概述
本文基于Volley 1.1.1版本的源码。
Volley是Google官方出的一套小而巧的异步请求库，该框架封装的扩展性很强，支持HttpClient、HttpUrlConnection，甚至支持OkHttp。Volley不适用于下载大量内容的操作或流式传输操作，因为在解析过程中，Volley会将所有响应存储在内存中，因此Volley不适合上传和下载大文件。
添加依赖：
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implementation 'com.android.volley:volley:1.1.1'
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使用：
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RequestQueue queue = Volley.newRequestQueue(context);
String url ="http://www.google.com";
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
// in UI Thread
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
// in UI Thread
}
});
stringRequest.setTag(TAG);
// Add the request to the RequestQueue.
queue.add(stringRequest);
// onStop
queue.cancelAll(TAG);
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创建请求队列
Volley类中有多个重载的newRequestQueue静态方法：
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public class Volley {
private static final String DEFAULT_CACHE_DIR = "volley";
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, (BaseHttpStack) null);
}
public static RequestQueue newRequestQueue(Context context, BaseHttpStack stack) {
BasicNetwork network;
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
network = new BasicNetwork(new HurlStack());
} else {
// ...
}
} else {
network = new BasicNetwork(stack);
}
return newRequestQueue(context, network);
}
private static RequestQueue newRequestQueue(Context context, Network network) {
final Context appContext = context.getApplicationContext();
DiskBasedCache.FileSupplier cacheSupplier =
new DiskBasedCache.FileSupplier() {
private File cacheDir = null;
@Override
public File get() {
if (cacheDir == null) {
cacheDir = new File(appContext.getCacheDir(), DEFAULT_CACHE_DIR);
}
return cacheDir;
}
};
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheSupplier), network);
queue.start();
return queue;
}
}
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RequestQueue类构造方法如下：
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public class RequestQueue {
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
private final Cache mCache;
private final Network mNetwork;
private final ResponseDelivery mDelivery;
private final NetworkDispatcher[] mDispatchers;
private CacheDispatcher mCacheDispatcher;
public RequestQueue(
Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize, new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}
}
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RequestQueue#start()方法如下，可以看出就是启动了一个Cache Dispatcher和指定数目的Network Dispatcher：
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/** Starts the dispatchers in this queue. */
public void start() {
// Make sure any currently running dispatchers are stopped.
stop();
// 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();
}
}
/** Stops the cache and network dispatchers. */
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (final NetworkDispatcher mDispatcher : mDispatchers) {
if (mDispatcher != null) {
mDispatcher.quit();
}
}
}
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RequestQueue事件
在RequestQueue类中定义了一些事件：
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public @interface RequestEvent {
/** The request was added to the queue. */
public static final int REQUEST_QUEUED = 0;
/** Cache lookup started for the request. */
public static final int REQUEST_CACHE_LOOKUP_STARTED = 1;
/** Cache lookup finished for the request and cached response is delivered or request is queued for network dispatching. */
public static final int REQUEST_CACHE_LOOKUP_FINISHED = 2;
/** Network dispatch started for the request. */
public static final int REQUEST_NETWORK_DISPATCH_STARTED = 3;
/** The network dispatch finished for the request and response (if any) is delivered. */
public static final int REQUEST_NETWORK_DISPATCH_FINISHED = 4;
