第一次接觸到Volley，給我的印象就是用法很簡潔，簡單。上手很快，使用起來毫無壓力。所以我決定弄清楚裡面的原始碼是如何進行網路請求的，其實表面越簡單的東西，其內部實現起來會越複雜，這次我主要從整體流程來分析。
Volley的使用：首先新建一個管理類，實現單例模式，獲取Volley的網路請求佇列
public static RequestManager getInstance(Context context) {
if (mInstance == null) {
synchronized(RequestManager.class)
{
if (mInstance == null) {
mInstance = new RequestManager(context);
}
}
}
return mInstance;
}
複製程式碼public RequestQueue getRequestQueue() {
if (reqQueue == null){
synchronized(RequestManager.class)
{
if (reqQueue == null){
reqQueue = Volley.newRequestQueue(mContext);
}
}
}
return reqQueue;
}
複製程式碼加入請求佇列
public void addRequest(Request<?> request, Object tag) {
if (tag != null) {
request.setTag(tag);
}
getRequestQueue().add(request);
}
複製程式碼這時候使用起來就很簡單了，只需要new一個Request物件，設定相應的回撥，然後將Request加入到網路請求佇列就行了。
StringRequest stringRequest = new StringRequest(url,
new Listener<String>() {
@Override
public void onResponse(String response) {
// TODO 會調響應完成在這裡做相應的處理
}
}, new ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// TODO 訪問出現錯誤處理
}
});
RequestManager.getInstance(this).addRequest(stringRequest, this);
複製程式碼以上就是一般的Volley網路請求使用
現在開始我們的Volley原始碼解析
首先我們從獲取網路請求佇列開始Volley.newRequestQueue(mContext);
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
複製程式碼public static RequestQueue newRequestQueue(Context context, HttpStack stack)
{
return newRequestQueue(context, stack, -1);
}
複製程式碼public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
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;
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start();
return queue;
}
複製程式碼可以看到這三個過載的方法，最終會呼叫 newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);這個方法。
這個方法主要做的事情就是，如果Android版本大於2.2的，stack就會使用HurlStack，否則使用HttpClientStack。
HurlStack是使用HttpURLConnection做網路訪問，HttpClientStack是使用HttpClient網路訪問。這兩個有什麼區別呢？可以看郭霖大神的 Android訪問網路，使用HttpURLConnection還是HttpClient？這篇文章。
然後新建物件Network network = new BasicNetwork(stack);
最後建立網路請求佇列queue = new RequestQueue(new DiskBasedCache(cacheDir), network);，RequestQueue佇列的構造方法也是一個過載方法，最終的構造方法進行賦值。其中threadPoolSize變數預設為開啟4條執行緒。
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
複製程式碼我們回到 newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);中，看到此方法最後會呼叫queue.start();，我們看看queue.start();到底做了什麼操作，我們猜想一下，這裡面應該是做了開啟網路請求的操作。下面我們開啟這個方法看看。
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();
}
}
複製程式碼從上面方法中可以看出，首先呼叫stop()方法來確保所有當前的正在執行的網路排程都停止，然後建立一個新的快取分發物件CacheDispatcher，CacheDispatcher是一個繼承Thread的執行緒，然後開始執行CacheDispatcher，看看CacheDispatcher所做的事情，主要看run()方法
@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();
Request<?> request;
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
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.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}
複製程式碼我們看run()方法的主要做的事情：首先拿出佇列頭request = mCacheQueue.take();，然後設定標誌request.addMarker("cache-queue-take"); ，接著從快取中取出響應結果，如果響應結果為空if (entry == null)或者快取失效if (entry.isExpired())，都會加入到網路請求佇列中mNetworkQueue.put(request);，呼叫網路請求獲取結果。如果快取執行緒中取得結果會呼叫ResponseDelivery的postResponse方法。
而ResponseDelivery的實現就是RequestQueue構造方法new出來的ExecutorDelivery物件，我們看下ExecutorDelivery中的postResponse方法的實現。
@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));
}
複製程式碼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);
}
};
}
複製程式碼從ExecutorDelivery可以看出，其主要做的事情是：將結果傳送到主執行緒。再看下ResponseDeliveryRunnable做了什麼操作
@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();
}
}
複製程式碼上面的run方法中最重要的一句是
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
複製程式碼如果響應成功的話就會呼叫Request的deliverResponse方法，如果錯誤就會呼叫deliverError方法，而這個Request方法就是我們使用時new請求物件，而文章開頭我們使用的時StringRequest這個物件，那麼我們看看StringRequest的deliverResponse方法。
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}
複製程式碼deliverResponse方法的mListener物件就是我們使用時傳進來的回撥介面，所以最終結果我們可以在設定回撥的onResponse方法中使用。
分析到這裡我們就知道了Volley的呼叫流程了，就知道為什麼我們只是簡單的new一個請求物件然後設定回撥，並把請求物件加入請求佇列就可以完成整個網路請求了。
接下來我們在回看queue.start()的網路請求。主要看下面的語句
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
複製程式碼前面我們知道預設的執行緒數設定時4，所以這裡回迴圈4次，主要還是看回NetworkDispatcher類，同樣NetworkDispatcher也是繼承Thread的執行緒類。
主要看run()方法
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
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) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
}
}
}
複製程式碼run()：首先回取出佇列頭request = mQueue.take();獲取request，接著看網路請求最最主要的一句NetworkResponse networkResponse = mNetwork.performRequest(request);這一句呢主要是網路訪問，獲取得到結果。從RequestQueue的newRequestQueue方法中我們可以知道：如果Android版本大於2.2的會使用HttpURLConnection進行網路訪問，否則使用HttpClient。
網路訪問完成之後會對響應結果進行解析Response<?> response = request.parseNetworkResponse(networkResponse);，接著之後將結果放到快取執行緒mCache.put(request.getCacheKey(), response.cacheEntry);，跟快取執行緒一樣
最終呼叫mDelivery.postResponse(request, response);將結果回撥。
至此，整個volley解剖完成，大家應該也知道整體呼叫流程了。如果還有不明白的，可以看下volley官方給出來的這張圖，結合我的分析就一目瞭然了。
該文章是我看了好幾天的原始碼分析得來的，哪裡分析得不好請大家指出來，相互學習。網上也有很多分析volley的文章，如有雷同，純屬偶合。