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Volley框架源码浅析（一）
尊重原创http://blog.csdn.net/yuanzeyao/article/details/25837897
从今天开始，我打算为大家呈现关于Volley框架的源码分析的文章，Volley框架是Google在2013年发布的，主要用于实现频繁而且粒度比较细小的Http请求，在此之前Android中进行Http请求通常是使用HttpUrlConnection和HttpClient进行，但是使用起来非常麻烦，而且效率比较地下，我想谷歌正式基于此种原因发布了Volley框架，其实出了Volley框架意外，也有一些http请求开源项目，比如使用比较广泛的有async-http,UniversImageLoader等等，其中async-http主要用来实现异步http请求，而后者主要用来请求图片。Volley具有以上两种框架的功能，并且是Google公司发布，我想作为Android开发者，很有必要研究一下该框架。
1、下载Volley框架
git clone https://android.googlesource.com/platform/frameworks/volley
2、引用Volley框架
引用该框架的方式主要有两种：
(1):直接导入Volley框架，作为lib使用
(2):编译Volley成jar包
3、Volley的功能：
前面已经提及了Volley是一个用于http请求的框架，其主要功能如下：
json,xml,String，Image等资源的请求，当然我们还可以根据自己的需要来改写Volley框架源码，从而实现自己的功能
4、Volley源码分析
温馨提醒：如果是第一次看Volley源码，第一遍没看懂没关系，将源码copy下来，跟着我的思路慢慢分析，将文章从头到后多看几遍就ok了，因为Volley的一些关键类都互相应用，我只能一个一个的分析了，等你看完我所有的文章，然后再从头看一变，相信你定有所收获
当然阅读次文章之前，最好学会知道Volley框架的基本使用，由于网络上很多类似的教程，我在此处就不再描述了，后期如果有时间我也会讲解一下Volley的应用
我们就从Volley这个类开始吧
(1) Volley.java
public class Volley {
//缓存目录
private static final String DEFAULT_CACHE_DIR = "volley";
//创建一个默认的请求队列，我们的请求创建好后，放入该队列即可
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
//缓存目录
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) {
/**
如果我们没有传入stack，那么自己创建一个，如果sdk>9(就是2.3以上)，那么使用
HttpURLConnection实现http请求，如果2.3以前使用HttpClient实现，因为在2.3以前httpURLConnection不稳定
*/
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类是一个网络请求类，使用stack进行网络请求
Network network = new BasicNetwork(stack);
//真正创建一个请求队列，传入一个磁盘缓存和网络请求类
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
//启动请求队列，其实里面就是启动了一些线程，不断监听是否有请求
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);
}
}(2) HttpStack.java
下面看看HttpStack是何方神圣
public interface HttpStack {
/**
名字挺吓人的，呵呵，其实就是一个接口，它有两个实现，分别是HurlStack,HttpClientStack，通过名字大家
可以猜出来一个基于HttpClient，一个基于HttpURLConnection
* @return the HTTP response
*/
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError;
}直接查看它的子类方法吧
首先看 HurlStack.java类
这个类是基于HttpURLConnection实现的
由于这个类比较长，我就重点讲解一下
(3) HurlStack.java
/**
继承自HttpStack,我们暂时就把Request抽象成一个请求，包括url，method等信息，后期我们会重点分析这个类
第二个参数就是一些请求头信息
*/
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
String url = request.getUrl();
HashMap<String, String> map = new HashMap<String, String>();
map.putAll(request.getHeaders());
map.putAll(additionalHeaders);
//此处一般为空，我们直接忽略掉
if (mUrlRewriter != null) {
String rewritten = mUrlRewriter.rewriteUrl(url);
if (rewritten == null) {
throw new IOException("URL blocked by rewriter: " + url);
}
url = rewritten;
}
URL parsedUrl = new URL(url);
HttpURLConnection connection = openConnection(parsedUrl, request);
//添加请求头
for (String headerName : map.keySet()) {
connection.addRequestProperty(headerName, map.get(headerName));
}
//这个方法名字很长，其实功能很简单，就是为connection设置请求方法 如get post等等
setConnectionParametersForRequest(connection, request);
// Initialize HttpResponse with data from the HttpURLConnection.
ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
int responseCode = connection.getResponseCode();
if (responseCode == -1) {
// -1 is returned by getResponseCode() if the response code could not be retrieved.
// Signal to the caller that something was wrong with the connection.
throw new IOException("Could not retrieve response code from HttpUrlConnection.");
}
StatusLine responseStatus = new BasicStatusLine(protocolVersion,
connection.getResponseCode(), connection.getResponseMessage());
BasicHttpResponse response = new BasicHttpResponse(responseStatus);
response.setEntity(entityFromConnection(connection));
for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
if (header.getKey() != null) {
Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
response.addHeader(h);
}
}
//http的返回结果
return response;
}(4) HttpClientStack.java
/**
相对比上一个方法简单，相信使用过httpClient的同学一看就明白
*/
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
//这个方法见名知意，就是创建一个HttpGet或者HttpPost
HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);
//添加头信息
addHeaders(httpRequest, additionalHeaders);
addHeaders(httpRequest, request.getHeaders());
//在请求之前进行准备工作，其实是个空方法，很想AsyncTask的onPreExecute
onPrepareRequest(httpRequest);
HttpParams httpParams = httpRequest.getParams();
int timeoutMs = request.getTimeoutMs();
// TODO: Reevaluate this connection timeout based on more wide-scale
// data collection and possibly different for wifi vs. 3G.