/** All the work associated with the request is finished and request is removed from all the queues. */
public static final int REQUEST_FINISHED = 5;
}
public interface RequestEventListener {
void onRequestEvent(Request<?> request, @RequestEvent int event);
}
private final List<RequestEventListener> mEventListeners = new ArrayList<>();
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可以通过下面几个方法操作这些事件：
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void sendRequestEvent(Request<?> request, @RequestEvent int event) {
synchronized (mEventListeners) {
for (RequestEventListener listener : mEventListeners) {
listener.onRequestEvent(request, event);
}
}
}
public void addRequestEventListener(RequestEventListener listener) {
synchronized (mEventListeners) {
mEventListeners.add(listener);
}
}
public void removeRequestEventListener(RequestEventListener listener) {
synchronized (mEventListeners) {
mEventListeners.remove(listener);
}
}
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发送请求
RequestQueue#add()方法如下：
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private final AtomicInteger mSequenceGenerator = new AtomicInteger();
private final Set<Request<?>> mCurrentRequests = new HashSet<>();
private final PriorityBlockingQueue<Request<?>> mCacheQueue = new PriorityBlockingQueue<>();
private final PriorityBlockingQueue<Request<?>> mNetworkQueue = new PriorityBlockingQueue<>();
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");
sendRequestEvent(request, RequestEvent.REQUEST_QUEUED);
// 如果请求不需要缓存的话，直接交给网络请求队列去执行，默认是需要缓存的
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
mCacheQueue.add(request);
return request;
}
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在这个方法中把请求交给了缓存队列或者网络队列去处理。
CacheDispatcher
CacheDispatcher继承自Thread，在RequestQueue#start()方法中已经将其启动了，因此直接看run方法：
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public class CacheDispatcher extends Thread {
private final BlockingQueue<Request<?>> mCacheQueue;
private final BlockingQueue<Request<?>> mNetworkQueue;
private final Cache mCache;
private final ResponseDelivery mDelivery;
private volatile boolean mQuit = false;
/** Manage list of waiting requests and de-duplicate requests with same cache key. */
private final WaitingRequestManager mWaitingRequestManager;
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue,
BlockingQueue<Request<?>> networkQueue,
Cache cache,
ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
mWaitingRequestManager = new WaitingRequestManager(this);
}
public void quit() {
mQuit = true;
interrupt();
}
@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 {
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
Thread.currentThread().interrupt();
return;
}
VolleyLog.e("Ignoring spurious interrupt of CacheDispatcher thread; " + "use quit() to terminate it");
}
}
}
private void processRequest() throws InterruptedException {
// 阻塞式队列
final Request<?> request = mCacheQueue.take();
processRequest(request);
}
void processRequest(final Request<?> request) throws InterruptedException {
request.addMarker("cache-queue-take");
request.sendEvent(RequestQueue.RequestEvent.REQUEST_CACHE_LOOKUP_STARTED);
try {
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
return;
}
// 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.
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// 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 (!response.isSuccess()) {
request.addMarker("cache-parsing-failed");
mCache.invalidate(request.getCacheKey(), true);
request.setCacheEntry(null);
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
if (!entry.refreshNeeded()) {
// 缓存不需要刷新，直接交给ResponseDelivery去处理
mDelivery.postResponse(request, response);
} else {
// 软过期的缓存命中，可以分发缓存的response，但是我们还需要将请求发送到网络以进行刷新。
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);
// Mark the response as intermediate.
