Volley源码分析

时间:2024-09-18 18:08:08

取消请求的源码分析:

public void cancelAll(RequestFilter filter) {
synchronized (mCurrentRequests) {
for (Request<?> request : mCurrentRequests) {
if (filter.apply(request)) {
request.cancel();
}
}
}
} /**
* Cancels all requests in this queue with the given tag. Tag must be non-null
* and equality is by identity.
*/
public void cancelAll(final Object tag) {
if (tag == null) {
throw new IllegalArgumentException("Cannot cancelAll with a null tag");
}
cancelAll(new RequestFilter() {
@Override
public boolean apply(Request<?> request) {
return request.getTag() == tag;
}
});
}

使用Volley的第一步,首先要调用Volley.newRequestQueue(context)方法来获取一个RequestQueue对象,那么我们自然要从这个方法开始看起了,代码如下所示:

public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}

这个方法仅仅只有一行代码,只是调用了newRequestQueue()的方法重载,并给第二个参数传入null。那我们看下带有两个参数的newRequestQueue()方法中的代码,如下所示:

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,这里在第10行判断如果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.

创建好了HttpStack之后,接下来又创建了一个Network对象,它是用于根据传入的HttpStack对象来处理网络请求的,紧接着new出一个RequestQueue对象,并调用它的start()方法进行启动,然后将RequestQueue返回,这样newRequestQueue()的方法就执行结束了。

那么RequestQueue的start()方法内部到底执行了什么逻辑呢?

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();
}
}

这里先是创建了一个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。

得到了RequestQueue之后,我们只需要构建出相应的Request,然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了,那么不用说,add()方法的内部肯定有着非常复杂的逻辑,我们来一起看一下:

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;
}
}

可以看到,会判断当前的请求是否可以缓存,如果不能缓存则在第12行直接将这条请求加入网络请求队列,可以缓存的话则在第33行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。

OK,那么既然默认每条请求都是可以缓存的,自然就被添加到了缓存队列中,于是一直在后台等待的缓存线程就要开始运行起来了,我们看下CacheDispatcher中的run()方法,代码如下所示:

public class CacheDispatcher extends Thread {

    ........
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;
}
}
}

首先看到一个while(true)循环,说明缓存线程始终是在运行的,接着在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。之后会在第39行调用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,这部分代码我们先跳过,因为它的逻辑和NetworkDispatcher后半部分的逻辑是基本相同的,那么我们等下合并在一起看就好了,先来看一下NetworkDispatcher中是怎么处理网络请求队列的,代码如下所示:

public class NetworkDispatcher extends Thread {
........
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); // 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);
}
}
} private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
}

需要注意的是在第14行调用了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,默认情况下如果系统版本号大于9就创建的HurlStack对象,否则创建HttpClientStack对象。前面已经说过,这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的,我们就不再跟进去阅读了,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。还记得我们在上一篇文章中学习的自定义Request的方式吗?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,代码如下所示:

public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}

其中,在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象,就可以保证该对象中的run()方法就是在主线程当中运行的了,我们看下run()方法中的代码是什么样的:

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();
}
}
}

代码虽然不多,但我们并不需要行行阅读,抓住重点看即可。其中在第22行调用了Request的deliverResponse()方法,有没有感觉很熟悉?没错,这个就是我们在自定义Request时需要重写的另外一个方法,每一条网络请求的响应都是回调到这个方法中,最后我们再在这个方法中将响应的数据回调到Response.Listener的onResponse()方法中就可以了。

好了,到这里我们就把Volley的完整执行流程全部梳理了一遍,你是不是已经感觉已经很清晰了呢?对了,还记得在文章一开始的那张流程图吗,刚才还不能理解,现在我们再来重新看下这张图:

Volley源码分析

其中蓝色部分代表主线程,绿色部分代表缓存线程,橙色部分代表网络线程。我们在主线程中调用RequestQueue的add()方法来添加一条网络请求,这条请求会先被加入到缓存队列当中,如果发现可以找到相应的缓存结果就直接读取缓存并解析,然后回调给主线程。如果在缓存中没有找到结果,则将这条请求加入到网络请求队列中,然后处理发送HTTP请求,解析响应结果,写入缓存,并回调主线程。