Volley网络框架学习笔记（四）
看完了前三篇，大家对Volley的使用基本是没有问题的。
知其然要知其所以然，所以现在就可以去了解一下源码了，走一遍Volley的工作流程。
首先还是先上图：
其中蓝色部分代表主线程，绿色部分代表缓存线程，橙色部分代表网络线程。我们在主线程中调用RequestQueue的add()方法来添加一条网络请求，这条请求会先被加入到缓存队列当中，如果发现可以找到相应的缓存结果就直接读取缓存并解析，然后回调给主线程。如果在缓存中没有找到结果，则将这条请求加入到网络请求队列中，然后处理发送HTTP请求，解析响应结果，写入缓存，并回调主线程。
大家现在应该还是对这张图想要表达的意思还是模糊的，看了源码后才后更加的理解。这里注重的不是源码本身，而是从分析源码的切入角度入手。
一.RequestQueue
首先我们可以先理顺一下使用Volley请求的顺序：
1.新建一个队列queue
2.新建一个请求Request
3.使用add方法queue.add(Request)
现在我们根据这个步骤来学习一下，重要的是自己去Volley包中找到相应的位置，遇到不懂的方法或函数就跳转到相应的类中去寻找答案，这样对理解是很有帮助的：
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
} 可以看到，调用了newRequestQueue()的方法重载。所以我们现在去看newRequestQueue()方法
#Volley
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
public static RequestQueue newRequestQueue(Context context, HttpStack stack)
{
return newRequestQueue(context, stack, -1);
}
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) {
}
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;
}上面主要是一些文件缓存之类的东西，最重要的是stack.如果stack是等于null的，则去创建一个HttpStack对象，这里会判断如果手机系统版本号是大于9的，则创建一个HurlStack的实例，否则就创建一个HttpClientStack的实例。实际上HurlStack的内部就是使用HttpURLConnection进行网络通讯的，而HttpClientStack的内部则是使用HttpClient进行网络通讯的.http是如何工作的，我们这里暂时不去关注。
接下来初始化了RequestQueue，然后调用了start（）方法。
来看看RequestQueue的构造：
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}
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;
}初始化主要就是4个参数：mCache、mNetwork、mDispatchers、mDelivery。第一个是硬盘缓存；第二个主要用于Http相关操作；第三个用于转发请求的；第四个参数用于把结果转发到UI线程（ps:你可以看到new Handler(Looper.getMainLooper())）。
这里的参数有什么含义，我们暂时不知道，先不管，接着往下看。
接着就去看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();
}
} 先用stop确保那几个线程都退出。有兴趣的可以参考下Volley中是怎么处理线程退出的（几个线程都是while(true）{//doSomething})。
这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的，而默认情况下for循环会执行四次，也就是说当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中CacheDispatcher是缓存线程，NetworkDispatcher是网络请求线程。
二、add方法
接下来看比较复杂的add方法：
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;
} 会判断当前的请求是否可以缓存，如果不能缓存则直接将这条请求加入网络请求队列，可以缓存的话则将这条请求加入缓存队列。在默认情况下，每条请求都是可以缓存的，当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。
OK，那么既然默认每条请求都是可以缓存的，自然就被添加到了缓存队列中，于是一直在后台等待的缓存线程就要开始运行起来了，我们看下CacheDispatcher中的run()方法，既然涉及到线程，最主要的部分就是run( )方法了，我们直接看代码如下所示：
三、CacheDispatcher
public class CacheDispatcher extends Thread {
……
@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;
}
}
}
} ok，首先要明确这个缓存指的是硬盘缓存（目录为context.getCacheDir()/volley）
可以看到这里是个无限循环，不断的从mCacheQueue去取出请求，如果请求已经被取消就直接结束；
接下来从缓存中获取：
=>如果没有取到，则加入mNetworkQueue
=>如果缓存过期，则加入mNetworkQueue
否则，就是取到了可用的缓存了；调用request.parseNetworkResponse解析从缓存中取出的data和responseHeaders；接下来判断TTL（主要还是判断是否过期），如果没有过期则直接通过mDelivery.postResponse转发，然后回调到UI线程；如果ttl不合法，回调完成后，还会将该请求加入mNetworkQueue。
好了，这里其实就是如果拿到合法的缓存，则直接转发到UI线程；反之，则加入到NetworkQueue.
四.NetworkDispatcher.
现在我们再来NetworkDispatcher
# NetworkDispatcher
//new NetworkDispatcher(mNetworkQueue, mNetwork,mCache, mDelivery)
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
Request<?> request;
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) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
}
}
}看代码前，我们首先想一下逻辑，正常情况下我们会取出请求，让network去请求处理我们的请求，处理完成以后呢：加入缓存，然后转发。
代码是顺着思路的：
首先取出请求；然后通过mNetwork.performRequest(request)处理我们的请求，拿到NetworkResponse；接下来，使用request去解析我们的NetworkResponse。
拿到Response以后，判断是否应该缓存，如果需要，则缓存。
最后mDelivery.postResponse(request, response);转发；
五. 总结
首先初始化RequestQueue，主要就是开启几个Dispatcher线程(应该是4个），线程会不断读取请求（使用的阻塞队列，没有消息则阻塞）
当我们发出请求以后，会根据url属性等，构造出一个cacheKey，然后首先从LruCache中获取（这个缓存我们自己构建的，凡是实现ImageCache接口的都合法）；如果没有取到，则判断是否存在硬盘缓存，这一步是从getCacheDir里面获取（默认5M）；如果没有取到，则从网络请求；
网络请求等都是线程，主要内容在run（）中，关注run我们就可以掌握核心。