Android常用开源网络框架（一）—— Volley篇

在Android开发中，网络请求是最重要的模块之一，Android中大部分网络请求使用的是HTTP连接，包括原生的HttpClient和HttpUrlConnection两种访问网络方式。需要注意的是，HttpClient方式在Android6.0以后，很多类已经不支持了。
目前主流的开源网络框架，主要有OkHttp，Volley，Retrofit三种，我本人在短暂的开发经历中基本也只接触过这几个，在此简单分析这三个框架，仅作为本人记录。

Volley框架

Volley框架是由Google在2013年发布的较为轻量级的网络框架，主要适用于处理请求次数较多，量级较小的网络请求。

一、Volley使用方式

Volley的使用方式十分简单，主要分为以下几步：

Step1: 引入Volley（Android Studio）

方法一：在项目的build.gradle 添加依赖

implementation 'com.mcxiaoke.volley:library:1.0.+'

方法二：引入volley.jar包

方法三：通过git下载volley包，之后添加为项目的module，并为主工程添加依赖

需要注意的是，如果Volley返回Object，需要对其进行解析，转换为Gson对象，那么还需要引入Gson依赖

implementation 'com.google.code.gson:gson:2.8.2'  

注：引入的版本根据sdk版本而定

Step2: 创建请求队列实例

requestQueue = Volley.newRequestQueue(context); 

一般而言，请求队列不需要每次进行网络请求时创建，通常一个Activity创建一个，或者对于较少请求的轻量级的应用，也可以一个应用只创建一个请求队列，主要视请求的多少而定。

Step3: 创建请求Request

Volley本身已封装好几种常用的Request，包括StringRequest，JsonRequest，JsonObjectRequest，JsonArrayRequest
值得一提的是，Volley封装了ImageLoader和ImageRequest，可以方便地支持图片的获取和加载，不需要额外自己添加图片的加载
以StringRequest为例，创建Request的代码也十分简单：

StringRequest stringRequest = new StringRequest(url, new Listener<String>() {
      @Override
      public void onResponse(String response) {
        Log.e("xxxx", "onStringResponse: " + response);
      }
    }, new ErrorListener() {
      @Override
      public void onErrorResponse(VolleyError error) {
        Log.e("xxxx", "onStringErrorResponse: " + error);
      }
    });

Step4: 将请求加入请求队列

 requestQueue.add(stringRequest);

经过以上几个步骤，就可以实现网络请求，成功和失败结果均返回。

二、Volley源码分析

Volley作为一个轻量级的网络框架，源码实际上并不复杂，接下来将针对其主要的代码进行分析。

1. Volley.java 的主要代码分析

图中截出的就是我们在使用Volley时的第一步创建请求队列的代码，事实上Volley文件中也只有这一个重要的函数，主要步骤如下：

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;
    }

• 创建了一个用于存储Volley缓存的文件，使用的是默认的缓存路径，然后获取包名、包的信息以及用户信息等
• 根据sdk版本，sdk版本大于等于9的，即Android系统2.3版本以上，创建HurlStack，9以下创建HttpClientStack
• 创建了请求队列，maxDiskCacheBytes指的是缓存的最大容量，在创建队列时如果定义了该参数，则按指定的容量，否则缓存容量为按照默认的-1
• RequestQueue.start()

2. RequestQueue的代码分析部分

RequestQueue最重要的是add和start两个函数。

start函数：

    /**
     * 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();
        }
    }

可以看到主要的步骤是创建并启动了两种分发器，其中mNetworkDispatchers包含多个NetworkDispatcher，数量默认为4。

add函数的主要代码如图所示：

    /**
     * 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;
        }
    }

• 将要add的请求加入当前请求的列表，并设置一个序列号码和一个已加入队列的tag，用来让分发器按请求的顺序处理请求。
• 如果请求设置了不缓存（默认是缓存），那么将其加入网络请求队列。
• 用cachekey来作为请求的唯一标识，如果有相同key的请求在waitingRequests中，标识有相同的请求已经执行并且还没有返回结果，为避免重复请求，则将该请求存入；如果没有，则将该请求加入缓存队列中。

3. CacheDispatcher 和 NetworkDispatcher

在RequestQueue的代码分析中，我们可以看到队列并没有对网络请求进行处理，接下来我们看看这两个分发器是如何处理加入队列的网络请求的。

① CacheDispatcher

run()函数解析：

    @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) {
        	...
        }
    }

• 设置线程的优先级为最高，并初始化缓存
• 接下来进入一个无限循环，按序取出列表中的request，对request队列中的每一个request进行处理

对request进行处理的步骤：

            // 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());
            }
        }

• 从请求列表中取出request，判断是否已取消，如未取消，则通过request的cacheKey从缓存中获取
• 如果缓存中没有对应key的request或者已经过期，则将该request发送到mNetworkQueue，等待NetworkDispatcher进行处理
• 如果有对应的缓存request，并且没有过期，那么CacheDispatcher会将该请求发送到主线程（发送到主线程的具体方法后面会说明）。

② NetworkDispatcher

同样，NetworkDispatcher的run()函数也有一个无限循环对request进行处理，在确认request未被中断（通常发生在请求超时的情况）并且未被取消后，进行处理。

    @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");

                // 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);
            }
        }
    }

• 取出request后，通过Network的performRequest请求网络
• 在获取到response后，通过parseNetworkResponse对response进行解析
• 如果request需要缓存，则存储到mCache中，等待CacheDispatcher进行处理
• 最后将获取到的response返回给主线程并标记request完成即可

4. NetworkDispatcher 请求网络方式

上面run函数中可以看到请求网络是通过mNetwork.performRequest函数请求网络，该函数在BasicNetwork中实现，实现方式如下图：

    @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);
                }
                ...
            } 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);
                }
                if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || 
                		statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
                	VolleyLog.e("Request at %s has been redirected to %s", request.getOriginUrl(), request.getUrl());
                } else {
                	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 if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || 
                    			statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
                        attemptRetryOnException("redirect",
                                request, new RedirectError(networkResponse));
                    } else {
                        // TODO: Only throw ServerError for 5xx status codes.
                        throw new ServerError(networkResponse);
                    }
                } else {
                    throw new NetworkError(e);
                }
            }
        }
    }

• 获取headers，之后通过mHttpStack请求网络
• mHttpStack也是一个接口类，通过之前提到过的HurlStack和HttpClientStack实现
• 获取到 httpResponse后，根据响应的状态码，返回不同的NetworkResponse给NetworkDispatcher

5. 返回response给主线程的方式

在请求网络获取到NetworkResponse后，通过mDeliver.postResponse将结果返回给主线程，代码如下图：

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

ResponseRunnable代码：

        @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();
            }
       }

根据Request请求的结果，Runnable分别调用了Request不同的函数。

以StringRequest为例：

    @Override
    protected void deliverResponse(String response) {
        if (mListener != null) {
            mListener.onResponse(response);
        }
    }

请求成功时调用mListener（创建StringRequest时的参数）的回调函数，在回调函数中对获取到的response进行需要的操作即可。

至此，Volley实现网络请求的基本流程就梳理完成了，最后再通过流程图进行总结：

对Volley框架的总结就到这里，下一篇将继续总结另一种重要的网络框架——OkHttp的相关内容。