Volley源码解析

首先回顾一下Volley的一般使用方法：

        RequestQueue requestQueue = Volley.newRequestQueue(this);        StringRequest stringRequest = new StringRequest("http://www.i-test.com.cn", new Response.Listener<String>() {            @Override            public void onResponse(String response) {            }        }, new Response.ErrorListener() {            @Override            public void onErrorResponse(VolleyError error) {            }        });        requestQueue.add(stringRequest);

我们根据使用方法一步一步分析。

1、首先看Volley的newRequestQueue(Context context)做了哪些工作：

    public static RequestQueue newRequestQueue(Context context) {        return newRequestQueue(context, null);    }

很简单，它只是进一步调用了他的另一个同名方法只不过第二个参数值传入了null，我们继续看他的这个同名方法：

    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;    }

这个方法是一个Volley中十分重要的方法，它首先声明了cacheDir缓存文件夹，然后声明了一个userAgent给接下来可能要使用的AndroidHttpClient准备参数。接下来就要判断第二个参数是否为null，如果用户调用的是newRequestQueue(Context context)这个单参数的方法，那么stack显然为null，那么Volley会根据版本选择相应的HTTP请求库，可以看到在API9之后都是采用的HurlStack，使用的是java原生的HttpURLConnection库；而在API9之前使用的是HttpClientStack，使用的是AndroidHttpClient，它是Google进一步改造Apache的HttpClient库。当然用户可以知己使用双参数的newRequestQueue方法，从而指定使用哪一个库。

然后创建了network接口对象，并指向一他的一个重要的实现类BasicNetwork，这个实现类具体实现了提交请求并获得返回值的逻辑。DiskBasedCache是一个磁盘缓存类的具体实现类，将这两个具体实现类传给一个RequestQuene对象，并调用该对象的start方法，最终返回这个RequestQueue。

2、下面我们关注Network接口：

public interface Network {    /**     * Performs the specified request.     * @param request Request to process     * @return A {@link NetworkResponse} with data and caching metadata; will never be null     * @throws VolleyError on errors     */    public NetworkResponse performRequest(Request<?> request) throws VolleyError;}

可以看到，他只有一个方法就是performRequest(Request request)，把请求穿进去，返回一个结果。他最重要的一个实现类是BasicNetwork，他实现了具体的提交请求并得到返回值的逻辑。
以下是BasicNetWork类：

    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();                responseHeaders = convertHeaders(httpResponse.getAllHeaders());                // Handle cache validation.                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {                    Entry entry = request.getCacheEntry();                    if (entry == null) {                        return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,                                responseHeaders, true,                                SystemClock.elapsedRealtime() - requestStart);                    }                    // A HTTP 304 response does not have all header fields. We                    // have to use the header fields from the cache entry plus                    // the new ones from the response.                    // http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5                    entry.responseHeaders.putAll(responseHeaders);                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,                            entry.responseHeaders, true,                            SystemClock.elapsedRealtime() - requestStart);                }                // 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,                        SystemClock.elapsedRealtime() - requestStart);            } 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;                if (httpResponse != null) {                    statusCode = httpResponse.getStatusLine().getStatusCode();                } else {                    throw new NoConnectionError(e);                }                VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());                NetworkResponse networkResponse;                if (responseContents != null) {                    networkResponse = new NetworkResponse(statusCode, responseContents,                            responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);                    if (statusCode == HttpStatus.SC_UNAUTHORIZED ||                            statusCode == HttpStatus.SC_FORBIDDEN) {                        attemptRetryOnException("auth",                                request, new AuthFailureError(networkResponse));                    } else if (statusCode >= 400 && statusCode <= 499) {                        // Don't retry other client errors.                        throw new ClientError(networkResponse);                    } else if (statusCode >= 500 && statusCode <= 599) {                        if (request.shouldRetryServerErrors()) {                            attemptRetryOnException("server",                                    request, new ServerError(networkResponse));                        } else {                            throw new ServerError(networkResponse);                        }                    } else {                        // 3xx? No reason to retry.                        throw new ServerError(networkResponse);                    }                } else {                    attemptRetryOnException("network", request, new NetworkError());                }            }        }    }

这个方法有些长，不过最关键的一是行就是调用了mHttpStack对象的performRequest对象，根据httpstack的不同选择不同的http库来进行请求，并返回响应结果。其余逻辑只不过是进行http头的封装和一些异常处理。

3、接下来我们关注RequestQueue，首先看几个重要的成员变量

    /** The cache triage queue. */    private final PriorityBlockingQueue<Request<?>> mCacheQueue =        new PriorityBlockingQueue<Request<?>>();    /** The queue of requests that are actually going out to the network. */    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =        new PriorityBlockingQueue<Request<?>>();

    /** The network dispatchers. */    private NetworkDispatcher[] mDispatchers;    /** The cache dispatcher. */    private CacheDispatcher mCacheDispatcher;

他定义了两个优先级阻塞队列，一个用来存放缓存请求，一个用来存放网络请求。接下来引出两个Dispatcher即分发器，一个是NetWorkDisptacher，它负责分发网络请求，可以看到他是一个数组（默认是大小是4），用来实现一个线程池；另一个是CacheDispather，用来分发缓存请求。这两个分发器功能不同，但原理类似，我们只看NetWorkDispacher中的关键代码：

public class NetworkDispatcher extends Thread {//其余代码    @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;                }                NetworkResponse networkResponse = mNetwork.performRequest(request);//其余代码            } 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);            }        }    }//其余代码}

可以看到，他是Thread的子类，也就是一个线程，其run方法是一个死循环，不断的从networkQueue网络任务队列中获取请求，并使用network接口对象，发送请求并获取结果。

回到RequestQueue，我们看在一开就提到的start方法：

    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();        }    }

非常简单，他分别实例化了CacheDispather和NetworkDispather两个分发器，并将相应的请求队列和network对象传入。并调用start方法，开启线程。

接下来我们关注RequestQueue中的入队方法：

    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;        }    }

也十分清楚，根据请求是否需要缓存，放入不同的队列，从而使用不同的分发器进行处理。

4、最终我们再关注一下HurlStack，他是整个Volley最底层的类，用来直接提交一个HTTP请求并获得结果，另外HttpClinetStack类似，只是使用的具体库不同

    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);        if (hasResponseBody(request.getMethod(), responseStatus.getStatusCode())) {            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;    }

使用过java的HttpURLConnection的同学一定不陌生，这是非常典型一个请求方法。

Volley的整个架构如下：

至此，整个Volley的原理应该读者就已经大概了解，Volley的设计非常的简洁有力，读起来也非常舒服，是我们平常编程十分良好的一个学习对象。