response.intermediate = true;
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
// 将intermediate响应发回给用户，然后将请求转发到网络。
mDelivery.postResponse(
request,
response,
new Runnable() {
@Override
public void run() {
try {
// 如果缓存需要刷新，这个地方还需要再重新请求网络
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Restore the interrupted status
Thread.currentThread().interrupt();
}
}
});
} else {
// 该请求已添加到等待请求的列表中，以便在第一个请求返回时从第一个请求接收网络响应。
mDelivery.postResponse(request, response);
}
}
} finally {
request.sendEvent(RequestQueue.RequestEvent.REQUEST_CACHE_LOOKUP_FINISHED);
}
}
}
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如果有缓存，就取出来去处理，没有或者过期等，就加入到网络请求队列中去请求，需要网络请求的有几个地方：
- 请求不需要缓存
- 请求在缓存中没有找到
- 请求的缓存过期
- 请求的缓存需要刷新
NetworkDispatcher
NetworkDispatcher
NetworkDispatcher也继承自Thread，它们在RequestQueue#start()方法中已经被启动了，因此直接看run方法：
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public class NetworkDispatcher extends Thread {
private final BlockingQueue<Request<?>> mQueue;
private final Network mNetwork;
private final Cache mCache;
private final ResponseDelivery mDelivery;
private volatile boolean mQuit = false;
public NetworkDispatcher(
BlockingQueue<Request<?>> queue,
Network network,
Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
public void quit() {
mQuit = true;
interrupt();
}
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
try {
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
Thread.currentThread().interrupt();
return;
}
VolleyLog.e("Ignoring spurious interrupt of NetworkDispatcher thread; " + "use quit() to terminate it");
}
}
}
private void processRequest() throws InterruptedException {
Request<?> request = mQueue.take();
processRequest(request);
}
void processRequest(Request<?> request) {
long startTimeMs = SystemClock.elapsedRealtime();
request.sendEvent(RequestQueue.RequestEvent.REQUEST_NETWORK_DISPATCH_STARTED);
try {
request.addMarker("network-queue-take");
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
request.notifyListenerResponseNotUsable();
return;
}
addTrafficStatsTag(request);
// 执行网络请求
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");
request.notifyListenerResponseNotUsable();
return;
}
// 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);
request.notifyListenerResponseReceived(response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
request.notifyListenerResponseNotUsable();
} 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);
request.notifyListenerResponseNotUsable();
} finally {
request.sendEvent(RequestQueue.RequestEvent.REQUEST_NETWORK_DISPATCH_FINISHED);
}
}
}
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可以看到，在NetworkDispatcher中，最主要的就是调用了mNetwork.performRequest(request)执行网络请求，而在Network是个接口，具体实现类是BasicNetwork。
BasicNetwork
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public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
// ...
try {
// ...
httpResponse = mBaseHttpStack.executeRequest(request, additionalRequestHeaders);
// return NetworkResponse
} catch (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode;
if (httpResponse != null) {
statusCode = httpResponse.getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
NetworkResponse networkResponse;
if (responseContents != null) {
networkResponse =
new NetworkResponse(
statusCode,
responseContents,
/* notModified= */ false,
SystemClock.elapsedRealtime() - requestStart,
responseHeaders);
if (statusCode == HttpURLConnection.HTTP_UNAUTHORIZED
|| statusCode == HttpURLConnection.HTTP_FORBIDDEN) {
attemptRetryOnException(
"auth", request, new AuthFailureError(networkResponse));
} else if (statusCode >= 400 && statusCode <= 499) {
// Don't retry other client errors.
throw new ClientError(networkResponse);
} else if (statusCode >= 500 && statusCode <= 599) {
if (request.shouldRetryServerErrors()) {
attemptRetryOnException(
"server", request, new ServerError(networkResponse));
} else {
throw new ServerError(networkResponse);
}
} else {
// 3xx? No reason to retry.
throw new ServerError(networkResponse);
}
} else {
attemptRetryOnException("network", request, new NetworkError());
}
}
}
}
// 请求重试
private static void attemptRetryOnException(
String logPrefix, Request<?> request, VolleyError exception) throws VolleyError {
RetryPolicy retryPolicy = request.getRetryPolicy();
int oldTimeout = request.getTimeoutMs();
try {
retryPolicy.retry(exception);
} catch (VolleyError e) {
request.addMarker(String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout));
throw e;
}
request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));
}
public class DefaultRetryPolicy implements RetryPolicy {
@Override
public void retry(VolleyError error) throws VolleyError {
mCurrentRetryCount++;
mCurrentTimeoutMs += (int) (mCurrentTimeoutMs * mBackoffMultiplier);
if (!hasAttemptRemaining()) {
throw error;
}
}
/** Returns true if this policy has attempts remaining, false otherwise. */
protected boolean hasAttemptRemaining() {
return mCurrentRetryCount <= mMaxNumRetries;
}
}
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在BasicNetwork#performRequest()中调用了httpResponse = mBaseHttpStack.executeRequest(request, additionalRequestHeaders);，这里的mBaseHttpStack就是前面创建RequestQueue时新建的HurlStack和HttpClientStack(@Deprecated，上面RequestQueue的构造函数中还有个新建了HttpClientStack对象的方法，不过已经废弃了，不做解析)。因此在 HttpStack的executeRequest()中就是具体的网络请求。
HurlStack
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public class HurlStack extends BaseHttpStack {
@Override
public HttpResponse executeRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
// 这个类是使用HttpURLConnection来实现真的的网络请求，可以自定义
}
}
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ResponseDelivery
ResponseDelivery是一个接口：
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public interface ResponseDelivery {
/** Parses a response from the network or cache and delivers it. */
void postResponse(Request<?> request, Response<?> response);
/**
* Parses a response from the network or cache and delivers it. The provided Runnable will be
* executed after delivery.
