在上一篇文章Android-使用Volley 连接网络中，学习了简单使用Volley连接网络并获取数据，今天就来学习一下Volley的源码。毕竟刚开始接触Volley的时候，谁都难免会对其实现过程有所迷惑，我以自己刚接触Volley时心中的几点疑惑为例：
1)Volley 是靠什么去连接网络的？
2)Volley要连接网络，就需要异步类的操作，Volley是如何处理这步的？
3)Volley的流程是如何跑的？
在阅读源代码之前，可以先观看一下Android官方文档提供的一张图（如下图）：
先可以按照图上显示的内容，从字面上了解Volley的流程：首先是Request在主线程被添加到一个优先顺序的队列，然后交给了CacheDispatcher，
CacheDispatcher从字面上意思猜测是个缓存调度器，先不用管它到底干了什么，但是可以肯定它对Request做了处理，如果处理结果存在，就从Cache中请求数据并解析，再将得到的结果返回给主线程，让主线程解析去解析这个结果，如果处理结果不存在，就将Request丢给NetworkDispatcher（也是一个调度器），NetworkDispatcher就处理这个request，并把结果返回给主线程，让主线程去解析这个结果，根据图了解的简单流程就这样，现在就带着这个简单的流程和上面的问题去看Volley源代码：
-->程序获得RequestQueue的时候，Volley源代码实现了一些什么：
/**
* 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.
* @param stack An {@link HttpStack} to use for the network, or null for default.
* @return A started {@link RequestQueue} instance.
*/
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);
}@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);
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;
}
在得到RequestQueue以后，就启动这个RequestQueue。RequestQueue的start()方法又做了那些操作：
/**
* 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();
}
}
/**
* Stops the cache and network dispatchers.
*/
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
start()方法第一步就是先stop cache 和 network两个调度器，然后再重新创建这两个调度器。而这两个调度器都是继承于Thread，是两个线程。并且在它们的run()方法中设置了后台运行属性：
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
代码看到这里就解除了第二个疑惑。此时queue也start了，在写代码的时候在queue被start之后，就要把request添加到queue中，如：mRequestQueue.add(jsObjRequest)。
->RequestQueue中的add()方法实现了那些操作（这个一步也是流程图上的第一步）：
/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
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;
}
}
在代码中可以看到一个if语句的判断(!request.shouldCache())，判断request是否缓存，shouldCache默认值为true，但是可以通过下面方法来改变它的值：
/**
* Set whether or not responses to this request should be cached.
*
* @return This Request object to allow for chaining.
*/
public final Request<?> setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
return this;
}
默认是需要缓存的，就通过mCacheQueue.add(request)将request加入到mCacheQueue队列，如果不需要缓存就会通过mNetworkQueue.add(request)将request加入到network队列。
request被add到mCacheQueue以后，由于之前在RequestQueue的start()方法中，将mCacheQueue传给了CacheDispatcher，mCacheDispatcher.start()之后，就会执行CacheDispatcher重写于Thread类的run()方法：
@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;
}
}
}
这个run()方法里边有个while(true)循环，是一个无限循环，只要cache线程不停止，就不停的从缓存队列中取出请求结果,获得请求结果之后就通过mDelivery.postResponse()方法，将结果传递到主线程去处理。为什么说这里传递到了主线程呢？在前面获得RequestQueue对象的时候，代码是这么实现的：
...
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
...
其实new RequestQueue对象最终调用的方法是：
/**
* 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
* @param threadPoolSize Number of network dispatcher threads to create
*/
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
其中threadPoolSize = 4，而Looper.getMainLooper()就相当于主线程。这样在cache线程中获得结果最终传递给了主线程，让主线程来解析这个response,再回到CacheDispatcher的run()方法中，从缓存队列不一定能取出结果，即使取出了结果也可能不是最新的，这个时候就需要从新从网络获取数据，run()方法中有这么两个判断：
// 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;
}
源码看到此处，request总共有几种情况会被加入到network队列？我总结的是总共有三种情况request被加入到network队列，第一次是判断request是否需要缓存，不要需要缓存，request会被加入到network队列，后面两种情况如上面所述的两种情况了（流程图上第二步在这里就完成）。
经过上面的分析，cache线程对request的处理就完成了，但是由于上面所说的三种情况，request会被加入到network队列，下面就继续分析network，同样看看NetworkDispatcher的run()方法：
@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;
}
addTrafficStatsTag(request);
// 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));
}
}
}
从run()方法中可见，network线程会去执行network request：
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);
先来看看mNetwork是个什么东东，mNetwork是Network,它从newRequestQueue的时候就被创建：
Network network = new BasicNetwork(stack); /**
* @param httpStack HTTP stack to be used
*/
public BasicNetwork(HttpStack httpStack) {
// If a pool isn't passed in, then build a small default pool that will give us a lot of
// benefit and not use too much memory.
this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
}
/**
* @param httpStack HTTP stack to be used
* @param pool a buffer pool that improves GC performance in copy operations
*/
public BasicNetwork(HttpStack httpStack, ByteArrayPool pool) {
mHttpStack = httpStack;
mPool = pool;
}
所以mNetwork实际上是new了一个BasicNetwork对象，而BasicNetwork实现了接口Network。其performRequest方法如下：
@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());
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() == 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) {
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);
}
}
}
}
在此方法中会去连接网络：
httpResponse = mHttpStack.performRequest(request, headers);
经过前面的分析mHttpStack是stack，而对于SDK高于9的版本 stack是HurlStack类的一个对象，HurlStack又实现了HttpStack接口，所以mHttpStack.performRequest(request, headers)执行的是HurlStack中的performRequest()方法，此方法最终使用HttpURLConnection来连接网络并获取网络数据，代码在前面贴了。至此，Volley连接网络的流程也显出端倪了。
回到network调度器的run()方法中，经过上面根据request从网络获取到数据（response），接下来就会解析response，把解析的结果写入cache（如果可以），最后返回给主线程。
...
// 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);
...
至此，整个流程就分析完了，回想前面的流程图，根据这些源码就显得清晰不少了。
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