HttpConnectionParams.setConnectionTimeout(httpParams, 5000);
HttpConnectionParams.setSoTimeout(httpParams, timeoutMs);
//执行请求并返回结果
return mClient.execute(httpRequest);
}看到这里大家肯定觉得这个框架也没有什么了不起嘛，和使用HttpURLConnection和HttpClient差不多嘛，如果你真的这样觉得那么你就大错特错了，其实这个框架的核心在于线程的调度和缓存上面，后期我们会介绍的
回到Volley类，我们看下一个陌生的类就是Network，其实Network不过是个接口而已，它的实现类是BaskNetwork
(5) BaskicNetwork.java
从名字我们就可以看出来，这个类就是进行网络请求的，其实他就是对HttpurlStack或者HttpClientStack的一个封装，真正实现请求的还是上面两个类。
最核心的方法：
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = new HashMap<String, String>();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
//调用mHttpStack执行http请求
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) {
//将返回结果封装成一个NetworkResponse
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
request.getCacheEntry() == null ? null : 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) {
//超时重新请求
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
//超时重新请求
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);
}
}
}
}这个方法调用了mHttpStack的同名方法，只不过在mHttpStack中返回的是HttpResponse，在这里返回的是NetworkResponse。
然后再看看本篇文章的最后一个类：
RequestQueue.java
我保留了一些关键字段，删除不影响理解的字段
public class RequestQueue {
...
//本地缓存队列，如果一个请求能够缓存，那么先放到这个队列中，如果本地缓存没有命中，则加入网络队列，见后面
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();
//网络请求队列
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();
//默认的网络请求线程个数 默认四个，这个我们可以改动
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
//本地缓存的接口
private final Cache mCache;
//这个类相信大家并不陌生
private final Network mNetwork;
//由于网络请求在子线程中执行，这个对象将请求结果发送到ui线程，功能很像Handler
private final ResponseDelivery mDelivery;
//网络线程数组
private NetworkDispatcher[] mDispatchers;
//本地线程数组 只有一条
private CacheDispatcher mCacheDispatcher;
/**
创建一个请求队列
参数1：本地缓存
参数2: network 进行网络进行的包装类
参数3：网络请求线程池大小
参数4：就是一个将子线程的数据发送到ui线程的功能类，先可以不用关心
*/
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
/**
* Creates the worker pool. Processing will not begin until {@link #start()} is called.
*
* @param cache A Cache to use for persisting responses to disk
* @param network A Network interface for performing HTTP requests
*/
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}
//启动本地和网络线程
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();
}
}
//关闭本地和网络线程
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
//相当于一个过滤器，对于apply方法返回true的Request可以从Queue中删除
public interface RequestFilter {
public boolean apply(Request<?> request);
}
//借助上面的方法实现对没有执行的Request进行删除
public void cancelAll(RequestFilter filter) {
synchronized (mCurrentRequests) {
for (Request<?> request : mCurrentRequests) {
if (filter.apply(request)) {
request.cancel();
}
}
}
}
//取消所有的请求
public void cancelAll(final Object tag) {
if (tag == null) {
throw new IllegalArgumentException("Cannot cancelAll with a null tag");
}
cancelAll(new RequestFilter() {
@Override
public boolean apply(Request<?> request) {
return request.getTag() == tag;
}
});
}
/**
将Request放入两个队列中的一个
*/
public <T> Request<T> add(Request<T> 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");
// 如果该Request不能缓存，那么直接放入网络队列
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
// 把Request放入具有相同CacheKey的链表中，如果没有相同的CacheKey的Request请求存在，则放入本地队列
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
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;
}
}
/**
* 结束一个Request
*/
void finish(Request<?> request) {
// Remove from the set of requests currently being processed.
synchronized (mCurrentRequests) {
mCurrentRequests.remove(request);
}
if (request.shouldCache()) {
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
if (waitingRequests != null) {
if (VolleyLog.DEBUG) {
VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
waitingRequests.size(), cacheKey);
}
// Process all queued up requests. They won‘t be considered as in flight, but
// that‘s not a problem as the cache has been primed by ‘request‘.
mCacheQueue.addAll(waitingRequests);
}
}
}
}
}写到这里先高一段落吧，来个小小的总结：Volley中有一个RequestQueue（包含本地队列和网络队列），就是请求队列，每一个http请求都被封装成了一个Request,通过队列的add方法加入队列，如果一个Request可以缓存，那么先加入本地队列，如果不能缓存则加入网络队列
待续。。。。。。。
欢迎留言讨论。。。
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