Google Volley框架源码走读

 

PS一句：最终还是选择CSDN来整理发表这几年的知识点，该文章平行迁移到CSDN。因为CSDN也支持MarkDown语法了，牛逼啊！

【工匠若水 http://blog.csdn.net/yanbober】 阅读前一篇《Google Volley使用之自定义》 http://blog.csdn.net/yanbober/article/details/45307099

开源项目链接

Volley自定义 Android Developer文档

Volley主页：https://android.googlesource.com/platform/frameworks/volley

Volley仓库：Git clone https://android.googlesource.com/platform/frameworks/volley

Volley GitHub Demo：在GitHub主页搜索Volley会有很多，不过建议阅读Android Developer文档。

背景知识

在Volley使用基础那一篇最后一个知识点说到了Volley的请求架构，这里再搬过来说说。

在Android Developer上看到的这幅图：

RequestQueue会维护一个缓存调度线程（cache线程）和一个网络调度线程池（net线程），当一个Request被加到队列中的时候，cache线程会把这个请求进行筛选：如果这个请求的内容可以在缓存中找到，cache线程会亲自解析相应内容，并分发到主线程（UI）。如果缓存中没有，这个request就会被加入到另一个NetworkQueue，所有真正准备进行网络通信的request都在这里，第一个可用的net线程会从NetworkQueue中拿出一个request扔向服务器。当响应数据到的时候，这个net线程会解析原始响应数据，写入缓存，并把解析后的结果返回给主线程。

硬着头皮开始吧

直接这么看好空洞，所以直接把clone的工程导入IDE边看代码边看这个图吧。

还是按照前边的顺序分析吧，使用Volley的第一步首先是通过Volley.newRequestQueue(context)得到RequestQueue队列，那么先看下toolbox下的Volley.Java中的这个方法吧。

/**
 * 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);
}

先看如上注释所示，创建一个默认的worker pool，并且调运RequestQueue的start方法。在这个方法里又调运了该类的另一个连个参数的重载方法，如下所示。

/**
 * 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 (PackageManager.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;
}

如上所示，该方法有两个参数，第一个Context是为了拿到当前App的Activity的一些信息，第二个参数在默认的RequestQueue newRequestQueue(Context context)方法中传递为null，也就是说在这段代码中的if (stack == null)会被执行。在这个if中对版本号进行了判断，如果版本号大于等于9就使得HttpStack对象的实例为HurlStack，如果小于9则实例为HttpClientStack。至于这里为何进行版本号判断，实际代码中的(Click Me To See)注释已经说明了。实际上HurlStack类也在toolbox中，他实现了toolbox的HttpStack接口中的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，其实现过程使用的是HttpURLConnection。而HttpClientStack也在toolbox中，他也实现了toolbox的HttpStack接口的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，不过其实现过程使用的是HttpClient而已。

其中的userAgent就是App的包名加版本号而已，传入new HttpClientStack(AndroidHttpClient.newInstance(userAgent));作为name TAG使用。

如上HttpStack创建完成之后创建了Network实例。BasicNetwork是Network接口的实现，他们都在toolbox中，BasicNetwork实现了public NetworkResponse performRequest(Request<?> request)方法，其作用是根据传入的HttpStack对象来处理网络请求。紧接着new出一个RequestQueue对象，并调用它的start()方法进行启动，然后将RequestQueue返回。RequestQueue是根目录下的一个类，其作用是一个请求调度队列调度程序的线程池。这样newRequestQueue()的方法就执行结束了。

现在再来看下根目录下RequestQueue队列的start方法，如下所示：

/**
 * 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();
    }
}

通过注释可以看出来这里在派发队列的事务。先是创建了一个CacheDispatcher的实例，然后调用了它的start()方法，接着在一个for循环里去创建NetworkDispatcher的实例，并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的，而默认情况下for循环会执行（DEFAULT_NETWORK_THREAD_POOL_SIZE）四次，也就是说当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中一个CacheDispatcher是缓存线程，四个NetworkDispatcher是网络请求线程。

