volley 网络框架解析

Posted 2022-12-05 静思浅行

    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