开题篇中最后提到了使用volley的三个步骤，这一篇就从步骤一的源码看起：建立RequestQueue对象。建立RequestQueue对象是同过Volley的静态方法建立的。
Volley类中，，提供了三个生成RequestQueue的方法。两个利用默认的缓存目录生成RequestQueue的重载方法newRequestQueue，和一个用户自定义缓存目录的newRequestQueueInDisk方法。
下面看看源码public class Volley {private static final String DEFAULT_CACHE_DIR = "volley";//默认的缓存目录//这里将context对象传入，保证了网络访问过程中activity销毁时，网络访问也会中断public static RequestQueue newRequestQueue(Context context, HttpStack stack) {File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);//建立缓存文件目录String userAgent = "volley/0";try {String packageName = context.getPackageName();//获得包名PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);//获取包的信息userAgent = packageName + "/" + info.versionCode; //构造新的userAgent} catch (NameNotFoundException e) {}//这里利用接口httpStack指向其实现此接口的类对象（HttpClientStack、HurlStack）if (stack == null) {if (Build.VERSION.SDK_INT >= 9) {stack = new HurlStack();//根据httpURLconnection建立一个httpStack} else {stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));//根据httpclient建立httpstack}}//这里利用接口network指向实现此接口的类对象BasicNetwork，构建Network对象network。Network network = new BasicNetwork(stack);/*利用匿名对象建立cache对象（多态的又一次应用，实际上引用的是DiskBasedCache对象）*在这不难看出建立requestQueue对象的两个参数为cache缓存对象和network对象。其中缓存对象*负责从缓存文件中来获取对应请求的相应response，而network则是通过其内部封装的HttpStack*（HurlStack或者是HttpClientStack）来进行网络访问*/RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);queue.start();//请求队列工作return queue;}/*** context* dir sdcard's root dire* null* 用给定的文件目录建立缓存文件系统* */public static RequestQueue newRequestQueueInDisk(Context context, String dir, HttpStack stack) {File cacheDir = new File(dir, 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 {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);}}
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