Android-Volley源码分析

       在上一篇文章Android-使用Volley 连接网络中，学习了简单使用Volley连接网络并获取数据，今天就来学习一下Volley的源码。毕竟刚开始接触Volley的时候，谁都难免会对其实现过程有所迷惑，我以自己刚接触Volley时心中的几点疑惑为例：

1)Volley 是靠什么去连接网络的？

2)Volley要连接网络，就需要异步类的操作，Volley是如何处理这步的？

3)Volley的流程是如何跑的？

在阅读源代码之前，可以先观看一下Android官方文档提供的一张图（如下图）：

    

       先可以按照图上显示的内容，从字面上了解Volley的流程：首先是Request在主线程被添加到一个优先顺序的队列，然后交给了CacheDispatcher，CacheDispatcher从字面上意思猜测是个缓存调度器，先不用管它到底干了什么，但是可以肯定它对Request做了处理，如果处理结果存在，就从Cache中请求数据并解析，再将得到的结果返回给主线程，让主线程解析去解析这个结果，如果处理结果不存在，就将Request丢给NetworkDispatcher（也是一个调度器），NetworkDispatcher就处理这个request，并把结果返回给主线程，让主线程去解析这个结果，根据图了解的简单流程就这样，现在就带着这个简单的流程和上面的问题去看Volley源代码：

 -->程序获得RequestQueue的时候，Volley源代码实现了一些什么：

/**     * 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;    }    /**     * 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);    }

       从源代码中可以看出程序获得RequestQueue，最终执行的方法是newRequestQueue(Context context, HttpStack stack)，其内部第一步就为高速缓存创建了一个file 。后面还选择stack，这个stack是HurlStack,在Android-HttpClient连接网络获取数据 中介绍了Android当前版本中HttpClient的使用情况，大多选择都是HttpURLConnection，而这里对于SDK 版本高于9都是选择HurlStack，HurlStack也是通过HttpURLConnection来实现访问网络的，如源代码：

@Override    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)            throws IOException, AuthFailureError {        String url = request.getUrl();        HashMap<String, String> map = new HashMap<String, String>();        map.putAll(request.getHeaders());        map.putAll(additionalHeaders);        if (mUrlRewriter != null) {            String rewritten = mUrlRewriter.rewriteUrl(url);            if (rewritten == null) {                throw new IOException("URL blocked by rewriter: " + url);            }            url = rewritten;        }        URL parsedUrl = new URL(url);        HttpURLConnection connection = openConnection(parsedUrl, request);        for (String headerName : map.keySet()) {            connection.addRequestProperty(headerName, map.get(headerName));        }        setConnectionParametersForRequest(connection, request);        // Initialize HttpResponse with data from the HttpURLConnection.        ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);        int responseCode = connection.getResponseCode();        if (responseCode == -1) {            // -1 is returned by getResponseCode() if the response code could not be retrieved.            // Signal to the caller that something was wrong with the connection.            throw new IOException("Could not retrieve response code from HttpUrlConnection.");        }        StatusLine responseStatus = new BasicStatusLine(protocolVersion,                connection.getResponseCode(), connection.getResponseMessage());        BasicHttpResponse response = new BasicHttpResponse(responseStatus);        response.setEntity(entityFromConnection(connection));        for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {            if (header.getKey() != null) {                Header h = new BasicHeader(header.getKey(), header.getValue().get(0));                response.addHeader(h);            }        }        return response;    }

  看到这个就解除了第一个疑惑，Volley通过HttpURLConnection访问网络。

  在得到RequestQueue以后，就启动这个RequestQueue。RequestQueue的start()方法又做了那些操作：  

  /**     * 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();        }    }   /**     * Stops the cache and network dispatchers.     */    public void stop() {        if (mCacheDispatcher != null) {            mCacheDispatcher.quit();        }        for (int i = 0; i < mDispatchers.length; i++) {            if (mDispatchers[i] != null) {                mDispatchers[i].quit();            }        }    }

        start()方法第一步就是先stop cache 和 network两个调度器，然后再重新创建这两个调度器。而这两个调度器都是继承于Thread，是两个线程。并且在它们的run()方法中设置了后台运行属性：    

Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

 代码看到这里就解除了第二个疑惑。此时queue也start了，在写代码的时候在queue被start之后，就要把request添加到queue中，如：mRequestQueue.add(jsObjRequest)。

->RequestQueue中的add()方法实现了那些操作（这个一步也是流程图上的第一步）：

/**     * 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;        }    }

      在代码中可以看到一个if语句的判断(!request.shouldCache())，判断request是否缓存，shouldCache默认值为true，但是可以通过下面方法来改变它的值：  

  /**     * Set whether or not responses to this request should be cached.     *     * @return This Request object to allow for chaining.     */    public final Request<?> setShouldCache(boolean shouldCache) {        mShouldCache = shouldCache;        return this;    }

  默认是需要缓存的，就通过mCacheQueue.add(request)将request加入到mCacheQueue队列，如果不需要缓存就会通过mNetworkQueue.add(request)将request加入到network队列。

  request被add到mCacheQueue以后，由于之前在RequestQueue的start()方法中，将mCacheQueue传给了CacheDispatcher，mCacheDispatcher.start()之后，就会执行CacheDispatcher重写于Thread类的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();        while (true) {            try {                // Get a request from the cache triage queue, blocking until                // at least one is available.                final Request<?> request = mCacheQueue.take();                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.                    mDelivery.postResponse(request, response, new Runnable() {                        @Override                        public void run() {                            try {                                mNetworkQueue.put(request);                            } catch (InterruptedException e) {                                // Not much we can do about this.                            }                        }                    });                }            } catch (InterruptedException e) {                // We may have been interrupted because it was time to quit.                if (mQuit) {                    return;                }                continue;            }        }    }

  这个run()方法里边有个while(true)循环，是一个无限循环，只要cache线程不停止，就不停的从缓存队列中取出请求结果,获得请求结果之后就通过mDelivery.postResponse()方法，将结果传递到主线程去处理。为什么说这里传递到了主线程呢？在前面获得RequestQueue对象的时候，代码是这么实现的： 

   ...    RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);    queue.start();    return queue;    ...

