android网络开源框架volley(三)——请求队列是主线
忽忽,很意外,第二篇大家反映还不错哈,谢谢咯~~对于这篇,要写好信心不大,不过我会尽量的。
1、从哪里开始:
从哪里开始,菜鸟能想到的当然是从最先接触的地方开始啦~~
首先我们看volley.java这个类:
/** * 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; }回忆下之前我们就是通过它拿到我们的请求队列,在对newRequestQueue的描述中我们也得到验证:创建一个工作池(带优先级的阻塞队列)的实例。在这里有两个静态的构造方法,除去我们之前使用过的,另一个多了一个参数HttpStack(HttpStack稍后分析)。而我们之前并没有使用这个带HttpStack的构造方法。因此就有了上面对stack==null的判断:根据不同的版本创建不同的实例。判断的原因注释中也写的很明确: Gingerbread(2.3)之前,HttpUrlConnection不可用。这里就很自然的想到这样一个问题:Gingerbread(2.3)之前和之后的版本在网络请求上有何区别(或者HTTP Client与HttpURLConnection的区别)?如果那个网页你打不开,可以在这里下载英文原版,或者去看大牛翻译好的。完成HttpStack的创建之后紧接着有创建了一个Network和RequestQueue对象:Network执行网络请求;RequestQueue是A request dispatch queue with a thread pool of dispatchers。下面我们将围绕着RequestQueue进行展开。对了,还有一个HttpStack接口,只有一个方法,这里简单提一下:
public interface HttpStack { /** * Performs an HTTP request with the given parameters. * * <p>A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise, * and the Content-Type header is set to request.getPostBodyContentType().</p> * * @param request the request to perform * @param additionalHeaders additional headers to be sent together with * {@link Request#getHeaders()} * @return the HTTP response */ public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders) throws IOException, AuthFailureError; }使用给定的参数执行HTTP请求。通过上面的判断我们知道它有两个实现类:HurlStack和HttpClientStack。这样我们就得到下面的一个关系图:
这样他们的关系就相当明了了。
2、请求队列:RequestQueue
/** * 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 * @param delivery A ResponseDelivery interface for posting responses and errors */ public RequestQueue(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) { mCache = cache; mNetwork = network; mDispatchers = new NetworkDispatcher[threadPoolSize]; mDelivery = delivery; }这里我们传入了一个cache、Network、线程池大小和传递响应的对象。默认的线程池大小是4。创建这个RequestQueue对象之后,我们需要调用它的start()方法,完成对调度器的初始化工作,如根据传入线程池大小创建相应数量的Dispatcher(这里我们暂时只关注网络处理线程),然后启动:
/** * 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(); } }NetworkDispatcher的构造函数中传入了一个mNetworkQueue对象,这是一个带优先级的阻塞队列:
/** The queue of requests that are actually going out to the network. */ private final PriorityBlockingQueue<Request> mNetworkQueue = new PriorityBlockingQueue<Request>();另一个需要关注的就是add方法:
/** * Adds a Request to the dispatch queue. * @param request The request to service * @return The passed-in request */ public Request add(Request 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; } }通过上面的代码我们可以看出,volley是将我们创建的Request对象直接加到上面的mNetworkQueue中。这样我们很自然的联想到,对request的处理应该是由start中创建的NetworkDispatcher完成的。NetworkDispatcher继承了Thread,run()方法是这样实现的:
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; } // Tag the request (if API >= 14) if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) { TrafficStats.setThreadStatsTag(request.getTrafficStatsTag()); } // 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)); } } }首先从请求队列中取出一个请求;然后通过Network开始执行这个request请求,这个时候会拿到请求返回的结果;接着把请求的结果返给request进行处理:
Response<?> response = request.parseNetworkResponse(networkResponse);request处理之后会返回一个response,这个response和request最后会传递到ResponseDeliveryRunnable中处理:
/** * A Runnable used for delivering network responses to a listener on the * main thread. */ @SuppressWarnings("rawtypes") 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(); } } }这里我们可以看到,如果请求的响应是成功的,会去调用request的deliverResponse,这样一个完整的请求响应过程就完成了。
如上图(这个图我想修改下,但是不知道怎么改,都是拖出来的)。
3、请求的细节
3.1 第一次取消
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; }此时状态是刚从队列中取出:network-queue-take。首先,判断此请求是否已经取消,若取消直接调用request的finish()方法。这个时候我们会想到,怎么知道已经取消,或者用户怎么取消请求呢?到request中我们可以找到一个cancel()方法,就是通过此方法来取消请求的。注销 这个方法,我们可以看到在RequestQueue中有调用此方法,它们是:
/** * Cancels all requests in this queue for which the given filter applies. * @param filter The filtering function to use */ public void cancelAll(RequestFilter filter) { synchronized (mCurrentRequests) { for (Request<?> request : mCurrentRequests) { if (filter.apply(request)) { request.cancel(); } } } } /** * Cancels all requests in this queue with the given tag. Tag must be non-null * and equality is by identity. */ public void cancelAll(final Object tag) { if (tag == null) { throw new IllegalArgumentException("Cannot cancelAll with a null tag"); } cancelAll(new RequestFilter() { @Override public boolean apply(Request<?> request) { return request.getTag() == tag; } }); }这样,我们就可以知道,取消一个请求可以通过RequestQueue中的cancelAll()方法进行。这个时候取消的话,我们的请求是不会发出去的。当然,取消在任何时候都会发生,下面还会讲到,请求返回后取消的话结果是不会传递给用户的。
3.2 发起网络请求
// Perform the network request. NetworkResponse networkResponse = mNetwork.performRequest(request); request.addMarker("network-http-complete");通过执行network的performRequest()方法拿到一个请求结果。执行完这个时候,我们就得到了请求的结果。此处是可以自定义的,我们看下默认实现:
@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().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); } } } }关键部分:httpResponse = mHttpStack.performRequest(request, headers);通过这个执行网络请求。具体的细节还是可以自定义的。代码贴的太多就不继续贴了,两个实现大家可以具体去看看,比如网络请求超时时间的设置都是通过:request.getTimeoutMs();得到的。再到Request中你就可以知道设置超时是通过下面代码实现的:
/** * Sets the retry policy for this request. */ public void setRetryPolicy(RetryPolicy retryPolicy) { mRetryPolicy = retryPolicy; }
3.3 解析请求结果
// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
这个是不是很熟悉,我们在request中实现的parseNetworkResponse()。3.4 最后结果的处理
这就说了,大家可以继续看默认的代码实现。值得提的是,在这里我们会发现,又一次调用了mRequest.isCanceled()决定是否将结果传递给用户。4、总结
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