簡介：
Volley 是Google 在2013 年的IO大會上面推出的HTTP的操作的類庫。 其主要提供了類似於瀏覽器響應HTTP 304中緩存的設計。
我們可以參考 理解HTTP/304響應 ，發出一個url的http請求，如果緩存的有這個請求，帶上 個If-Modified-Since的請求頭，其值爲服務器上次返回的Last-Modified響應頭中的日期值。 通過響應碼200 和304 來區別，是否讀取本地的緩存數據。（服務端通過，客服端發過來的If-Modified-Since 判斷，客服端緩存的數據是否過期，過期響應200和新的數據，未過期響應304）
Volley的使用：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Initialise Volley Request Queue.
mVolleyQueue = Volley.newRequestQueue(this);
StringRequest stringRequest=new StringRequest(Request.Method.GET,url,new Response.Listener<String>() {
@Override
public void onResponse(String response) {
// TODO Auto-generated method stub
}
},new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// TODO Auto-generated method stub
}
});
mVolleyQueue.add(stringRequest); public void onStop() {
super.onStop();
mVolleyQueue.cancelAll(TAG_REQUEST);Volley源碼解讀（側重與網絡的請求的運行的流程）：
1、mVolleyQueue = Volley.newRequestQueue(this);
public class Volley {
/** Default on-disk cache directory. */
private static final String DEFAULT_CACHE_DIR = "volley";
/**
* 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) {
//請求的緩存文件的cache文件存放的位置
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) {
}
//網絡請求處理，（解決在低版本上面可能出現的問題2.3 Gingerbread）
/**
* HurlStack HttpClientStack 實現了接口HttpStack ,兩者都實現了方法performRequest
* <br> 後面的code 通過，調用這個接口的方法，就可以實現在不同SDK平臺上面的實現
*/
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;
}
/**
* 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.
* @return A started {@link RequestQueue} instance.
*/
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
}
/**
* 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.處理緩存中的數據的一個線程
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// Create network dispatchers (and corresponding threads) up to the pool size.
//創建處理網絡請求的線程 ，默認的情況下創建了4個線程處理網絡的請求.
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}在 RequestQueue中的add(Request request) 方法
/**
* Adds a Request to the dispatch queue.
* 添加 需要處理的 Request 到 dispath 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.
//判斷這個請求是否是需要cache的，如果不需要cache 直接去網絡隊列中，去下載.
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;
}
}當添加了一個網絡的Request 到queue 。在前面的start中的
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);mNetwork 在Volley 中 傳入的參數，實現的具體類，可以在此查看
mCache cache的實現
mDelivery dispatch 網絡的具體實現。
NetworkDispatcher 總run的的code
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request request;
while (true) {
try {
// 不停的循環在此處，當沒有數據的的時候，執行mQueue.take() 的時候，將會阻塞在這個地方 直到有數據到達，執行下一步.
// 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());
}
/**
* 通過前面的 Network network = new BasicNetwork(stack);
*/
// 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();
// dispatch 響應的結果 到 ui的線程中，具體實現是通過 ExecutorDelivery 實現的
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));
}
}
}
在此處，我們分別分析下，mNetwork.performRequest(request)和 mDelivery.postResponse(request, response) 的調用的過
1、mNetwork.performRequest(request)
詳見源碼 其網絡的請求又通過接口的調用.. 具體的實現（version>9）在HulStack中。
2、 mDelivery.postResponse(request, response)
public class ExecutorDelivery implements ResponseDelivery {
/** Used for posting responses, typically to the main thread. */
private final Executor mResponsePoster;
/**
* Creates a new response delivery interface.
* @param handler {@link Handler} to post responses on
*/
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
// 將其執行在UI的主線程中，達到具體的數據網絡的返回數據 傳遞到UI的線程
handler.post(command);
}
};
}
/**
* Creates a new response delivery interface, mockable version
* for testing.
* @param executor For running delivery tasks
*/
public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}
@Override
public void postResponse(Request<?> request, Response<?> response) {
postResponse(request, response, null);
}
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
@Override
public void postError(Request<?> request, VolleyError error) {
request.addMarker("post-error");
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}
/**
* 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();
}
}
}
}
總結：
本文，簡單的講述了Volley 的網絡請求的在代碼中運行的一個大概的流程（不包括緩存的實現）。 在這個volley中，對接口的設計和調用，設計的很好。值的多多回顧和學習。