Android开发学习之路-Volley源码解析
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从简单的StringRequest入手看看Volley的工作机制。
先简单说下Volley的用法:
① 获取一个RequestQueue
mRequestQueue = Volley.newRequestQueue(this);
② 构造一个StringRequest对象
mStringRequest = new StringRequest(url, new Response.Listener<String>() { @Override public void onResponse(String response) { mTextView.setText(response); } }, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError error) { Log.d(TAG, "onErrorResponse: " + error.getMessage()); } });
③ 将StringRequest对象add进RequestQueue
mRequestQueue.add(mStringRequest);
下面通过源码跟踪一下Volley处理请求的过程:
用过Volley都知道,请求一般是继承自Request这个抽象类,那么StringRequest自然也是。在构造方法中需要几个参数,具体的构造方法如下
public StringRequest(int method, String url, Listener<String> listener, ErrorListener errorListener) { super(method, url, errorListener); mListener = listener; }
可以看到我们需要给它指定请求的方法、url、成功返回的回调类和错误的回调类。
接着我们在需要请求的时候,把这个对象传递给RequestQueue的add方法。这个add方法是做什么的我们其实都能猜到,就是把这个请求放到队列中。
1 public <T> Request<T> add(Request<T> request) { 2 // Tag the request as belonging to this queue and add it to the set of current requests. 3 request.setRequestQueue(this); 4 synchronized (mCurrentRequests) { 5 mCurrentRequests.add(request); 6 } 7 8 // Process requests in the order they are added. 9 request.setSequence(getSequenceNumber()); 10 request.addMarker("add-to-queue"); 11 12 // If the request is uncacheable, skip the cache queue and go straight to the network. 13 if (!request.shouldCache()) { 14 mNetworkQueue.add(request); 15 return request; 16 } 17 18 // Insert request into stage if there\'s already a request with the same cache key in flight. 19 synchronized (mWaitingRequests) { 20 String cacheKey = request.getCacheKey(); 21 if (mWaitingRequests.containsKey(cacheKey)) { 22 // There is already a request in flight. Queue up. 23 Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey); 24 if (stagedRequests == null) { 25 stagedRequests = new LinkedList<Request<?>>(); 26 } 27 stagedRequests.add(request); 28 mWaitingRequests.put(cacheKey, stagedRequests); 29 if (VolleyLog.DEBUG) { 30 VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey); 31 } 32 } else { 33 // Insert \'null\' queue for this cacheKey, indicating there is now a request in 34 // flight. 35 mWaitingRequests.put(cacheKey, null); 36 mCacheQueue.add(request); 37 } 38 return request; 39 } 40 }
代码可能有点长,但是核心的地方也比较简单,就是进行线程同步之后将请求放入请求的集合mNetworkQueue(在不需要缓存的情况下)。如果需要缓存请求,则同步等待序列,判断请求是否已经被发出过,根据情况返回请求。
因为这里会根据是否需要缓存进行区别处理,下面按照不需要缓存来讲解源码。而StringRequest是会进行缓存的。
接着我们看看RequestQueue是怎么工作的。这里其实我们也是可以知道,既然命名为Queue,肯定是会通过一个线程来不停的遍历队列中的等待者然后进行处理,跟Handler中的MessageQueue是很类似的。
我们先看RequestQueue的构造,我们一般通过下面这个方法来获得一个RequestQueue
mRequestQueue = Volley.newRequestQueue(this);
而实际上这个构造的方法内容是下面这样的
1 public static RequestQueue newRequestQueue(Context context, HttpStack stack) { 2 File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR); 3 4 String userAgent = "volley/0"; 5 try { 6 String packageName = context.getPackageName(); 7 PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0); 8 userAgent = packageName + "/" + info.versionCode; 9 } catch (NameNotFoundException e) { 10 } 11 12 if (stack == null) { 13 if (Build.VERSION.SDK_INT >= 9) { 14 stack = new HurlStack(); 15 } else { 16 // Prior to Gingerbread, HttpUrlConnection was unreliable. 17 // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html 18 stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent)); 19 } 20 } 21 22 Network network = new BasicNetwork(stack); 23 24 RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); 25 queue.start(); 26 27 return queue; 28 }
从注释可以看到,在api等级大于9的时候,使用HttpUrlConnection来进行主要的网络请求工作,到这里已经很明显了,Volley底层是使用HttpUrlConnection进行的。我们看到这里是使用了一个HttpClientStack来包装根据userAgent得到的HttpClient(这个类在最新的源码中已经被移除了),而HttpClient实际上就是使用HttpUrlConnection来实现的。最后被包装为一个BasicNetwork对象。