其实new RequestQueue对象最终调用的方法是：  

   /**     * 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     */    public RequestQueue(Cache cache, Network network, int threadPoolSize) {        this(cache, network, threadPoolSize,                new ExecutorDelivery(new Handler(Looper.getMainLooper())));    }

  其中threadPoolSize = 4，而Looper.getMainLooper()就相当于主线程。这样在cache线程中获得结果最终传递给了主线程，让主线程来解析这个response,再回到CacheDispatcher的run()方法中，从缓存队列不一定能取出结果，即使取出了结果也可能不是最新的，这个时候就需要从新从网络获取数据，run()方法中有这么两个判断：                    

            // 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;                }

    源码看到此处，request总共有几种情况会被加入到network队列？我总结的是总共有三种情况request被加入到network队列，第一次是判断request是否需要缓存，不要需要缓存，request会被加入到network队列，后面两种情况如上面所述的两种情况了（流程图上第二步在这里就完成）。

    经过上面的分析，cache线程对request的处理就完成了，但是由于上面所说的三种情况，request会被加入到network队列，下面就继续分析network，同样看看NetworkDispatcher的run()方法：    

@Override    public void run() {        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);        Request<?> request;        while (true) {            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) {                parseAndDeliverNetworkError(request, volleyError);            } catch (Exception e) {                VolleyLog.e(e, "Unhandled exception %s", e.toString());                mDelivery.postError(request, new VolleyError(e));            }        }    }

    从run()方法中可见，network线程会去执行network request：        

// Perform the network request.    NetworkResponse networkResponse = mNetwork.performRequest(request);

    先来看看mNetwork是个什么东东，mNetwork是Network,它从newRequestQueue的时候就被创建：    

Network network = new BasicNetwork(stack);

   

 /**     * @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;    }

    所以mNetwork实际上是new了一个BasicNetwork对象，而BasicNetwork实现了接口Network。其performRequest方法如下：

     @Override    public NetworkResponse performRequest(Request<?> request) throws VolleyError {        long requestStart = SystemClock.elapsedRealtime();        while (true) {            HttpResponse httpResponse = null;            byte[] responseContents = null;            Map<String, String> responseHeaders = new HashMap<String, String>();            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();                responseHeaders = convertHeaders(httpResponse.getAllHeaders());                // Handle cache validation.                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,                            request.getCacheEntry() == null ? null : request.getCacheEntry().data,                            responseHeaders, true);                }                // Some responses such as 204s do not have content.  We must check.                if (httpResponse.getEntity() != null) {                  responseContents = entityToBytes(httpResponse.getEntity());                } else {                  // Add 0 byte response as a way of honestly representing a                  // no-content request.                  responseContents = new byte[0];                }                // if the request is slow, log it.                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;                logSlowRequests(requestLifetime, request, responseContents, statusLine);                if (statusCode < 200 || statusCode > 299) {                    throw new IOException();                }                return new NetworkResponse(statusCode, responseContents, responseHeaders, false);            } catch (SocketTimeoutException e) {                attemptRetryOnException("socket", request, new TimeoutError());            } catch (ConnectTimeoutException e) {                attemptRetryOnException("connection", request, new TimeoutError());            } catch (MalformedURLException e) {                throw new RuntimeException("Bad URL " + request.getUrl(), e);            } catch (IOException e) {                int statusCode = 0;                NetworkResponse networkResponse = null;                if (httpResponse != null) {                    statusCode = httpResponse.getStatusLine().getStatusCode();                } else {                    throw new NoConnectionError(e);                }                VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());                if (responseContents != null) {                    networkResponse = new NetworkResponse(statusCode, responseContents,                            responseHeaders, false);                    if (statusCode == HttpStatus.SC_UNAUTHORIZED ||                            statusCode == HttpStatus.SC_FORBIDDEN) {                        attemptRetryOnException("auth",                                request, new AuthFailureError(networkResponse));                    } else {                        // TODO: Only throw ServerError for 5xx status codes.                        throw new ServerError(networkResponse);                    }                } else {                    throw new NetworkError(networkResponse);                }            }        }    }

在此方法中会去连接网络：   

httpResponse = mHttpStack.performRequest(request, headers);

  经过前面的分析mHttpStack是stack，而对于SDK高于9的版本 stack是HurlStack类的一个对象，HurlStack又实现了HttpStack接口，所以mHttpStack.performRequest(request, headers)执行的是HurlStack中的performRequest()方法，此方法最终使用HttpURLConnection来连接网络并获取网络数据，代码在前面贴了。至此，Volley连接网络的流程也显出端倪了。

回到network调度器的run()方法中，经过上面根据request从网络获取到数据（response），接下来就会解析response，把解析的结果写入cache（如果可以），最后返回给主线程。

    ...// 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);  ...

  至此，整个流程就分析完了，回想前面的流程图，根据这些源码就显得清晰不少了。