按照之前使用Volley可以知道，得到了RequestQueue之后，我们只需要构建出相应的Request，然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了。也就是说add()方法的内部是核心代码了。现在看下RequestQueue的add方法，具体如下：

/**
 * 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;
    }
}

可以看到注释所示，添加一个Request到派发队列。Request是所有请求的基类，是一个抽象类。request.setRequestQueue(this);的作用就是将请求Request关联到当前RequestQueue。然后同步操作将当前Request添加到RequestQueue对象的mCurrentRequests HashSet中做记录。通过request.setSequence(getSequenceNumber());得到当前RequestQueue中请求的个数，然后关联到当前Request。request.addMarker("add-to-queue");添加调试的Debug标记。if (!request.shouldCache())判断当前的请求是否可以缓存，如果不能缓存则直接通过mNetworkQueue.add(request);将这条请求加入网络请求队列，然后返回request；如果可以缓存的话则在通过同步操作将这条请求加入缓存队列。在默认情况下，每条请求都是可以缓存的，当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。OK，那么既然默认每条请求都是可以缓存的（shouldCache返回为true），自然就被添加到了缓存队列中，于是一直在后台等待的缓存线程就要开始运行起来了。现在来看下CacheDispatcher中的run()方法，代码如下所示：

@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;
        }
    }
}

首先通过Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);设置线程优先级，然后通过mCache.initialize(); 初始化缓存块，其中mCache是由Volley.java中的newRequestQueue(Context context, HttpStack stack)方法中实例化传入的，其Cache接口的实现为new DiskBasedCache(cacheDir)，其中cacheDir默认在Volley.java中不设置为/data/data/app-package/cache/volley/。

接下来由while (true)可以发现缓存线程是一直在执行，其中通过mQuit标记进制是否结束线程的操作。mCacheQueue.take()从阻塞队列获取最前面的一个request，没有request就阻塞等待。接着通过mCache.get(request.getCacheKey());尝试从缓存中取出响应结果，如何为空的话则把这条请求加入到网络请求队列中，如果不为空的话再判断该缓存是否已过期，如果已经过期了则同样把这条请求加入到网络请求队列中，否则就认为不需要重发网络请求，直接使用缓存中的数据即可。在这个过程中调运了parseNetworkResponse()方法来对数据进行解析，再往后就是将解析出来的数据进行回调了。现在先来看下Request抽象基类的这部分代码：

/**
 * Subclasses must implement this to parse the raw network response
 * and return an appropriate response type. This method will be
 * called from a worker thread.  The response will not be delivered
 * if you return null.
 * @param response Response from the network
 * @return The parsed response, or null in the case of an error
 */
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

通过注释可以看到他就是一个解析模块的功能。

上面说了，当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中一个CacheDispatcher是缓存线程，四个NetworkDispatcher是网络请求线程。CacheDispatcher的run方法刚才已经大致分析了，解析来看下NetworkDispatcher中是怎么处理网络请求队列的，具体代码如下所示：

@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);
        }
    }
}

和CacheDispatcher差不多，如上可以看见一个类似的while(true)循环，说明网络请求线程也是在不断运行的。

如上通过mNetwork.performRequest(request);代码来发送网络请求，而Network是一个接口，这里具体的实现之前已经分析是BasicNetwork，所以先看下它的performRequest()方法，如下所示：

NetWork接口的代码：

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;
}

上面说了，就是执行指定的请求。他的BasicNetwork实现子类如下：

@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
    long requestStart = SystemClock.elapsedRealtime();
    while (true) {
        HttpResponse httpResponse = null;
        byte[] responseContents = null;
        Map<String, String> responseHeaders = 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 = 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, SystemClock.elapsedRealtime() - requestStart);
                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);
            }
        }
    }
}

