【安卓笔记】Volley全方位解析,带你从源码的角度彻底理解
转载请声明原出处(blog.csdn.net/chdjj)，谢谢!
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参考资料
1.http://www.codekk.com/open-source-project-analysis/detail/Android/grumoon/Volley%20%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90
2.http://blog.csdn.net/guolin_blog/article/details/17656437
部分配图及文字来自上面链接中的文章，谢谢原作者的辛勤劳动！
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花了两天时间看完了Volley的源码，我觉得有必要总结一下，故有此文。
主要特点：
(1). 扩展性强。Volley 中大多是基于接口的设计，可配置性强。
(2). 一定程度符合 Http 规范，包括返回 ResponseCode(2xx、3xx、4xx、5xx）的处理，请求头的处理，缓存机制的支持等。并支持重试及优先级定义。
(3). 默认 Android2.3 及以上基于 HttpURLConnection，2.3 以下基于 HttpClient 实现。
(2). 一定程度符合 Http 规范，包括返回 ResponseCode(2xx、3xx、4xx、5xx）的处理，请求头的处理，缓存机制的支持等。并支持重试及优先级定义。
(3). 默认 Android2.3 及以上基于 HttpURLConnection，2.3 以下基于 HttpClient 实现。
HttpURLConnection 和 AndroidHttpClient(HttpClient 的封装)如何选择及原因：
在 Froyo(2.2) 之前，HttpURLConnection 有个重大 Bug，调用 close() 函数会影响连接池，导致连接复用失效，所以在 Froyo 之前使用 HttpURLConnection 需要关闭 keepAlive。另外在 Gingerbread(2.3) HttpURLConnection 默认开启了 gzip 压缩，提高了 HTTPS 的性能，Ice Cream Sandwich(4.0) HttpURLConnection 支持了请求结果缓存。再加上 HttpURLConnection 本身 API 相对简单，所以对 Android 来说，在 2.3 之后建议使用 HttpURLConnection，之前建议使用 AndroidHttpClient。
(4). 提供简便的图片加载工具。
(4). 提供简便的图片加载工具。
（一）.基本使用
(a)各种Request的使用
volley使用及其简单，我们只需要创建一个RequestQueue请求队列，然后往队列里面扔http请求即可，volley会
不断从队列里面取出请求然后交给一堆工作线程处理。这里的http请求是通过Request类来封装的，我们只用创建Request对象，然后提供诸如url之类的参数即可。网络操作全部是在子线程中处理的，我们不必担心阻塞UI线程。
网络请求的结果会异步返回给我们，我们只需要处理Request的回调即可。
Request本身是一个抽象类，不能直接创建实例，volley为我们实现了一些Request，比如StringRequest是普通http请求，JsonRequest可以封装json数据，并将服务端的返回数据封装成JsonObject，ImageRequest可以请求一张网络图片，并将服务端的返回数据封装成Bitmap。
Request的使用分为三步:
1.创建RequestQueue队列；2.创建Request对象,并加入队列中；3.处理回调事件。
创建RequestQueue很简单，调用Volley类的静态方法newRequestQueue，并指定Context即可:
private RequestQueue mQueue = null;
// create request queue...
mQueue = Volley.newRequestQueue(this);//this代表当前的上下文
1.StringRequest
StringRequest的默认请求方式为GET,使用其他请求方式可以用其另一种重载形式。
String url = "http://192.168.56.1:8080";
StringRequest request = new StringRequest(url,new Response.Listener()
{
@Override
public void onResponse(String response)//success callbacks
{
//handle it
}
}, new Response.ErrorListener()//error callbacks
{
@Override
public void onErrorResponse(VolleyError error)
{
error.printStackTrace();
}
});
//add request to queue...
