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说起分析源码,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,那么我们自然要从这个方法开始看起了,代码如下所示:
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
。那我们看下带有两个参数的newRequestQueue()
方法中的代码,如下所示:
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) { // ① 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; }
可以看到,这里在①判断如果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,这里为什么这样选择呢?可以参考我之前翻译的一篇文章Android访问网络,使用HttpURLConnection还是HttpClient?
创建好了HttpStack
之后,接下来又创建了一个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,然后将RequestQueue
返回,这样newRequestQueue()
的方法就执行结束了。
那么RequestQueue
的start()
方法内部到底执行了什么东西呢?我们跟进去瞧一瞧:
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();
}
}
这里先是创建了一个CacheDispatcher
的实例,然后调用了它的start()
方法,接着在一个for
循环里去创建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。
得到了RequestQueue
之后,我们只需要构建出相应的Request
,然后调用RequestQueue
的add()
方法将Request
传入就可以完成网络请求操作了,那么不用说,add()
方法的内部肯定有着非常复杂的逻辑,我们来一起看一下:
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()) { // ② mNetworkQueue.add(request); // ③ 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); // ④ } return request; } }
可以看到,在②会判断当前的请求是否可以缓存,如果不能缓存则在③直接将这条请求加入网络请求队列,可以缓存的话则在④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,代码如下所示:
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) { // ⑤ 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()); // ⑥ 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( // ⑦ 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,我们只挑重点看。首先在⑤可以看到一个while(true)
循环,说明缓存线程始终是在运行的,接着在⑥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,如果不为空的话再判断该缓存是否已过期,如果已经过期了则同样把这条请求加入到网络请求队列中,否则就认为不需要重发网络请求,直接使用缓存中的数据即可。之后会在⑦调用Request
的parseNetworkResponse()
方法来对数据进行解析,再往后就是将解析出来的数据进行回调了,这部分代码我们先跳过,因为它的逻辑和NetworkDispatcher
后半部分的逻辑是基本相同的,那么我们等下合并在一起看就好了,先来看一下NetworkDispatcher
中是怎么处理网络请求队列的,代码如下所示:
public class NetworkDispatcher extends Thread { …… @Override public void run() { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); Request<?> request; while (true) { // ⑧ 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); // ⑨ 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)); } } } }
同样地,在⑧我们看到了类似的while(true)
循环,说明网络请求线程也是在不断运行的。在⑨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
,我们来看下它的performRequest()
方法,如下所示:
public class BasicNetwork implements Network { …… @Override public NetworkResponse performRequest(Request<?> request) throws VolleyError { long requestStart = SystemClock.elapsedRealtime(); while (true) { // ⑤ 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); //⑩ 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,我们并不需要太多关心,需要注意的是在⑩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。
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,因为不同种类的Request
解析的方式也肯定不同。还记得我们在上一篇文章中学习的自定义Request
的方式吗?其中parseNetworkResponse()
这个方法就是必须要重写的。
在解析完了NetworkResponse
中的数据之后,又会调用ExecutorDelivery
的postResponse()
方法来回调解析出的数据,代码如下所示:
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,就可以保证该对象中的run()
方法就是在主线程当中运行的了,我们看下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); // ⑪ } 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(); } } }
代码虽然不多,但我们并不需要行行阅读,抓住重点看即可。其中在⑪调用了Request
的deliverResponse()
方法,有没有感觉很熟悉?没错,这个就是我们在自定义Request
时需要重写的另外一个方法,每一条网络请求的响应都是回调到这个方法中,最后我们再在这个方法中将响应的数据回调到Response.Listener
的onResponse()
方法中就可以了。
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。
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