volley 网络框架解析

    volley网络框架是Google在2013年Google I/O大会上推出了一个新的网络通信框架。Volley既可以访问网络取得数据，也可以加载图片，它的设计目标就是非常适合去进行数据量不大，但通信频繁的网络操作，而对于大数据量的网络操作，比如说下载文件等，Volley的表现就会非常糟糕。

一.volley的基本用法

volley内部实现是基于队列的，开发者只需要创建请求放到请求队列中就可以了，请求队列会依次执行队列中的请求。

一个应用程序如果请求不是太频繁的话完全可以使用一个请求队列（对应于application），也可以每个activity使用一个请求队列（对应于activity）。

 1）创建一个请求队列

     RequestQueue mQueue= Volley.newRequestQueue(getApplicationContext());

2).创建一个请求

volley提供了两个默认的request实现 StringRequest、JsonObjectRequest

StringRequest返回的数据类型是String类型， 

 //创建StringRequest请求        StringRequest mStringRequest = new StringRequest(Request.Method.GET, "http://www.baidu.com",                new Response.Listener<String>() {                    @Override                    public void onResponse(String response) {                        Log.i("wangshu", response);                    }                }, new Response.ErrorListener() {            @Override            public void onErrorResponse(VolleyError error) {                Log.e("wangshu", error.getMessage(), error);            }        });

//创建JsonObjectRequest请求        JsonObjectRequest mJsonObjectRequest = new JsonObjectRequest(Request.Method.POST,"http://api.1-blog.com/biz/bizserver/article/list.do",                new Response.Listener<JSONObject>() {                    @Override                    public void onResponse(JSONObject response) {                        Log.d("wangshu", response.toString());                    }                }, new Response.ErrorListener() {            @Override            public void onErrorResponse(VolleyError error) {                Log.e("wangshu", error.getMessage(), error);            }        }        );

3).加入到请求队列中

     mQueue.add(mJsonObjectRequest);

二、源码分析

首先看看Volley这个类，Volley位于volley.toolbox包下

public class Volley {    /** Default on-disk cache directory. */    private static final String DEFAULT_CACHE_DIR = "volley";    /**     * 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);    }}

newRequestQeue(context,stack);方法会创建一个缓存目录，如果当前Android版本>=9就创建一个HurlStack实例（内部使用httpUrlContation实现）否则创建httpClintStack实例（内部使用HttpClint实现）,然后创建一个NetWork根据传入的Stack进行网络请求。最后创建RequestQueue实例然后调用start（）方法启动。

下面看看RequestQueue的start（）方法内部做了哪些事情：

    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();        }    }

关闭当前的正在执行的调度器，创建一个缓存调度器CacheDispatcher并启动。然后在一个for循环中创建并启动NetWorkDispatcher (默认启动4个)

CacheDispatcher和NetWorkerDispatcher都是继承与线程。

下面看看CacheDispatcher内部做了些什么：

 @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;            }        }

CacheDispatcher的run()方法是一个while(true)死循环，说明这个线程一直在执行，每次从请求队列中取出一个请求，根据这条请

求从缓存中取出对应的缓存如果=null 就把这条request加到NetWorkQueue中，不为空则判断请求是否过期，过期也加到

NetWorkQueue中，否则直接使用缓存数据不进行网络请求，直接解析缓存数据然后回调response接口。

NetWorkerDispatcher内部实现：

 @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) {                parseAndDeliverNetworkError(request, volleyError);            } catch (Exception e) {                VolleyLog.e(e, "Unhandled exception %s", e.toString());                VolleyError volleyError = new VolleyError(e);                mDelivery.postError(request, volleyError);            }        }    }

可见NetWorkerDispatcher调用了NetWorker.performRequest(),实际是NerWorker实现类BasicNetWorker的performRequest()方法，在其内部又调用了

mHttpStack.performRequest(request, headers);获取请求结果，之后会将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，以及将数据写入到缓存，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。

在解析完了NetworkResponse中的数据之后，又会调用ExecutorDelivery的postResponse()方法来回调解析出的数据，

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

其中，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

三、自定义不同类型Request请求

自定义网络请求格式定义一个类继承与Request然后重写Request类中的deliverResponse()和parseNetworkResponse()方法，注意在构造函数中一定要调用super()方法将这几个参数传给父类，因为HTTP的请求和响应都是在父类中自动处理的。deliverResponse()方法仅仅是调用了mListener中的onResponse()方法，并将response内容传入即可，这样就可以将服务器响应的数据进行回调了。parseNetworkResponse()方法中则应该对服务器响应的数据进行解析，其中数据是以字节的形式存放在NetworkResponse的data变量中的，然后转换成需要的数据格式（json、xml等格式）就可以了。

四、图片Request