在Android Volley分析（一）——结构中主要分析了Volley的基本组件和框架结构，组件主要是定义的接口，也就是说我们可以实现这些接口来定制自己的Volley版本，比如NetWork、Cache、Request等等。Android Volley在com.android.volley.toolbox下已经做了这些工作，下面就看看这些具体的实现内容
先看一个Volley使用的例子
final TextView mTextView = (TextView) findViewById(R.id.text);...// Instantiate the RequestQueue.RequestQueue queue = Volley.newRequestQueue(this);String url ="http://www.google.com";// Request a string response from the provided URL.StringRequest stringRequest = new StringRequest(Request.Method.GET, url, new Response.Listener() { @Override public void onResponse(String response) { // Display the first 500 characters of the response string. mTextView.setText("Response is: "+ response.substring(0,500)); }}, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError error) { mTextView.setText("That didn't work!"); }});// Add the request to the RequestQueue.queue.add(stringRequest);这是官方volley教程的一个例子，使用Volley的步骤
1、Volley.newRequestQueue();
2、new Request();
3、queue.add(request);
在newRequestQueue的时候会初始化基本的组件
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; }1、BasicNetwork
BasicNetwork是对Network接口的实现，同时还将联网操作部分提取出来形成HttpStack，原因是Android 2.3之前网络操作推荐使用HttpClient，2.3之后Android做了优化推荐使用HttpUrlConnection。
2、DiskBasedCache
DiskBasedCache是对Cache对实现，用于硬盘缓存，也就是将数据保存为特定的文件。
结合LruCache试着总结一下缓存的一般方法：
映射结构——LinkedHashMap，内部实现了Lru的算法排序，可以直接使用
put——存储，容量计数增加，判断是否超出最大容量，超出则删除最少使用的
get——key到entry的映射
remove——删除，容量计数减小
3、ImageLoader
ImageLoader是一个图片加载类，封装了请求，提供了内存缓存机制，批处理等，让图片加载更容易使用，下面看一下它的用法：
mImageLoader = new ImageLoader(mRequestQueue, new ImageLoader.ImageCache() { private final LruCache<String, Bitmap> cache = new LruCache<String, Bitmap>(20); @Override public Bitmap getBitmap(String url) { return cache.get(url); } @Override public void putBitmap(String url, Bitmap bitmap) { cache.put(url, bitmap); }mImageLoader.get(IMAGE_URL, ImageLoader.getImageListener(mImageView, R.drawable.def_image, R.drawable.err_image));
（示例来源 Making a Standard Request ）
内存缓存推荐使用LruCache，需要实现ImageLoader.ImageCache接口。图片的加载通过imageLoader.get()方法实现，
public ImageContainer get(String requestUrl, ImageListener imageListener, int maxWidth, int maxHeight) { // only fulfill requests that were initiated from the main thread. throwIfNotOnMainThread(); final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight); // Try to look up the request in the cache of remote images. Bitmap cachedBitmap = mCache.getBitmap(cacheKey); if (cachedBitmap != null) { // Return the cached bitmap. ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null); imageListener.onResponse(container, true); return container; } // The bitmap did not exist in the cache, fetch it! ImageContainer imageContainer = new ImageContainer(null, requestUrl, cacheKey, imageListener); // Update the caller to let them know that they should use the default bitmap. imageListener.onResponse(imageContainer, true); // Check to see if a request is already in-flight. BatchedImageRequest request = mInFlightRequests.get(cacheKey); if (request != null) { // If it is, add this request to the list of listeners. request.addContainer(imageContainer); return imageContainer; } // The request is not already in flight. Send the new request to the network and // track it. Request<?> newRequest = new ImageRequest(requestUrl, new Listener<Bitmap>() { @Override public void onResponse(Bitmap response) { onGetImageSuccess(cacheKey, response); } }, maxWidth, maxHeight, Config.RGB_565, new ErrorListener() { @Override public void onErrorResponse(VolleyError error) { onGetImageError(cacheKey, error); } }); mRequestQueue.add(newRequest); mInFlightRequests.put(cacheKey, new BatchedImageRequest(newRequest, imageContainer)); return imageContainer; }首先得到一个cacheKey，这个是统一的规则
private static String getCacheKey(String url, int maxWidth, int maxHeight) { return new StringBuilder(url.length() + 12).append("#W").append(maxWidth) .append("#H").append(maxHeight).append(url).toString(); }然后在缓存cache中查找，没找到就讲请求添加到批处理任务BitmapImageRequest中，这里有两个类，ImageContainer和BitmapImageRequest：
ImageContainer是一个包装类，里面包含了bitmap、url、cacheKey和listener，当得到bitmap后会通过listener将bitmap返回；
BitmapImageRequest是将相同url请求放在一起，这样就可以只向网络请求一次，返回结果后再分别设置到目标上，从而达到减少网络请求对目的。