Android -- Volley解析
2015-07-15 22:57
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Volley设计
Dispatch Thread不断从
RequestQueue中取出请求,根据是否已缓存调用
Cache或
Network这两类数据获取接口之一,从内存缓存或是服务器取得请求的数据,然后交由
ResponseDelivery去做结果分发及回调处理。
Volley中的类简介
Volley:过 newRequestQueue(…) 函数新建并启动一个请求队列RequestQueue。
Request:表示一个请求的抽象类。
StringRequest、
JsonRequest、
ImageRequest都是它的子类,表示某种类型的请求。
RequestQueue:表示请求队列,里面包含一个
CacheDispatcher(用于处理走缓存请求的调度线程)、
NetworkDispatcher数组(用于处理走网络请求的调度线程),一个
ResponseDelivery(返回结果分发接口),通过 start() 函数启动时会启动
CacheDispatcher和
NetworkDispatchers。
CacheDispatcher:一个线程,用于调度处理走缓存的请求。启动后会不断从缓存请求队列中取请求处理,队列为空则等待,请求处理结束则将结果传递给
ResponseDelivery去执行后续处理。当结果未缓存过、缓存失效或缓存需要刷新的情况下,该请求都需要重新进入
NetworkDispatcher去调度处理。
NetworkDispatcher:一个线程,用于调度处理走网络的请求。启动后会不断从网络请求队列中取请求处理,队列为空则等待,请求处理结束则将结果传递给
ResponseDelivery去执行后续处理,并判断结果是否要进行缓存。
ResponseDelivery:返回结果分发接口,目前只有基于
ExecutorDelivery的在入参 handler 对应线程内进行分发。
HttpStack:处理 Http 请求,返回请求结果。目前 Volley 中有基于 HttpURLConnection 的
HurlStack和 基于 Apache HttpClient 的
HttpClientStack。
Network:调用
HttpStack处理请求,并将结果转换为可被
ResponseDelivery处理的
NetworkResponse。
Cache:缓存请求结果,Volley 默认使用的是基于 sdcard 的
DiskBasedCache。
NetworkDispatcher得到请求结果后判断是否需要存储在 Cache,
CacheDispatcher会从 Cache 中取缓存结果。
类图
Volley
public static RequestQueue newRequestQueue(Context context) { return newRequestQueue(context, null); }
调用了
newRequestQueue()的方法重载,并给第二个参数传入null。
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; }
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。
得到了 HttpStack,然后通过它构造一个代表网络(Network)的具体实现
BasicNetwork。
接着构造一个代表缓存(Cache)的基于 Disk 的具体实现
DiskBasedCache。
最后将网络(Network)对象和缓存(Cache)对象传入构建一个 RequestQueue,启动这个 RequestQueue,并返回。
Volley 会将请求头中的 User-Agent 字段设置为 App 的 ${packageName}/${versionCode},如果异常则使用 "volley/0"
Request
我们通过构建一个Request类的非抽象子类(StringRequest、JsonRequest、ImageRequest 或自定义)对象,并将其加入到·RequestQueue·中来完成一次网络请求操作。
Volley 支持 8 种 Http 请求方式 GET, POST, PUT, DELETE, HEAD, OPTIONS, TRACE, PATCH
因为是抽象类,子类必须重写的两个方法。
abstract protected Response<T> parseNetworkResponse(NetworkResponse response); abstract protected void deliverResponse(T response);
以下两个方法也经常会被重写
public byte[] getBody(); protected Map<String, String> getParams();
RequestQueue
RequestQueue 中维护了两个基于优先级的 Request 队列,缓存请求队列和网络请求队列。private final PriorityBlockingQueue<Request<?>> mCacheQueue = new PriorityBlockingQueue<Request<?>>(); private final PriorityBlockingQueue<Request<?>> mNetworkQueue = new PriorityBlockingQueue<Request<?>>();
维护了一个正在进行中,尚未完成的请求集合。
private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();
维护了一个等待请求的集合,如果一个请求正在被处理并且可以被缓存,后续的相同 url 的请求,将进入此等待队列。
private final Map<String, Queue<Request<?>>> mWaitingRequests = new HashMap<String, Queue<Request<?>>>();
在Volley类中启动了RequestQueue,queue.start();
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(); }
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。
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; } }
程序会判断当前的请求是否可以缓存,如果不能缓存则直接将这条请求加入网络请求队列,可以缓存的话则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。
CacheDispatcher
一个线程,用于调度处理走缓存的请求。启动后会不断从缓存请求队列中取请求处理,队列为空则等待,请求处理结束则将结果传递给ResponseDelivery去执行后续处理。当结果未缓存过、缓存失效或缓存需要刷新的情况下,该请求都需要重新进入
NetworkDispatcher去调度处理。
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; } } } }
首先可以看到一个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.
NetworkDispatcher
一个线程,用于调度处理走网络的请求。启动后会不断从网络请求队列中取请求处理,队列为空则等待,请求处理结束则将结果传递给 ResponseDelivery 去执行后续处理,并判断结果是否要进行缓存。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)); } } } }
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。访问网络的时候会调用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。
BasicNetwork
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) { …… } } } }
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,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。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。
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) { request.markDelivered(); request.addMarker("post-response"); mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable)); }
ResponseDeliveryRunnable
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(); } } }
ByteArrayPool
缓存接口,代表了一个可以获取请求结果,存储请求结果的缓存。byte[] 的回收池,用于 byte[] 的回收再利用,减少了内存的分配和回收。 主要通过一个元素长度从小到大排序的
ArrayList作为 byte[] 的缓存,另有一个按使用时间先后排序的
ArrayList属性用于缓存满时清理元素。
public synchronized void returnBuf(byte[] buf)
将用过的 byte[] 回收,根据 byte[] 长度按照从小到大的排序将 byte[] 插入到缓存中合适位置。
public synchronized byte[] getBuf(int len)
获取长度不小于 len 的 byte[],遍历缓存,找出第一个长度大于传入参数len的 byte[],并返回;如果最终没有合适的 byte[],new 一个返回。
private synchronized void trim()
当缓存的 byte 超过预先设置的大小时,按照先进先出的顺序删除最早的 byte[]。
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