編輯：關於Android編程
volley官方文檔給出的執行一個請求的步驟是：
RequestQueue mQueue = Volley.newRequestQueue(context);
mQueue.add(request);
為什麼創建一個mQueue以後，直接放入一個request，就可以執行該request了呢？發起網絡請求的代碼在哪裡呢？這裡看下newRequestQueue()做了哪些操作：
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;
}
可以看到創建RequestQueue後，調用了start()方法，再看下start()方法：
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;
VolleyCompat.getInstance().setNetworkDispatcherPriorityMax(networkDispatcher);
networkDispatcher.start();
}
}
通過源碼可以查看到,CacheDispatcher,NetworkDispatcher都是繼承了Thread類，這裡相當於啟動了一個緩存調度線程，四個網絡調度線程，這裡先不分析緩存，看網絡調度線程做了些什麼操作：
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
Request request;
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);
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) {
// volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
// parseAndDeliverNetworkError(request, volleyError);
//volley擴展:(2015-11-10)當出現錯誤，如設備無網絡，請求服務器出錯，並且有緩存時，需要將緩存內容展示出來
VolleyCompat.getInstance().showCacheWhenErrorOccur(mCache,request,mDelivery,volleyError,startTimeMs);
} catch (Exception e) {
e.printStackTrace();
// VolleyLog.e(e, "Unhandled exception %s", e.toString());
// VolleyError volleyError = new VolleyError(e);
// volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
// mDelivery.postError(request, volleyError);
//volley擴展:(2015-11-10)當出現錯誤，如設備無網絡，請求服務器出錯，並且有緩存時，需要將緩存內容展示出來
VolleyError volleyError = new VolleyError(e);
VolleyCompat.getInstance().showCacheWhenErrorOccur(mCache,request,mDelivery,volleyError,startTimeMs);
}
}
}
很簡單，網絡調度線程先是從隊列mQueue裡取出一個request,然後判斷該request是否已經取消，如果沒有被用戶取消，則調用mNetwork的performRequest(request)發起網絡請求，這裡的mNetWork是一個BasicNetWrok,看下performRequest()的實現，
@Override
public NetworkResponse performRequest(Request request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map responseHeaders = Collections.emptyMap();
try {
// Gather headers.
Map headers = new HashMap();
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) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// A HTTP 304 response does not have all header fields. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// 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,
SystemClock.elapsedRealtime() - requestStart);
} 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, SystemClock.elapsedRealtime() - requestStart);
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);
}
}
}
}
首先通過調用mHttpStack.performRequest(request,headers) 發起請求，返回的首先是一個org.apache.http.HttpResponse對象,根據statusCode判斷，如果是SC_NOT_MODIFIED,並且支持緩存，那麼直接加載本地緩存，否則根據httpResponse.getEntity()方法來構建一個NetworkResponse的對象。
現在回到NetWorkDispatcher的run()方法裡，當得到一個NetWorkResponse對象後，接著調用request.parseNetWorkResponse(networkResponse)來對結果數據進行解析，解析數據的過程是什麼樣的呢？查看下request的parseNetWorkResponse()方法。
abstract protected Response parseNetworkResponse(NetworkResponse response);
是給抽象方法，再看Request，也是一個抽象類，說明要想發起請求，必須創建一個Request的子類，並且自己實現該方法，由該Requet來決定如何解析這個networkResponse。好，那先看下Volley自己提供的StringRequest中該方法是怎麼實現的：
@Override
protected Response parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}
對比下另外一個Volley提供的JsonObjectRequest的parseNetWorkResponse實現：
@Override
protected Response parseNetworkResponse(NetworkResponse response) {
try {
String jsonString = new String(response.data,
HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET));
return Response.success(new JSONObject(jsonString),
HttpHeaderParser.parseCacheHeaders(response));
} catch (UnsupportedEncodingException e) {
return Response.error(new ParseError(e));
} catch (JSONException je) {
return Response.error(new ParseError(je));
}
}
其中Response.success()會new 出一個Response對象，該對象中的result變量也是泛型，並且該result的類型也是由新建Request類的子類的時候制定的，這樣這個請求就得到了一個Response對象。
接著回到NetWorkDispatcher的run()方法，接著分析下邊流程：