mQueue.add(request);
Response.Listener处理请求成功时的回调。Response.ErrorListener处理失败时的回调。代码很简单，不过多介绍。
2.JsonRequest
这里的JsonObject是android内置的org.json库，而不是自家的Gson，这点需要注意。
Map params = new HashMap();
params.put("name","zhangsan");
params.put("age","17");
JSONObject jsonRequest = new JSONObject(params);
Log.i(TAG,jsonRequest.toString());
//如果json数据为空则是get请求，否则是post请求
//如果jsonrequest不为null，volley会将jsonObject对象转化为json字符串原封不动的发给服务器,并不会转成k-v对,因为volley不知道应该如何转化
String url = "http://192.168.56.1:8080/volley_test/servlet/JsonServlet";
JsonObjectRequest request = new JsonObjectRequest(url, jsonRequest, new Response.Listener()
{
@Override
public void onResponse(JSONObject response)
{
//handle it
}
},new Response.ErrorListener()
{
@Override
public void onErrorResponse(VolleyError error)
{
error.printStackTrace();
}
});
mQueue.add(request);
3.ImageRequest
ImageRequest可以控制图片的宽高、照片品质。如果宽高比原始宽高小的话，将会进行压缩。
ImageRequest request = new ImageRequest("http://192.168.56.1:8080/volley_test/image.jpg",new Response.Listener()
{
@Override
public void onResponse(Bitmap response)
{
mImageView.setImageBitmap(response);
}
},0,0, Bitmap.Config.ARGB_8888,new Response.ErrorListener()
{//参数0 0 代表不压缩
@Override
public void onErrorResponse(VolleyError error)
{
show(error.getMessage());
//可以去显示默认图片
}
});
mQueue.add(request);
当然，图片加载还有另外两种方式，我们放到下一部分再介绍。
4.添加请求头
有时候我们需要为Request添加请求头，这时候可以去重写Request的getHeaders方法。
String url = "http://192.168.56.1:8080/volley_test/servlet/JsonServlet";
JsonObjectRequest request = new JsonObjectRequest(url, null,resplistener,errlistener)
{
//添加自定义请求头
@Override
public Map getHeaders() throws AuthFailureError
{
Map map = new HashMap();
map.put("header1","header1_val");
map.put("header2","header2_val");
return map;
}
};
5.添加post请求参数
添加Post请求参数可以重写Request的GetParams方法，另需 修改请求参数为POST。
String url = "http://192.168.56.1:8080/volley_test/servlet/PostServlet";
StringRequest request = new StringRequest(Method.POST,url,listener, errorListener)
{
//post请求需要复写getParams方法
@Override
protected Map getParams() throws AuthFailureError
{
Map map = new HashMap();
map.put("KEY1","value1");
map.put("KEY2", "value2");
return map;
}
};
Request里面还有一些getXXX方法，大家参考代码自己琢磨吧。
6.取消请求
当Activity销毁时，我们可能需要去取消一些网络请求，这时候可以通过如下方式:
Request req = ...;
request.setTag("MAIN_ACTIVITY");
onDestroy()
{
...
mQueue.cancelAll("MAIN_ACTIVITY");
}
为属于该Activity的请求全部加上Tag，然后需要销毁的时候调用cancelAll传入tag即可。
RequestQueue#cancelAll还有另一种重载形式，可以传入RequestFilter，自己指定一个过滤策略。
如果我们需要干掉所有请求，并且后续不再有网络请求，可以干掉RequestQueue，调用其stop方法即可。
7.全局共享RequestQueue
RequestQueue没有必要每个Activity里面都创建，全局保有一个即可。这时候自然想到使用Application了。我们可以在Application里面创建RequestQueue，并向外暴露get方法。代码很简单，相信大家都会写。
(b)图片加载框架的使用
上面介绍了ImageRequest加载网络图片，但是这还不够精简，volley另提供了ImageLoader和NetworkImageView。当然，它们内部都是使用ImageRequest。
1.ImageLoader
RequestQueue mQueue = ...;
ImageCache mCache = ...;
loader = new ImageLoader(mQueue,mImageCache);
ImageListener listener = ImageLoader.getImageListener(mImageView/*关联的iamgeView*/,R.drawable.ic_launcher/*图片加载时显示*/, R.drawable.task_icon/*图片加载失败时显示*/);
loader.get("http://192.168.56.1:8080/volley_test/image.jpg", listener, 0, 0);
首先创建RequestQueue不用多说，然后创建ImageLoader实例，同时需要传入请求队列和ImageCache。这个ImageCache是一个接口，我们需要自己实现，它是一个图片缓存，通常我们都是结合LRUCache作为内存缓存使用，当然，如果你不想使用内存缓存，那就给它一个空实现即可。紧接着，需要为ImageLoader绑定ImageView以及图片加载时、图片加载失败时的图片资源。最后调用get方法传入url请求网络。
关于ImageCache缓存：
ImageCache接口：
public interface ImageCache {
public Bitmap getBitmap(String url);
public void putBitmap(String url, Bitmap bitmap);
}
可见，如果我们需要图片缓存，那么实现getBitmap/putBitmap即可。每次调用ImageLoader#get方法时，都会首先从ImageCache中寻找(getBitmap),如果没找到才将请求添加到队列中。
