Volley作为当年Google在2013年的Google I/O上的重点，是一个相当给力的框架。它从设计模式上来说，非常具有扩展性，也比较轻巧。关于Volley的使用，网上介绍的很多了，不再赘述。现在，我将记录我阅读Volley源码的过程，来学习Volley的设计思想和其中的一些小技巧。
值的一提的是，新版的gradle已经支持：
compile ‘com.android.volley:volley:1.0.0‘
这样导入Volley了。
从最简单的例子看起：
RequestQueue queue;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
queue = Volley.newRequestQueue(this);
final StringRequest request = new StringRequest("https://www.baidu.com/",
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
((TextView) findViewById(R.id.test_textview_id)).setText(response);
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
});
findViewById(R.id.test_btn_id).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
queue.add(request);
}
});
}
可以看到，一个简单的Volley的http请求，只需要三步，建立Request队列--->建立Request请求--->将请求加入Request队列。
今天就只看这三步是怎么运作的。
我们先来看看queue = Volley.newRequestQueue(this);
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;
}
从体来说这个方法，做了以下几件事：获取userAgent、创建HttpStack、创建NetWork、创建并启动RequestQueue。。
获取userAgetnt的方法比较简单 userAgent = packageName + "/" + info.versionCode; 一目了然。
接着创建HttpStack和NetWork比较复杂，会单独记录。
最后会创建一个RequestQueue,作为请求队列。并且在初始化时，请求队列会直接启动。
接下来，我们看一下RequestQueue的代码。我们先从构造方法开始阅读：
/**
* Creates the worker pool. Processing will not begin until {@link #start()} is called.
*
* @param cache A Cache to use for persisting responses to disk
* @param network A Network interface for performing HTTP requests
* @param threadPoolSize Number of network dispatcher threads to create
* @param delivery A ResponseDelivery interface for posting responses and errors
*/
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
在初始化时，RequestQueue对4个成员赋了值。缓存、NetWork、网络分发线程队列和网络响应分发接口。
下面我们看看在初始化Volley初始化时，第一个会调用的方法start()
/**
* 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.
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();
}
}
上面的代码比较简洁。首先调用stop();关于stop()：
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
它就是通过遍历，结束了线程中的所有任务。
我们可以在NetworkDispatcher.java中找到相关实现
public void quit() {
mQuit = true;
interrupt();
}
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;
}
......
}
}
继续看start方法：
/**
* 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.
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();
}
}
stop之后，就会启动CacheDispatcher和NetworkDispatcher。
关于NetworkDispatcher中start()方法的实现，NetworkDispatcher继承了Thread，所以看start()其实是看run方法：
@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);
// 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);
}
}
}
代码比较长，我们分段阅读。
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;
}
设置线程优先级为标准后台线程，然后进入死循环。虽然这一段代码没有什么实质的实现，但是却很重要，它基本确立了这个线程的工作模式。这个线程在while(true)的死循环中，每次循环有一次停止线程和一次暂停线程的机会，暂停在前。
mQueue是个阻塞型队列。每次循环会在这个队列里取一个请求，如果mQueue为空，取不到请求了，线程就会卡死在这里，这就是暂停的机会，等到mQueue加入请求时，线程又会重新跑起来。通过这样的设计，我们后面发起http请求，只需要往mQueue里加入请求就好了。当调用interrupt方法时，循环就会进入catch分支，然后判断是否真的要取消，所以我们在quit()方法中，看到它的实现是先将mQuit置为true，然后调用interrupt来结束这个线程。
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);
如果请求被取消，则调用请求的finish方法，进入下一轮循环。 然后加入流量标记。我们可以看到addTrafficStatsTag
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void addTrafficStatsTag(Request<?> request) {
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
}
我们可以对这个网络请求加上标记，方便调式时知晓它的流量使用情况。
// 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;
}
接着便了Network执行它的performRequest方法，方法的具体实现，将在后面阅读。通过这个方法，我们处理了网络请求，取得了响应——NetworkResponse。下面的一个判断语句，注释解释得很清楚。如果我们的请求返回304，并且我们已经分发过这个响应了。我们将不再处理响应，结束这次循环，开始下一轮。同时执行request.finish的方法。如果没有304，那么我们还要考虑如何处理响应。
// 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");
}
这里，进行解析网络响应，获得响应——Response。根据配置，如果需要缓存，则将响应的内容缓存在mCache中。
// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);
最后，根据请求，分析网络响应。
至此，我们将volley的网络请求及处理流程基本走通了一遍。后面的文章中，将仔细分析一些Volley对于网络请求的一些处理细节。
Done
时间： 2024-12-17 02:53:19