接着根据得到的BasicNetwork对象和一个DiskBasedCache对象(磁盘缓存)来构造一个RequestQueue,并且调用了它的start方法来启动这个线程。
再看看RequestQueue的start方法:
1 public void start() { 2 stop(); // Make sure any currently running dispatchers are stopped. 3 // Create the cache dispatcher and start it. 4 mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery); 5 mCacheDispatcher.start(); 6 7 // Create network dispatchers (and corresponding threads) up to the pool size. 8 for (int i = 0; i < mDispatchers.length; i++) { 9 NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork, 10 mCache, mDelivery); 11 mDispatchers[i] = networkDispatcher; 12 networkDispatcher.start(); 13 } 14 }
这里我们先不顾这个mCacheDispatcher,直接看到下面的for循环,这个for循环遍历了mDispatchers,这个mDispatcher其实相当于一个线程池,这个线程池的大小默认是4。然后分别让这里面的线程运行起来(调用了它们的start方法)。这里为什么要有多个线程来处理呢?原因很简单,因为我们每一个请求都不一定会马上处理完毕,多个线程进行同时处理的话效率会提高。
我们进入NetworkDispatcher看看它的run方法:
1 @Override 2 public void run() { 3 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 4 while (true) { 5 long startTimeMs = SystemClock.elapsedRealtime(); 6 Request<?> request; 7 try { 8 // Take a request from the queue. 9 request = mQueue.take(); 10 } catch (InterruptedException e) { 11 // We may have been interrupted because it was time to quit. 12 if (mQuit) { 13 return; 14 } 15 continue; 16 } 17 18 try { 19 request.addMarker("network-queue-take"); 20 21 // If the request was cancelled already, do not perform the 22 // network request. 23 if (request.isCanceled()) { 24 request.finish("network-discard-cancelled"); 25 continue; 26 } 27 28 addTrafficStatsTag(request); 29 30 // Perform the network request. 31 NetworkResponse networkResponse = mNetwork.performRequest(request); 32 request.addMarker("network-http-complete"); 33 34 // If the server returned 304 AND we delivered a response already, 35 // we\'re done -- don\'t deliver a second identical response. 36 if (networkResponse.notModified && request.hasHadResponseDelivered()) { 37 request.finish("not-modified"); 38 continue; 39 } 40 41 // Parse the response here on the worker thread. 42 Response<?> response = request.parseNetworkResponse(networkResponse); 43 request.addMarker("network-parse-complete"); 44 45 // Write to cache if applicable. 46 // TODO: Only update cache metadata instead of entire record for 304s. 47 if (request.shouldCache() && response.cacheEntry != null) { 48 mCache.put(request.getCacheKey(), response.cacheEntry); 49 request.addMarker("network-cache-written"); 50 } 51 52 // Post the response back. 53 request.markDelivered(); 54 mDelivery.postResponse(request, response); 55 } catch (VolleyError volleyError) { 56 volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs); 57 parseAndDeliverNetworkError(request, volleyError); 58 } catch (Exception e) { 59 VolleyLog.e(e, "Unhandled exception %s", e.toString()); 60 VolleyError volleyError = new VolleyError(e); 61 volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs); 62 mDelivery.postError(request, volleyError); 63 } 64 } 65 }
第三行设置了这些线程的优先级,这个优先级比较低,目的是为了尽量减少对UI线程的影响保证流畅度。
接着第9行,调用mQueue的take方法取出队列头的一个请求进行处理,这个mQueue是什么?其实它就是我们在上面add方法中添加进去的一个请求。
我们不看这些设置状态标记的地方,直接看到第31行,如果请求没有被取消,也就是正常的情况下,我们会调用mNetwork的performRequest方法进行请求的处理。不知道你还记的这个mNetwork不,它其实就是我们上面提到的那个由HttpUrlConnection层层包装的网络请求对象。
如果请求得到了结果,我们会看到54行调用了mDelivery的postResponose方法来回传我们的请求结果。
这里还有两个重要的地方需要再了解一下,一个是究竟postResponse是怎么传回我们的请求结果的,另一个就是performRequest是怎么去进行网络请求的。
先看第一个,结果的回传。我们先了解下这个mDelivery是怎么定义的。它其实是在RequestQueue中创建的,可以看到RequestQueue的其中一个构造方法:
1 public RequestQueue(Cache cache, Network network, int threadPoolSize) { 2 this(cache, network, threadPoolSize, 3 new ExecutorDelivery(new Handler(Looper.getMainLooper()))); 4 }