mDelivery.postResponse(request, response);
這個mDelivery又是什麼東西呢？查看NetWorkDispatcher源碼，其實是一個ResponseDelivery類型變量，而查看ResponseDelivery發現是一個接口，從這裡可以看到,Volley中處處用到了面向接口的編程思想，使得使用者可以自己定義子類型，提供相關服務。那麼這個mDelivery是在哪裡制定的呢？其實是在RequestQueue的構造方法中指定，然後在start()方法中傳遞給NetWorkDispatcher的:
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
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;
VolleyCompat.getInstance().setNetworkDispatcherPriorityMax(networkDispatcher);
networkDispatcher.start();
}
}
好，接著看ExecutorDelivery的postResponse()方法：
@Override
public void postResponse(Request request, Response response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
調用了mResponsePoster的execute()方法，接著看下mResponsePoster：
/** 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) {
handler.post(command);
}
};
}
可以看到看到handler，想必熟悉handler機制的童鞋們就特別想知道這個handler是哪個線程的handler了吧？會不會是UI Thread的呢？接著看
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
可以看到，確實是UI 線程的，這樣我們就可以將ResponseDeliveryRunnable放到UI線程裡去執行啦。好了，那看下讓UI線程執行什麼操作呢？
@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();
}
}
其中這個mRequest,就是我們這次網絡請求發送的Request,Response就是我們這次請求返回的結果封裝類，再次提醒，該Response裡的result變量的類型，就是我們在新建Request子類時候指定的類型。好的，萬裡長城差最後一步了，再看下Request的deliverResponse方法()
/**
* Subclasses must implement this to perform delivery of the parsed
* response to their listeners. The given response is guaranteed to
* be non-null; responses that fail to parse are not delivered.
* @param response The parsed response returned by
* {@link #parseNetworkResponse(NetworkResponse)}
*/
abstract protected void deliverResponse(T response);
也是一個抽象方法，那麼我們還是看下JsonRequest類的該方法是如何實現的：
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}
mListener是Response的一個內部接口：
public class Response {
/** Callback interface for delivering parsed responses. */
public interface Listener {
/** Called when a response is received. */
public void onResponse(T response);
}
那mListener是在哪裡賦值的呢？
public JsonRequest(int method, String url, String requestBody, Listener listener,
ErrorListener errorListener) {
super(method, url, errorListener);
mListener = listener;
mRequestBody = requestBody;
}
也就是在創建JsonRequest的時候，就要指定這個Response.Listener,這裡也是面相接口編程，因為不知道將來誰會對這個Request請求的結果數據感興趣，那麼就在設計框架的時候，調用listener的onResponse()方法，這樣以後誰對結果數據敢興趣，那就實現接口，並且在創建相應Request的時候，傳遞進來就可以了。
有一個細節值得注意的就是，在Volley原生Request類的構造函數裡，是只有Response.ErrorListener,並沒有Response.Listener的：
/**
* Creates a new request with the given method (one of the values from {@link Method}),
* URL, and error listener. Note that the normal response listener is not provided here as
* delivery of responses is provided by subclasses, who have a better idea of how to deliver
* an already-parsed response.
*/
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
setRetryPolicy(new DefaultRetryPolicy());
mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}
也就是說請求如果發生錯誤信息了，那麼必須要有能通知到的listener來處理，但是請求成功後，可以任何事情也不做。
正查數據請求過來的流程分析清楚後，最後再看下請求出錯後的處理邏輯：
在ExecutorDelivery的run()方法裡：
// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
而在Request的deliverError()方法中：
/**
* Delivers error message to the ErrorListener that the Request was
* initialized with.
*
* @param error Error details
*/
public void deliverError(VolleyError error) {
if (mErrorListener != null) {
mErrorListener.onErrorResponse(error);
}
}
也印證了上面的請求出錯必須有處理的猜測。
至此，Volley一個請求的執行流程就分析完成了，總結下：
Volley在主線程中創建一個request請求後，就會將該請求放到隊列中，然後後台線程(NetWorkDispatcher)從隊列中取出請求，調用網絡相關類，執行請求，得到返回結果後，通過ExecutorDelivery將一個Runnable對象放到UI線程的消息池裡去執行。 在創建Request的時候，需要制定Response.Listener,Response.ErroListener對象，因為在Request執行完後，Volley會通過handler機制，將在UI線程裡調用這些listener的相關方法，從而使應用可以在這些listener的相關方法裡，獲得這些請求結果數據，從而實現刷新UI等操作。
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