使用自己定义的HttpStack。

Network network = new BasicNetwork(stack);

Network对stack的进一步封装，然后创建队列和启动队列。

com/android/volley/RequestQueue.java:

    /**
     * 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();
        }
    }

启动队列，就是启动了一条cache thread 和4个 network thread。

然后分析CacheDispatcher 和NetworkDispatcher 这两个东东。

二：NetworkDispatcher

这一节我们分析网络请求，所以就忽略cache的部分，将在下一节分析：

public class NetworkDispatcher extends Thread

NetworkDispatcher是一个thread，可见network请求应该是从requestQueue队列中获取数据以后，已while（true）的形式不断的向服务器请求，

当requestQueue 为空时，线程讲block住，直到队列有数据，或者线程推出为止。

com/android/volley/NetworkDispatcher.java:

@Override
    public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        while (true) {
            long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
            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);
            }
        }
    }

request默认是存在 PriorityBlockingQueue队列里面，

这个队列 可以解决2个问题。

1）线程间的同步

2）当队列为空时，take方法将会block住。

                // If the request was cancelled already, do not perform the
                // network request.
                if (request.isCanceled()) {
                    request.finish("network-discard-cancelled");
                    continue;
                }

注释已经解释的很清楚了。

网络请求在这个进行：

NetworkResponse networkResponse = mNetwork.performRequest(request);

                // 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;
                }

是否Modified。

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

把networkResponse转化成Response<?>.

接下去，就是判读是否需要cache，需要就保存此次请求结果。

然后就是分发这次的结果。

以上就是volley的一次网络请求的过程。

三：CacheDispatcher & Cache

首先是com/android/volley/RequestQueue.java

的add 方法。

/**
     * 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;
        }
    }

private final Set<Request<?>> mCurrentRequests 这个集合用于记录当前正在队列里面的request，此处的集合应该是所有的request。

request.setSequence(getSequenceNumber());

用于request的排序。

        if (!request.shouldCache()) {
            mNetworkQueue.add(request);
            return request;
        }

request是否cache，如果否，讲直接放到network队列做请求操作。

// 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;
        }

如果cache队列里面已经有正在处理的request，就无需放入cache队列，直接等待返回结果时，会把所有重复的request加入cache。

下面我们分析cache线程的情况。

我们已经说过，request会在网络请求结束的时候，把可以cache的request放入cache库中。

com/android/volley/CacheDispatcher.java

                // 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;
                }

通过cachekey，获取缓存的数据。没有的话，将有network dispatcher来处理。

接下来判断是否过期。

过期有2中状态，软过期，和completely 过期。

completely 过期。直接请求网络。

软过期，先response到前台，然后后台进行更新。

这个就是cache线程在干的事情。

再看看刚才response时如何产生cache数据的。

// Parse the response here on the worker thread.
                Response<?> response = request.parseNetworkResponse(networkResponse);

可以看到居然是由每个request自己来决定如何产生cache数据。

Cache.Entry的获得将在：

com/android/volley/toolbox/HttpHeaderParser.java:

public static Cache.Entry parseCacheHeaders(NetworkResponse response)

中获得。这个方法时公共的，如何以后有自定义请求的时候，可以使用该方法。

自此一个volley的队列已经搭载启动了。

我们只需要把request放入requestQueue就可以了！