这里直接就new了一个ExecutorDelivery对象,并传入了一个不断从MainLooper中获取Message的Handler。再看看postResponse方法的内容:
1 @Override 2 public void postResponse(Request<?> request, Response<?> response, Runnable runnable) { 3 request.markDelivered(); 4 request.addMarker("post-response"); 5 mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable)); 6 }
这里看到第5行调用了mResponsePoster的execute方法并传入了一个ResponseDeliveryRunnable对象,再看mResponsePoster的定义:
1 public ExecutorDelivery(final Handler handler) { 2 // Make an Executor that just wraps the handler. 3 mResponsePoster = new Executor() { 4 @Override 5 public void execute(Runnable command) { 6 handler.post(command); 7 } 8 }; 9 }
也就是我们在这里把ResponseDeliveryRunnable对象通过Handler的post方法发送出去了。这里为什么要发送到MainLooper中?因为RequestQueue是在子线程中执行的,回调到的代码也是在子线程中的,如果在回调中修改UI,就会报错。再者,为什么要使用post方法?原因也很简单,因为我们在消息发出之后再进行回调,post方法允许我们传入一个Runnable的实现了,post成功会自动执行它的run方法,这个时候在run方法中进行结果的判断并且进行回调:
1 private class ResponseDeliveryRunnable implements Runnable { 2 private final Request mRequest; 3 private final Response mResponse; 4 private final Runnable mRunnable; 5 6 public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) { 7 mRequest = request; 8 mResponse = response; 9 mRunnable = runnable; 10 } 11 12 @SuppressWarnings("unchecked") 13 @Override 14 public void run() { 15 // If this request has canceled, finish it and don\'t deliver. 16 if (mRequest.isCanceled()) { 17 mRequest.finish("canceled-at-delivery"); 18 return; 19 } 20 21 // Deliver a normal response or error, depending. 22 if (mResponse.isSuccess()) { 23 mRequest.deliverResponse(mResponse.result); 24 } else { 25 mRequest.deliverError(mResponse.error); 26 } 27 28 // If this is an intermediate response, add a marker, otherwise we\'re done 29 // and the request can be finished. 30 if (mResponse.intermediate) { 31 mRequest.addMarker("intermediate-response"); 32 } else { 33 mRequest.finish("done"); 34 } 35 36 // If we have been provided a post-delivery runnable, run it. 37 if (mRunnable != null) { 38 mRunnable.run(); 39 } 40 } 41 }
可以看到,23行是调用Request的deleverResponse方法将结果回调给StringRequest。接着看看StringRequest中该方法是实现:
1 @Override 2 protected void deliverResponse(String response) { 3 mListener.onResponse(response); 4 }
直接通过我们构造StringRequest时传进来的Listener的回调方法onResponse来将结果回调给Activity。deleverError也是同样的做法。
再来看看performRequest是怎么进行网络请求的。
mNetwork是Network接口的对象,而这个接口只有一个实现类,就是BasicNetwork,我们看看这个BasicNetwork中的performRequest的代码:
1 @Override 2 public NetworkResponse performRequest(Request<?> request) throws VolleyError { 3 long requestStart = SystemClock.elapsedRealtime(); 4 while (true) { 5 HttpResponse httpResponse = null; 6 byte[] responseContents = null; 7 Map<String, String> responseHeaders = Collections.emptyMap(); 8 try { 9 // Gather headers. 10 Map<String, String> headers = new HashMap<String, String>(); 11 addCacheHeaders(headers, request.getCacheEntry()); 12 httpResponse = mHttpStack.performRequest(request, headers); 13 StatusLine statusLine = httpResponse.getStatusLine(); 14 int statusCode = statusLine.getStatusCode(); 15 16 responseHeaders = convertHeaders(httpResponse.getAllHeaders()); 17 // Handle cache validation. 18 if (statusCode == HttpStatus.SC_NOT_MODIFIED) { 19 20 Entry entry = request.getCacheEntry(); 21 if (entry == null) { 22 return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null, 23 responseHeaders, true, 24 SystemClock.elapsedRealtime() - requestStart); 25 } 26 27 // A HTTP 304 response does not have all header fields. We 28 // have to use the header fields from the cache entry plus 29 // the new ones from the response. 