这个方法是网络请求的具体实现，也是一个大while循环，其中mHttpStack.performRequest(request, headers);代码中的mHttpStack是Volley的newRequestQueue()方法中创建的实例，前面已经说过，这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的，然后把服务器返回的数据组装成一个NetworkResponse对象进行返回。在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据，同时将数据写入到缓存，这个方法的实现是交给Request的子类来完成的，因为不同种类的Request解析的方式也肯定不同。

前面你可以看到在NetWorkDispatcher的run中最后执行了mDelivery.postResponse(request, response);，也就是说在解析完了NetworkResponse中的数据之后，又会调用ExecutorDelivery（ResponseDelivery接口的实现类）的postResponse()方法来回调解析出的数据，具体代码如下所示：

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

这里可以看见在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：

/**
 * A Runnable used for delivering network responses to a listener on the
 * main thread.
 */
@SuppressWarnings("rawtypes")
private class ResponseDeliveryRunnable implements Runnable {
    private final Request mRequest;
    private final Response mResponse;
    private final Runnable mRunnable;

    public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
        mRequest = request;
        mResponse = response;
        mRunnable = runnable;
    }

    @SuppressWarnings("unchecked")
    @Override
    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();
        }
    }
}

这段代码里的run方法中可以看到如下一部分细节：

// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
    mRequest.deliverResponse(mResponse.result);
} else {
    mRequest.deliverError(mResponse.error);
}

这段代码是最核心的，明显可以看到通过mRequest的deliverResponse或者deliverError将反馈发送到回调到UI线程。这也是你重写实现的接口方法。

再来整体看下

现在是该回过头去看背景知识模块了，再看下那幅官方图，对比就明白咋回事了。结合上图和如上分析可以知道：

1. 当一个RequestQueue被成功申请后会开启一个CacheDispatcher和4个默认的NetworkDispatcher。

2. CacheDispatcher缓存调度器最为第一层缓冲，开始工作后阻塞的从缓存序列mCacheQueue中取得请求；对于已经取消的请求，标记为跳过并结束这个请求；新的或者过期的请求，直接放入mNetworkQueue中由N个NetworkDispatcher进行处理；已获得缓存信息（网络应答）却没有过期的请求，由Request的parseNetworkResponse进行解析，从而确定此应答是否成功。然后将请求和应答交由Delivery分发者进行处理，如果需要更新缓存那么该请求还会被放入mNetworkQueue中。

3. 将请求Request add到RequestQueue后对于不需要缓存的请求（需要额外设置，默认是需要缓存）直接丢入mNetworkQueue交给N个NetworkDispatcher处理；对于需要缓存的，新的请求加到mCacheQueue中给CacheDispatcher处理；需要缓存，但是缓存列表中已经存在了相同URL的请求，放在mWaitingQueue中做暂时处理，等待之前请求完毕后，再重新添加到mCacheQueue中。

4. 网络请求调度器NetworkDispatcher作为网络请求真实发生的地方，对消息交给BasicNetwork进行处理，同样的，请求和结果都交由Delivery分发者进行处理。

5. Delivery分发者实际上已经是对网络请求处理的最后一层了，在Delivery对请求处理之前，Request已经对网络应答进行过解析，此时应答成功与否已经设定；而后Delivery根据请求所获得的应答情况做不同处理；若应答成功，则触发deliverResponse方法，最终会触发开发者为Request设定的Listener；若应答失败，则触发deliverError方法，最终会触发开发者为Request设定的ErrorListener；处理完后，一个Request的生命周期就结束了，Delivery会调用Request的finish操作，将其从mRequestQueue中移除，与此同时，如果等待列表中存在相同URL的请求，则会将剩余的层级请求全部丢入mCacheQueue交由CacheDispatcher进行处理。

至此所有搞定。

PPPS一句：通过上面原理分析之后总结发现，推荐整个App全局持有一个RequestQueue的做法，这样会有相对比较高的性能效率。

---

走读代码时参考博客链接

• 上一篇Google Volley使用之自定义
• 下一篇android-async-http框架库使用基础