第一次接触到Volley，给我的印象就是用法很简洁，简单。上手很快，使用起来毫无压力。所以我决定弄清楚里面的源码是如何进行网络请求的，其实表面越简单的东西，其内部实现起来会越复杂，这次我主要从整体流程来分析。
Volley的使用：首先新建一个管理类，实现单例模式，获取Volley的网络请求队列
public static RequestManager getInstance(Context context) {
if (mInstance == null) {
synchronized(RequestManager.class)
{
if (mInstance == null) {
mInstance = new RequestManager(context);
}
}
}
return mInstance;
}
public RequestQueue getRequestQueue() {
if (reqQueue == null){
synchronized(RequestManager.class)
{
if (reqQueue == null){
reqQueue = Volley.newRequestQueue(mContext);
}
}
}
return reqQueue;
}
加入请求队列
public void addRequest(Request<?> request, Object tag) {
if (tag != null) {
request.setTag(tag);
}
getRequestQueue().add(request);
}
这时候使用起来就很简单了，只需要new一个Request对象，设置相应的回调，然后将Request加入到网络请求队列就行了。
StringRequest stringRequest = new StringRequest(url,
new Listener<String>() {
@Override
public void onResponse(String response) {
// TODO 会调响应完成在这里做相应的处理
}
}, new ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// TODO 访问出现错误处理
}
});
RequestManager.getInstance(this).addRequest(stringRequest, this);
以上就是一般的Volley网络请求使用
现在开始我们的Volley源码解析
首先我们从获取网络请求队列开始Volley.newRequestQueue(mContext);
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;
}
可以看到这三个重载的方法，最终会调用 newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);这个方法。
这个方法主要做的事情就是，如果Android版本大于2.2的，stack就会使用HurlStack，否则使用HttpClientStack。
HurlStack是使用HttpURLConnection做网络访问，HttpClientStack是使用HttpClient网络访问。这两个有什么区别呢？可以看郭霖大神的 Android访问网络，使用HttpURLConnection还是HttpClient？这篇文章。
然后新建对象Network network = new BasicNetwork(stack);
最后创建网络请求队列queue = new RequestQueue(new DiskBasedCache(cacheDir), network);，RequestQueue队列的构造方法也是一个重载方法，最终的构造方法进行赋值。其中threadPoolSize变量默认为开启4条线程。
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
我们回到 newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);中，看到此方法最后会调用queue.start();，我们看看queue.start();到底做了什么操作，我们猜想一下，这里面应该是做了开启网络请求的操作。下面我们打开这个方法看看。
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();
}
}
从上面方法中可以看出，首先调用stop()方法来确保所有当前的正在运行的网络调度都停止，然后创建一个新的缓存分发对象CacheDispatcher，CacheDispatcher是一个继承Thread的线程，然后开始执行CacheDispatcher，看看CacheDispatcher所做的事情，主要看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();
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());
}
}
}
我们看run()方法的主要做的事情：首先拿出队列头request = mCacheQueue.take();，然后设置标志request.addMarker("cache-queue-take"); ，接着从缓存中取出响应结果，如果响应结果为空if (entry == null)或者缓存失效if (entry.isExpired())，都会加入到网络请求队列中mNetworkQueue.put(request);，调用网络请求获取结果。如果缓存线程中取得结果会调用ResponseDelivery的postResponse方法。
而ResponseDelivery的实现就是RequestQueue构造方法new出来的ExecutorDelivery对象，我们看下ExecutorDelivery中的postResponse方法的实现。
@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));
}
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}
从ExecutorDelivery可以看出，其主要做的事情是：将结果发送到主线程。再看下ResponseDeliveryRunnable做了什么操作
@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();
}
}
上面的run方法中最重要的一句是
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
如果响应成功的话就会调用Request的deliverResponse方法，如果错误就会调用deliverError方法，而这个Request方法就是我们使用时new请求对象，而文章开头我们使用的时StringRequest这个对象，那么我们看看StringRequest的deliverResponse方法。
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}
deliverResponse方法的mListener对象就是我们使用时传进来的回调接口，所以最终结果我们可以在设置回调的onResponse方法中使用。
分析到这里我们就知道了Volley的调用流程了，就知道为什么我们只是简单的new一个请求对象然后设置回调，并把请求对象加入请求队列就可以完成整个网络请求了。
接下来我们在回看queue.start()的网络请求。主要看下面的语句
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
前面我们知道默认的线程数设置时4，所以这里回循环4次，主要还是看回NetworkDispatcher类，同样NetworkDispatcher也是继承Thread的线程类。
主要看run()方法
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
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);
}
}
}
run()：首先回取出队列头request = mQueue.take();获取request，接着看网络请求最最主要的一句NetworkResponse networkResponse = mNetwork.performRequest(request);这一句呢主要是网络访问，获取得到结果。从RequestQueue的newRequestQueue方法中我们可以知道：如果Android版本大于2.2的会使用HttpURLConnection进行网络访问，否则使用HttpClient。
网络访问完成之后会对响应结果进行解析Response<?> response = request.parseNetworkResponse(networkResponse);，接着之后将结果放到缓存线程mCache.put(request.getCacheKey(), response.cacheEntry);，跟缓存线程一样
最终调用mDelivery.postResponse(request, response);将结果回调。
至此，整个volley解剖完成，大家应该也知道整体调用流程了。如果还有不明白的，可以看下volley官方给出来的这张图，结合我的分析就一目了然了。
该文章是我看了好几天的源码分析得来的，哪里分析得不好请大家指出来，相互学习。网上也有很多分析volley的文章，如有雷同，纯属偶合。