*/
void postResponse(Request<?> request, Response<?> response, Runnable runnable);
/** Posts an error for the given request. */
void postError(Request<?> request, VolleyError error);
}
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具体实现类是ExecutorDelivery，它保证了客户端调用的回调运行在UI线程：
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public class ExecutorDelivery implements ResponseDelivery {
private final Executor mResponsePoster;
// 在上面的分析中可以看到这里的handle = new Handler(Looper.getMainLooper())
public ExecutorDelivery(final Handler handler) {
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}
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));
}
private static 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 (mRequest.isCanceled()) {
mRequest.finish("canceled-at-delivery");
return;
}
// 这里会调用在创建Request时传入的回调
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
if (mResponse.intermediate) {
mRequest.addMarker("intermediate-response");
} else {
mRequest.finish("done");
}
// 如果有需要执行的任务，如前面缓存需要刷新等，就会在这个地方进行调用
if (mRunnable != null) {
mRunnable.run();
}
}
}
}
|
总结
Volley中主要的类有这些：
- Volley: 提供了构建RequestQueue的统一方法，我们也可以不通过Volley类而是自己构建RequestQueue；
- RequestQueue: 负责分发请求到不同的请求队列中；
- CacheDispatcher: 处理缓存请求；
- NetworkDispatcher: 处理网络请求；
- ResponseDelivery: 获取请求后进行处理，具体实现类为ExecutorDelivery；
- Cache: 缓存接口，具体实现类有DiskBasedCache；
- Network: 网络接口，具体实现类有BasicNetwork；
- HttpStack: 真正执行请求，具体实现类有HttpClientStack(已废弃)，BaseHttpStack(子类有HurlStack，AdaptedHttpStack等)；
- Request: 封装请求信息并处理回调，具体实现类有StringRequest，JsonRequest，ImageRequest等；
- NetworkResponse：请求网络时的返回的response；
- Response: 定义了一些回调接口，用来封装返回给客户端的数据。
Volley框架的拆装性很强，可以使用许多我们自定义的类来扩展其功能：
- Request：框架默认提供了StringRequest，JsonRequest和ImageRequest等，我们可以继承Request并重写deliverResponse()和parseNetworkResponse()来自定义请求。
- Cache：框架默认提供了DiskBasedCache类来处理缓存，我们可以继承Cache接口来自定义缓存策略。
- HttpStack：框架默认使用的HurlStack类是通过HttpUrlConnection和HttpClient来实现网络请求，我们可以换成其它的比如说OkHttp，只需要继承HttpStack并重写performRequest()来处理请求就可以了。
总结一下Volley网络请求的流程：
- 首先创建请求队列RequestQueue，其中可以自定义相关类，在这里会启动了一个Cache Dispatcher和指定数目的Network Dispatcher；
- 然后在CacheDispatcher中处理请求，若缓存可用，则直接返回缓存，否则请求网络；
- 接着在NetworkDispatcher中处理网络请求，真正起作用的是HttpStack实现类；
- 最后通过ResponseDelivery实现类返回请求结果。
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