通过前面的学习我们已经掌握了Volley的基本用法，没看过的建议大家先去阅读我的博文【安卓网络请求开源框架Volley源码解析系列】初识Volley及其基本用法。如StringRequest用来请求一段文本信息，JsonRequest(JsonObjectRequest、JsonArrayRequest)用于请求一条JSON格式的数据，ImageRequest则是用于请求网络上的图片资源。但是我们知道在网络上传输的轻量级数据还包括另外一种即XML。但是Volley中目前还没提供用于请求XML格式的类，那我们如果我们要请求的是一段XML格式数据该怎们办？
答案很简单，自己动手写，还记得在初识Volley的博客中讲到的Volley的基本特点之一吗：可扩展性强。Volley提供了非常强的扩展机制，使得我们可以很轻松地定制出任意类型的Request。
一定制自己的Request请求
定制XMLRequest：去扩展不是自己开发的框架中的某些功能不是一件很容易的事，因为你很可能不知道从哪个地方作为切入点，因此我们首先要做的就是把谷歌官方写的一些与自己要定制的类功能相似的类好好研究一下，看能否看懂然后自己模仿。那么自然首先想到去研究一下StringRequest类的源码，代码如下：
/**
* A canned request for retrieving the response body at a given URL as a String.
*/
public class StringRequest extends Request {
private final Listener mListener;
/**
* Creates a new request with the given method.
*
* @param method the request {@link Method} to use
* @param url URL to fetch the string at
* @param listener Listener to receive the String response
* @param errorListener Error listener, or null to ignore errors
*/
public StringRequest(int method, String url, Listener listener,
ErrorListener errorListener) {
super(method, url, errorListener);
mListener = listener;
}
/**
* Creates a new GET request.
*
* @param url URL to fetch the string at
* @param listener Listener to receive the String response
* @param errorListener Error listener, or null to ignore errors
*/
public StringRequest(String url, Listener listener, ErrorListener errorListener) {
this(Method.GET, url, listener, errorListener);
}
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}
@Override
protected Response parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}
}
public class StringRequest extends Request 接下来是其构造器，这个在初识Volley的博客中已经讲过，因此不再赘述。
然后是两个重写方法，这也是我们关注的重点：
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}
@Override
protected Response parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
} 第一个为protected void deliverResponse(String response) ，它是在其父类Request中定义的一个抽象方法，即所有继承自Request的子类必须重写它，其定义如下：
abstract protected void deliverResponse(T response);第二个为 protected Response
abstract protected Response parseNetworkResponse(NetworkResponse response); public class NetworkResponse {
/**
* Creates a new network response.
* @param statusCode the HTTP status code
* @param data Response body
* @param headers Headers returned with this response, or null for none
* @param notModified True if the server returned a 304 and the data was already in cache
*/
public NetworkResponse(int statusCode, byte[] data, Map headers,
boolean notModified) {
this.statusCode = statusCode;
this.data = data;
this.headers = headers;
this.notModified = notModified;
}
public NetworkResponse(byte[] data) {
this(HttpStatus.SC_OK, data, Collections.emptyMap(), false);
}
public NetworkResponse(byte[] data, Map headers) {
this(HttpStatus.SC_OK, data, headers, false);
}
/** The HTTP status code. */
public final int statusCode;
/** Raw data from this response. */
public final byte[] data;
/** Response headers. */
public final Map headers;
/** True if the server returned a 304 (Not Modified). */
public final boolean notModified;
} 重点关注byte[] data这个成员属性， 它是用来存储从服务器端获取的数据，/** Raw data from this response. */，即从服务器端获取到数据将以字节的形式存放在data字节数组中。因此我们需要获取怎样类型的数据就只需要对该数组中的内容进行相应操作即可，如在上述的StringRequest中因为要获取的是String类型的数据，因此我们需要将其以字符串的形式表示， parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));然后调用Response.success（）方法将其返回return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));。其中success返回的是代泛型参数的Response对象，其定义如下：
/** Returns a successful response containing the parsed result. */
public static Response success(T result, Cache.Entry cacheEntry) {
return new Response(result, cacheEntry);
} 讲到这我们基本上就从源码的角度彻底理解了StringRequest类，那么接下来我们就按照上述我为大家分析的步骤模仿写一个XMLRequest，代码如下：
public class XMLRequest extends Request {//继承自Request类，指定泛型参数为XML解析器
private final Listener mListener;//指定监听器的泛型参数为XML解析器
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));
XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
XmlPullParser xmlPullParser = factory.newPullParser();
xmlPullParser.setInput(new StringReader(xmlString));
return Response.success(xmlPullParser, HttpHeaderParser.parseCacheHeaders(response));//返回包含该xml解析器的Response对象
} catch (UnsupportedEncodingException e) {
return Response.error(new ParseError(e));
} catch (XmlPullParserException e) {
return Response.error(new ParseError(e));
}
}
@Override
protected void deliverResponse(XmlPullParser response) {
mListener.onResponse(response);
}
} XMLRequest xmlRequest = new XMLRequest(
"http://flash.weather.com.cn/wmaps/xml/china.xml",
new Response.Listener() {
@Override