应该怎样实现ImageCache：
通常结合LRUCache。值得注意的是
ImageCache需要做成单例，全局共享。下面是ImageCache结合LRUCache的代码:
/**
* @author Rowandjj
*图片缓存需要做成单例,全局共享
*/
private static class LruImageCache implements ImageCache
{
private LruImageCache(){}
private static LruImageCache instance = new LruImageCache();
public static final LruImageCache getInstance()
{
return instance;
}
private static final String TAG = "LruImageCache";
private final int maxSize = (int) (Runtime.getRuntime().maxMemory()/8);
private LruCache mCacheMap = new LruCache(maxSize)
{
protected int sizeOf(String key, Bitmap value)
{
return value.getRowBytes()*value.getHeight();
}
};
@Override
public Bitmap getBitmap(String url)
{
Bitmap bitmap = mCacheMap.get(url);
Log.i(TAG, "url = "+url+",cache:"+bitmap);
return bitmap;
}
@Override
public void putBitmap(String url, Bitmap bitmap)
{
Log.i(TAG, "put url = "+url);
mCacheMap.put(url, bitmap);
}
}
2.NetworkImageView
这是一个自定义控件，使用上跟ImaegView类似。
ImageLoader loader = ...;
mNetImageView = findViewById(R.id.niv);
mNetImageView.setDefaultImageResId(R.drawable.ic_launcher);
mNetImageView.setErrorImageResId(R.drawable.task_icon);
mNetImageView.setImageUrl("http://192.168.56.1:8080/volley_test/image.jpg", loader);
代码很简单，不过多介绍。
(c)定制request
volley是一个高度可扩展性的框架，我们可以在其基础上，增加很多自己的东西，比如定制Request。
上面介绍了JsonRequest可以解析传递json数据，那么下面我们定制一个XMLRequest去解析XML数据。
定制Request需重写Request#parseNetworkResponse解析网络响应数据，Request#deliverResponse回调Listener.onResponse。
public class XMLRequest extends Request
{
private Listener mListener;
public XMLRequest(int method, String url, Listener listener,
ErrorListener errorListener)
{
super(method, url, errorListener);
mListener = listener;
}
public XMLRequest(String url, Listener listener,
ErrorListener errorListener)
{
this(Method.GET, url, listener, errorListener);
}
@Override
protected Response parseNetworkResponse(NetworkResponse response)
{
try
{
String xmlString = new String(response.data,HttpHeaderParser.parseCharset(response.headers));
XmlPullParser parser = Xml.newPullParser();
parser.setInput(new StringReader(xmlString));//将返回数据设置给解析器
return Response.success(parser,HttpHeaderParser.parseCacheHeaders(response));
} catch (UnsupportedEncodingException e)
{
return Response.error(new VolleyError(e));
} catch (XmlPullParserException e)
{
return Response.error(new VolleyError(e));
}
}
@Override
protected void deliverResponse(XmlPullParser response)
{
mListener.onResponse(response);
}
} /**
* xmlRequest 使用示例
*/
void test()
{
RequestQueue queue = Volley.newRequestQueue(context);
String url = "";
XMLRequest request = new XMLRequest(url,new Response.Listener()
{
@Override
public void onResponse(XmlPullParser response)
{
int type = response.getEventType();
while(type != XmlPullParser.END_DOCUMENT)
{
switch (type)
{
case XmlPullParser.START_TAG:
break;
case XmlPullParser.END_TAG:
break;
default:
break;
}
response.next();
}
}
},new Response.ErrorListener()
{
@Override
public void onErrorResponse(VolleyError error)
{
}
});
} public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery)
我们发现构造器需要指定缓存策略(默认硬盘缓存)，线程池大小，结果分发策略等等。所有这些都是可配置的。volley这个框架写的太牛逼了！
（二）.源码分析
仅知道如何使用那是远远不够的，我们应该分析源码看其内部实现原理，这样才能够进步。
1.主线
首先我们从整体上把握volley的工作流程，抓住其主线。
（1）请求队列(RequestQueue)的创建
创建请求队列的工作是从Volley#newRequestQueue开始的,这个方法内部会调用RequestQueue的构造器，同时指定一些基本配置，如缓存策略为硬盘缓存(DiskBasedCache),http请求方式为HttpURLConnection(level>9)和HttpClient(level<9)，默认线程池大小为4。最后，调用RequestQueue#start启动请求队列。
看详细代码:
//Volley.java
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}//volley.java
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
... ...