30 // http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5 31 entry.responseHeaders.putAll(responseHeaders); 32 return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data, 33 entry.responseHeaders, true, 34 SystemClock.elapsedRealtime() - requestStart); 35 } 36 37 // Some responses such as 204s do not have content. We must check. 38 if (httpResponse.getEntity() != null) { 39 responseContents = entityToBytes(httpResponse.getEntity()); 40 } else { 41 // Add 0 byte response as a way of honestly representing a 42 // no-content request. 43 responseContents = new byte[0]; 44 } 45 46 // if the request is slow, log it. 47 long requestLifetime = SystemClock.elapsedRealtime() - requestStart; 48 logSlowRequests(requestLifetime, request, responseContents, statusLine); 49 50 if (statusCode < 200 || statusCode > 299) { 51 throw new IOException(); 52 } 53 return new NetworkResponse(statusCode, responseContents, responseHeaders, false, 54 SystemClock.elapsedRealtime() - requestStart); 55 } catch (SocketTimeoutException e) { 56 attemptRetryOnException("socket", request, new TimeoutError()); 57 } catch (ConnectTimeoutException e) { 58 attemptRetryOnException("connection", request, new TimeoutError()); 59 } catch (MalformedURLException e) { 60 throw new RuntimeException("Bad URL " + request.getUrl(), e); 61 } catch (IOException e) { 62 int statusCode = 0; 63 NetworkResponse networkResponse = null; 64 if (httpResponse != null) { 65 statusCode = httpResponse.getStatusLine().getStatusCode(); 66 } else { 67 throw new NoConnectionError(e); 68 } 69 VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl()); 70 if (responseContents != null) { 71 networkResponse = new NetworkResponse(statusCode, responseContents, 72 responseHeaders, false, SystemClock.elapsedRealtime() - requestStart); 73 if (statusCode == HttpStatus.SC_UNAUTHORIZED || 74 statusCode == HttpStatus.SC_FORBIDDEN) { 75 attemptRetryOnException("auth", 76 request, new AuthFailureError(networkResponse)); 77 } else { 78 // TODO: Only throw ServerError for 5xx status codes. 79 throw new ServerError(networkResponse); 80 } 81 } else { 82 throw new NetworkError(networkResponse); 83 } 84 } 85 }
这段代码中,先10和11行代码将cache的属性设置给header,接着第9行调用mHttpStack对象的performRequest方法并传入请求对象和头部来进行请求,得到一个HttpResponse对象。
接着将HttpResponse对象中的状态吗取出,如果值为HttpStatus.SC_NOT_MODIFIED(也就是304),则表示请求得到的Response没有变化,直接显示缓存内容。
第39行表示请求成功并且获取到请求内容,将内容取出并作为一个NetworkResponse对象的属性并返回给NetworkDispatcher,接着将其转,接着就是上面介绍的回调给主线程了。
用过HttpClient的都知道,其实这里得到的HttpResponse就是由HttpClient返回的,我们直接看第12行调用的performRequest的源码:
1 @Override 2 public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders) 3 throws IOException, AuthFailureError { 4 HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders); 5 addHeaders(httpRequest, additionalHeaders); 6 addHeaders(httpRequest, request.getHeaders()); 7 onPrepareRequest(httpRequest); 8 HttpParams httpParams = httpRequest.getParams(); 9 int timeoutMs = request.getTimeoutMs(); 10 // TODO: Reevaluate this connection timeout based on more wide-scale 11 // data collection and possibly different for wifi vs. 3G. 12 HttpConnectionParams.setConnectionTimeout(httpParams, 5000); 13 HttpConnectionParams.setSoTimeout(httpParams, timeoutMs); 14 return mClient.execute(httpRequest); 15 }
这里14行的mClient其实就是一个HttpClient对象。
说到这里,是不是感觉很混乱呢?这里我做了一个图,可以参考看看
我们这里简单再解析下:
① 在RequestQueue创建的时候,会生成多个NetwrokDispatcher,接着这些NetwrokDispatcher会不断的从请求队列中读取请求,如果有就使用包装好的请求类来执行performRequest,接着将结果通过postResponse方法传包装好并通过post方法发送到MainLooper。
② 在MainLooper中,判断Response是否有内容,通过deliverResponse将结果回调给RequestQueue,RequestQueue通过我们构造时传入的Listener中的回调方法对结果进行回调。
③ 在我们需要请求的时候,构建一个StringRequest(设置好对应的回调接口和实现回调方法)并将其add到MessageQueue中即可自动完成请求。
我们应该掌握什么?
① 子线程中应该如何将结果回调给主线程
② 如果自己要设计一个类似的框架,知道如何进行设计保证低耦合和便于维护
③ 通过学习其他的Request子类定义我们自己的请求类
④ 阅读源代码的技巧,比如查看直接子类,方法和参数定义查看等等
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