public void onResponse(XmlPullParser response) {
try {
int eventType = response.getEventType();
while (eventType != XmlPullParser.END_DOCUMENT) {
switch (eventType) {
case XmlPullParser.START_TAG:
String nodeName = response.getName();
if ("city".equals(nodeName)) {
String pName = response.getAttributeValue(0);
Log.d("TAG", "pName is " + quName);
}
break;
}
eventType = response.next();
}
} catch (XmlPullParserException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("TAG", error.getMessage(), error);
}
});
mQueue.add(xmlRequest); 运行结果如下：
讲到这基本上就把Volley的用法及其扩展全部讲完了，相信大家对Volley使用已经相当熟练了，但这还不够，接下来我们不再是从单个类的源码角度来解析Volley，而是从整个框架体系角度来解析Volley，让你彻底了解Volley的整个框架体系。
二Volley框架源码剖析
首先我们来看一下谷歌官方文档中Volley的框架图：
从这个图可以看到整个框架体系包括至少三个线程：主线程（main thread），缓存调度线程（cache thread），网络调度线程（network thread）。那么我们首先从主线程入手。
可以看到在主线程中会根据优先级将请求添加到缓存队列。所谓缓存队列就是前面我们每次使用Volley时都要创建的RequestQueue对象。那么我们首先来看一下其源代码：
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);
}那么最核心的当然是start()方法，我们来看一下其源码：
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();
}
}创建了请求队列之后我们只需要构建出相应的Request，然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作，那么接下来我们来看一下请求添加
即add()的源码：
/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
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;
}
}
1.将请求加入mCurrentRequests集合（注意此行为是同步的）
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。
通过上述分析我们可以知道add方法仅仅只起到添加请求的作用，而请求的处理是CacheDispatcher和NetworkDispatcher来完成的。这也正好与上述给的Volley的框架流程图相符。我们来看一下其源码：
public class CacheDispatcher extends Thread {
private static final boolean DEBUG = VolleyLog.DEBUG;
/** The queue of requests coming in for triage. */
private final BlockingQueue mCacheQueue;
/** The queue of requests going out to the network. */
private final BlockingQueue mNetworkQueue;
/** The cache to read from. */
private final Cache mCache;
/** For posting responses. */
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new cache triage dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param cacheQueue Queue of incoming requests for triage
* @param networkQueue Queue to post requests that require network to
* @param cache Cache interface to use for resolution
* @param delivery Delivery interface to use for posting responses
*/
public CacheDispatcher(
BlockingQueue cacheQueue, BlockingQueue networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@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;
}
}
}
}
缓存线程是一直在运行的，在该while循环中的处理过程大致如下：
1首先从缓存中取出一个请求，如果结果为空，则把该请求加入到网络请求中，否则对缓存结果进行过期判断。
2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构造为Response对象（Request的parseNetworkResponse是抽象的，需要子类重写）。
在来看看网络请求线程的源码：
public class NetworkDispatcher extends Thread {
/** The queue of requests to service. */
private final BlockingQueue mQueue;
/** The network interface for processing requests. */
private final Network mNetwork;
/** The cache to write to. */
private final Cache mCache;
/** For posting responses and errors. */
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new network dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param queue Queue of incoming requests for triage
* @param network Network interface to use for performing requests
* @param cache Cache interface to use for writing responses to cache
* @param delivery Delivery interface to use for posting responses
*/
public NetworkDispatcher(BlockingQueue queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@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));
}
}
}
private void parseAndDeliverNetworkError(Request request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
}
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);即执行网络请求，在这个perfromRequest中会调用HttpClient或HttpUrlConnection去执行网络请求，然后将从服务器端获取到的数据构造成一个NetWorkResponse对象返回。
4请求结果的分发与处理：
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。还记得我们上述自定义Request的方式吗？其中parseNetworkResponse()这个方法就是必须要重写的。
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，代码如下
@Override
public void postResponse(Request request, Response response) {
postResponse(request, response, null);
}
@Override
public void postResponse(Request request, Response response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
} 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();
}
}
}
}
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);}至此Volley的整个框架体系就解析完了，我们来总结一下：
1首先在主线程中创建一个请求队列：Volley.newRequestQueue(context)然后通过add()添加一条网络请求。
2这条请求会被先添加到缓存队列，如果在缓存队列中找到该请求则读取缓存解析，然后将解析结果返回给主线程
3如果没找到该请求则将该请求加入网络请求队列，发送HTTP请求，获取服务器端返回的结果，解析之后写入缓存，然后将解析结果返回主线程。
如果你再仔细看一下上述Volley框架体系结构图，你会看到上述三个步骤正好完全对应谷歌官方文档的架构体系流程图。
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