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;
}
这里指定了硬盘缓存的位置为data/data/package_name/cache/volley/...，Network类（具体实现类是BasicNetwork）封装了请求方式，并且根据当前API版本来选用不同的http工具。最后启动了请求队列。
下面看RequestQueue的构造器:
//RequestQueue.java
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}//RequestQueue.java
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
这里的ResponseDelivery是请求结果的分发器（具体实现是ExecutorDelivery），内部将结果返回给主线程(
根据代码中使用了Handler和UI线程的Looper大家就应该能猜到了)，并处理回调事件。
下面看请求队列的开始后会发生什么,查看RequestQueue#start方法:
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();
}
}
（2）请求的添加
请求的添加是通过RequestQueue#add完成的，add方法的逻辑是这样的:
1.将请求加入mCurrentRequests集合
2.为请求添加序列号
3.判断是否应该缓存请求，如果不需要，加入网络请求队列
4.如果有相同请求正在被处理，加入到相同请求等待队列中，否则加入缓存请求队列。
public Request add(Request 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 stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList();
}
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;
}
} 通过这一方法，请求就被分发到两个队列中分别供CacheDispatcher和NetworkDispatcher处理。
（3）请求的处理
请求的处理是由CacheDispatcher和NetworkDispatcher来完成的，它们的run方法通过一个死循环不断去从各自的队列中取出请求，进行处理，并将结果交由ResponseDelivery。两者处理思想一致但是具体逻辑还是有点区别，我们分别看看。
1.走缓存的请求
CacheDispatcher.java#run
@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 reques
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;
}
}
}大体逻辑是这样的，首先从队列中取出请求，看其是否已被取消，若是则返回，否则继续向下走。接着从硬盘缓存中通过缓存的键找到值(Cache.Entry),如果找不到，那么将此请求加入网络请求队列。否则对缓存结果进行过期判断( 这个需要请求的页面指定了Cache-Control或者Last-Modified/Expires等字段,并且Cache-Control的优先级比Expires更高。否则请求一定是过期的 )，如果过期了，则加入网络请求队列。如果没有过期，那么通过request.parseNetworkResponse方法将硬盘缓存中的数据封装成Response对象（Request的parseNetworkResponse是抽象的，需要复写）。最后进行新鲜度判断，如果不需要刷新，那么调用ResponseDelivery结果分发器的postResponse分发结果。否则先将结果返回，再将请求交给网络请求队列进行刷新。【这段代码读起来很爽，google工程师写的太赞了！】 关于ResponseDelivery的具体过程我们留到下节讲。
2.走网络的请求
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request request;
while (true) {
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;
}
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
// 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());
mDelivery.postError(request, new VolleyError(e));
}
}
} NetworkResponse networkResponse = mNetwork.performRequest(request); public NetworkResponse performRequest(Request request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map responseHeaders = new HashMap();
try {
// Gather headers.
Map headers = new HashMap();
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) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
request.getCacheEntry().data, responseHeaders, true);
}
// Some responses such as 204s do not have content. We must check.
if (httpResponse.getEntity() != null) {
responseContents = entityToBytes(httpResponse.getEntity());
} else {
// Add 0 byte response as a way of honestly representing a
// no-content request.
responseContents = new byte[0];
}
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}
return new NetworkResponse(statusCode, responseContents, responseHeaders, false);
} 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);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(networkResponse);
}
}
}
} httpResponse = mHttpStack.performRequest(request, headers);它调用了HttpStack的performRequest，这个方法内部肯定会调用HttpURLConnection或者是HttpClient去请求网络。这里我们就不必继续向下跟源码了。
（4）请求结果的分发与处理
请求结果的分发处理是由ResponseDelivery实现类ExecutorDelivery完成的，ExecutorDelivery是在RequestQueue的构造器中被创建的，并且绑定了UI线程的Looper:
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}@Override
public void postResponse(Request request, Response response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
@Override
public void postResponse(Request request, Response response) {
postResponse(request, response, null);
}private class ResponseDeliveryRunnable implements Runnable {
private final Request mRequest;
private final Response mResponse;
private final Runnable mRunnable;
public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
mRequest = request;
mResponse = response;
mRunnable = runnable;
}
@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();
}
}
}
2.一些支线细节
（1）请求的取消
调用Request#cancel可以取消一个请求。cancel方法很简单，仅将mCanceled变量置为true。而CacheDispatcher/NetworkDispatcher的run方法中在取到一个Request后会判断是否请求取消了:
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
（2）请求队列的终止
调用RequestQueue#stop可以终止整个请求队列，并终止缓存请求线程与网络请求线程:
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
（3）ImageLoader
ImageLoader是对ImageRequest的封装，这里重点关注下get方法:
public ImageContainer get(String requestUrl, ImageListener imageListener,
int maxWidth, int maxHeight) {
...
