Android框架-Volley(四)
通过前三篇文章的学习，Volley的用法咱们已经掌握的差很少了，可是对于Volley的工做原理，恐怕有不少朋友还不是很清楚。所以，本篇文章中咱们就来一块儿阅读一下Volley的源码，将它的工做流程总体地梳理一遍。同时，这也是Volley系列的最后一篇文章了。java
其实，Volley的官方文档中自己就附有了一张Volley的工做流程图，以下图所示。缓存
多数朋友忽然看到一张这样的图，应该会和我同样，感受一头雾水吧？没错，目前咱们对Volley背后的工做原理尚未一个概念性的理解，直接就来看这张图天然会有些吃力。不过不要紧，下面咱们就去分析一下Volley的源码，以后再从新来看这张图就会好理解多了。服务器
提及分析源码，那么应该从哪儿开始看起呢？这就要回顾一下Volley的用法了，还记得吗，使用Volley的第一步，首先要调用Volley.newRequestQueue(context)方法来获取一个RequestQueue对象，那么咱们天然要从这个方法开始看起了，代码以下所示：网络
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
这个方法仅仅只有一行代码，只是调用了newRequestQueue()的方法重载，并给第二个参数传入null。那咱们看下带有两个参数的newRequestQueue()方法中的代码，以下所示：app
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 {
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork(stack);
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
}
能够看到，这里在第10行判断若是stack是等于null的，则去建立一个HttpStack对象，这里会判断若是手机系统版本号是大于9的，则建立一个HurlStack的实例，不然就建立一个HttpClientStack的实例。实际上HurlStack的内部就是使用HttpURLConnection进行网络通信的，而HttpClientStack的内部则是使用HttpClient进行网络通信的，这里为何这样选择呢？ide
建立好了HttpStack以后，接下来又建立了一个Network对象，它是用于根据传入的HttpStack对象来处理网络请求的，紧接着new出一个RequestQueue对象，并调用它的start()方法进行启动，而后将RequestQueue返回，这样newRequestQueue()的方法就执行结束了。post
那么RequestQueue的start()方法内部到底执行了什么东西呢？咱们跟进去瞧一瞧：学习
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();
}
}
这里先是建立了一个CacheDispatcher的实例，而后调用了它的start()方法，接着在一个for循环里去建立NetworkDispatcher的实例，并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的，而默认状况下for循环会执行四次，也就是说当调用了Volley.newRequestQueue(context)以后，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。ui
获得了RequestQueue以后，咱们只须要构建出相应的Request，而后调用RequestQueue的add()方法将Request传入就能够完成网络请求操做了，那么不用说，add()方法的内部确定有着很是复杂的逻辑，咱们来一块儿看一下：this
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;
}
}
能够看到，在第11行的时候会判断当前的请求是否能够缓存，若是不能缓存则在第12行直接将这条请求加入网络请求队列，能够缓存的话则在第33行将这条请求加入缓存队列。在默认状况下，每条请求都是能够缓存的，固然咱们也能够调用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能够缓存的，天然就被添加到了缓存队列中，因而一直在后台等待的缓存线程就要开始运行起来了，咱们看下CacheDispatcher中的run()方法，代码以下所示：
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;
}
}
}
}
代码有点长，咱们只挑重点看。首先在11行能够看到一个while(true)循环，说明缓存线程始终是在运行的，接着在第23行会尝试从缓存当中取出响应结果，如何为空的话则把这条请求加入到网络请求队列中，若是不为空的话再判断该缓存是否已过时，若是已通过期了则一样把这条请求加入到网络请求队列中，不然就认为不须要重发网络请求，直接使用缓存中的数据便可。以后会在第39行调用Request的parseNetworkResponse()方法来对数据进行解析，再日后就是将解析出来的数据进行回调了，这部分代码咱们先跳过，由于它的逻辑和NetworkDispatcher后半部分的逻辑是基本相同的，那么咱们等下合并在一块儿看就行了，先来看一下NetworkDispatcher中是怎么处理网络请求队列的，代码以下所示：
public class NetworkDispatcher extends Thread {
……
@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));
}
}
}
}
一样地，在第7行咱们看到了相似的while(true)循环，说明网络请求线程也是在不断运行的。在第28行的时候会调用Network的performRequest()方法来去发送网络请求，而Network是一个接口，这里具体的实现是BasicNetwork，咱们来看下它的performRequest()方法，以下所示：
public class BasicNetwork implements Network {
……
@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 (Exception e) {
……
}
}
}
}
这段方法中大多都是一些网络请求细节方面的东西，咱们并不须要太多关心，须要注意的是在第14行调用了HttpStack的performRequest()方法，这里的HttpStack就是在一开始调用newRequestQueue()方法是建立的实例，默认状况下若是系统版本号大于9就建立的HurlStack对象，不然建立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，咱们就再也不跟进去阅读了，以后会将服务器返回的数据组装成一个NetworkResponse对象进行返回。
在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据，以及将数据写入到缓存，这个方法的实现是交给Request的子类来完成的，由于不一样种类的Request解析的方式也确定不一样。还记得咱们在上一篇文章中学习的自定义Request的方式吗？其中parseNetworkResponse()这个方法就是必需要重写的。
在解析完了NetworkResponse中的数据以后，又会调用ExecutorDelivery的postResponse()方法来回调解析出的数据，代码以下所示：
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
其中，在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象，就能够保证该对象中的run()方法就是在主线程当中运行的了，咱们看下run()方法中的代码是什么样的：
private class ResponseDeliveryRunnable implements Runnable {
private final Request mRequest;
private final Response mResponse;
private final Runnable mRunnable;
public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
mRequest = request;
mResponse = response;
mRunnable = runnable;
}
@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();
}
}
}
代码虽然很少，但咱们并不须要行行阅读，抓住重点看便可。其中在第22行调用了Request的deliverResponse()方法，有没有感受很熟悉？没错，这个就是咱们在自定义Request时须要重写的另一个方法，每一条网络请求的响应都是回调到这个方法中，最后咱们再在这个方法中将响应的数据回调到Response.Listener的onResponse()方法中就能够了。
好了，到这里咱们就把Volley的完整执行流程所有梳理了一遍，你是否是已经感受已经很清晰了呢？对了，还记得在文章一开始的那张流程图吗，刚才还不能理解，如今咱们再来从新看下这张图：
其中蓝色部分表明主线程，绿色部分表明缓存线程，橙色部分表明网络线程。咱们在主线程中调用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，若是发现能够找到相应的缓存结果就直接读取缓存并解析，而后回调给主线程。若是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，而后处理发送HTTP请求，解析响应结果，写入缓存，并回调主线程。
怎么样，是否是感受如今理解这张图已经变得轻松简单了？好了，到此为止咱们就把Volley的用法和源码所有学习完了，相信你已经对Volley很是熟悉并能够将它应用到实际项目当中了，那么Volley彻底解析系列的文章到此结束，感谢你们有耐心看到最后。
- 1. android volley框架
- 2. Android框架-Volley(二)
- 3. Android框架-Volley(一)
- 4. Android框架之Volley
- 5. Android 网络框架---Volley
- 6. Android网络框架Volley
- 7. Volley 框架
- 8. Volley框架
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- 10. Android 学习笔记之Volley开源框架解析（四）
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每一个你不满意的现在，都有一个你没有努力的曾经。