volley是现在常用的网络请求库，使用也非常简单，在google的官网上就有一张图介绍volley的请求过程，
volley-request.png
本文主要从源码上分析volley的请求过程，达到知其然也知其所以然的目的。
1，step 1
在volley使用的最开始都需要调用
RequestQueue queue = Volley.newRequestQueue(this);
那么这句话的作用到底是什么呢，在源码中是这么写的：
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
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) {
e.printStackTrace();
}
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;
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start();
return queue;
}
这段代码非常简单，首先是见一个缓存文件，然后根据SDK的不同版本新建不同的HttpStack，在版本号大于9的时候使用的HurlStack，在后面可以看到也就是使用的HttpUrlConnection来进行网络请求，而在版本号小于9的时候使用的是HttpClientStack，也就是使用的是HttpClient进行网络请求。在然后就新建了一个RequestQueue，下面再看看这个RequestQueue的构造方法和start方法：
/**
* 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);
}
/**
* Starts the dispatchers in this queue.
*/
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();
}
}
从上面的代码可以看出，volley在启动的时候新建了一个CecheDispatcher和4个NetworkDispatcher，这两个都是继承自Thread，也就是说volley有一个缓存队列和4个网络请求队列，在不停的等待加入队列的请求。在请求队列完成之后，就是要将request加入到请求队列，下面看看请求队列是如何添加请求的，
RequestQueue添加请求的方法是这样的：
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");
// 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<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;
}
}
首先将request加入到当前的请求集合中，如何判断是否使用了缓存，如果没有使用请求缓存，那就很简单，直接加入到NetworkQueue中开始请求，如果使用了请求缓存，那么就先看WaitingRequest中是否包含request的cacheKey，如果有，就加入到这个WaitingRequest中，如果没有，那么就将这个request加入到cacheQueue中。现在我们已经知道了如何将request加入到RequestQueue中，那么现在就看看RequestQueue中的一个CacheDispatcher和4个NetworkDispatcher是如何工作的：
先看NetWorkDispatcher的run方法：
// 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");
省略了部分方法，源码也很简单，就是从请求队列中取出一个，如何调用mNetwork请求，获得response后，再对response进行转化。
再看CacheDispatcher的run方法：
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();
Request<?> request;
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
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.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}
首先还是从CacheQueue中取出一个request，然后从缓存中检查看是否存在包含request的cacheKey，如果没有，那么就直接加入到NetworkQueue中，如果有，还要看缓存中包含这个cacheKey的缓存是否已经过期，如果过期了，那么还是加入到NetworkQueue中，如果存在这个request的缓存，而且还没有过期，那么就直接从缓存中取出response返回，这个时候还要判断是否需要更新缓存中的 数据，如果需要，那么就把request加入到Networkqueue中再请求一次。
上面看了如何将request加入到NetworkQueue中，然后NetworkDispatcher就取出Networkqueue中的request进行请求，调用的是Network的performRequest，那么Network是怎么继续请求的呢，在volley中，Network的实现类是BasicNetwork，看看BasicNetwork的performRequest方法：
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
这段方法中大多都是一些网络请求细节方面的东西，我们并不需要太多关心，需要注意的是调用了HttpStack的performRequest()方法，这里的HttpStack就是在一开始调用newRequestQueue()方法是创建的实例，默认情况下如果系统版本号大于9就创建的HurlStack对象，否则创建HttpClientStack对象。前面已经说过，这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的，我们就不再跟进去阅读了，之后会将服务器返回的数据组装成一个NetworkResponse对象进行返回。
下面就看一下HurlStack的performRequest方法：
@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);
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);
for (String headerName : map.keySet()) {
connection.addRequestProperty(headerName, map.get(headerName));
}
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);
if (hasResponseBody(request.getMethod(), responseStatus.getStatusCode())) {
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);
}
}
return response;
}
以上就是volley请求的完整过程，看完源码再看官方给出的流程图，确实清晰多了。