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;
}
...
Request newRequest =
new ImageRequest(requestUrl, new Listener() {
@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;
}
首先会从缓存中获取如果没有则构造ImageRequest并添加到请求队列。
（4）关于缓存
Volley的CacheDispatcher工作时需要指定缓存策略，这个缓存策略即Cache接口，这个接口有两个实现类，DiskBasedCache和NoCache，默认使用DiskedBasedCache。它会将请求结果存入文件中，以备复用。Volley是一个高度灵活的框架，缓存是可以配置的。甚至你可以使用自己的缓存策略。
可惜这个DiskBasedCache很多时候并不能被使用，因为CacheDispatcher即使从缓存文件中拿到了缓存的数据，还需要看该数据是否过期，如果过期，将不使用缓存数据。这就要求服务端的页面可以被缓存，这个是由Cache-Control和Expires等字段决定的，服务端需要设定此字段才能使数据可以被缓存。否则缓存始终是过期的，最终总是走的网络请求。
服务端假如是servlet写的可以这样做:
Servlet#doPost/doGet()
/*设置缓存*/
resp.setDateHeader("Last-Modified",System.currentTimeMillis());
resp.setDateHeader("Expires", System.currentTimeMillis()+10*1000*60);
resp.setHeader("Cache-Control","max-age=10000");
resp.setHeader("Pragma","Pragma");public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
long now = System.currentTimeMillis();
Map headers = response.headers;
long serverDate = 0;
long serverExpires = 0;
long softExpire = 0;
long maxAge = 0;
boolean hasCacheControl = false;
String serverEtag = null;
String headerValue;
headerValue = headers.get("Date");
if (headerValue != null) {
serverDate = parseDateAsEpoch(headerValue);
}
headerValue = headers.get("Cache-Control");
if (headerValue != null) {
hasCacheControl = true;
String[] tokens = headerValue.split(",");
for (int i = 0; i < tokens.length; i++) {
String token = tokens[i].trim();
if (token.equals("no-cache") || token.equals("no-store")) {
return null;
} else if (token.startsWith("max-age=")) {
try {
maxAge = Long.parseLong(token.substring(8));
} catch (Exception e) {
}
} else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
maxAge = 0;
}
}
}
headerValue = headers.get("Expires");
if (headerValue != null) {
serverExpires = parseDateAsEpoch(headerValue);
}
serverEtag = headers.get("ETag");
// Cache-Control takes precedence over an Expires header, even if both exist and Expires
// is more restrictive.
if (hasCacheControl) {
softExpire = now + maxAge * 1000;
} else if (serverDate > 0 && serverExpires >= serverDate) {
// Default semantic for Expire header in HTTP specification is softExpire.
softExpire = now + (serverExpires - serverDate);
}
Cache.Entry entry = new Cache.Entry();
entry.data = response.data;
entry.etag = serverEtag;
entry.softTtl = softExpire;
entry.ttl = entry.softTtl;
entry.serverDate = serverDate;
entry.responseHeaders = headers;
return entry;
} Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response))
另外，在使用图片加载框架时还有个ImageCache，它是作为图片加载的一级缓存，跟上面的DiskedBasedCache没有任何关系，大家不要混淆。ImageCache需要我们自己实现，通常结合LRUCache，具体第一部分已经介绍过了。
到这里我们把整个Volley框架全部分析完了。最后贴上Volley的整体架构图:
下面这幅图也很好：
以上就是Volley的全部内容，谢谢大家看到最后！祝大家春节快乐！！
测试代码：http://download.csdn.net/detail/rowandjj/8448383