Volley 源码分析
图片分析
要说源码分析，我们得先看一下官方的配图： 
从这张图中我们可以了解到 volley 工作流程：
1.请求加入优先队列
2.从缓存调度器中查看是否存在该请求，如果有（没有进入第三步）直接缓存中读取并解析数据，最后分发到 UI 线程（主线程）。
3.从网络中获取数据（如果设置可以缓存，则写入缓存）并解析数据，最后分发到 UI 线程（主线程）。
从图中，我们还可以看到 volley 的工作其实就是三个线程之间的数据传递 主线程 缓存线程 网络线程。
## 代码分析
既然是源码分析，当然是得从源码开始啦（怎么下载源码我就不说了！不会的自行谷歌）！那我们从哪段源码开始嘞？我们就从我们使用 volley 框架的第一句代码开始。
Volley.newRequestQueue(context) 是我们使用 volley 框架的第一句代码！这句代码是用创建个 RequestQueue (请求队列。对，这个就是 volley 工作流程中的第一步的铺垫。这样请求才能有容器装啊！)。那好，我们现在就来看一下 newRequestQueue(context) 这个静态方法：
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
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;
}
从代码中可以看出，我们使用的是 newRequestQueue(Context context, HttpStack stack) 这个函数的重载方法！
我简要对这个方法说明下：在系统版本大于等于 9 的时候，我们创建 HurlStack(就是 HttpUrlConnection),在小于 9 的时候我们创建 HttpClientStack(就是 HttpClient) 至于为啥要要这么做。那就是 HttpUrlConnection 的性能要比HttpClient 的好。当然这里我还可以使用另外第三发的 HTTP 库 。比如说 okhttp 。这里我们只要调用 newRequestQueue(Context context, HttpStack stack) 这个方法就行了！当然 要对 okhttp 做简单的封装了！前提是要继承 HttpStack 这个接口啊！
接下来 我们直接看这句代码：RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); 这就是我们要创建的请求队列啦！看 RequestQueue 这个类：
我们先看构造函数（直接看最复杂的！哈哈）
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
可以看到，需要传入 缓存 ，网络执行器，线程池大小，返回结果分发器。这四个参数！接着 我们再来看一下 queue.start() 这句代码的含义！还是一样先看代码：
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 (mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
先来解释下这这段代码：先停止当前正在运行的所有的线程！接着 初始化 CacheDispatcher(缓存调度器) 并启动他！接着就是启动 NetworkDispatcher(网络调度器) 这有多个。他的个数全靠 threadPoolSize 这个变量控制（默认大小是4）。这样一来。请求队列就是初始化好了！就等待任务加入了啦！！那我们就趁热打铁，直接看加入任务队列的源码：
public Request {
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
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 {
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}
依旧是来解读这段代码，在解读这该段代码的时候，我们先来弄清楚其中三个变量的含义：
//当前 RequestQueue 中所有的请求队列
private final Set<Request> mCurrentRequests = new HashSet<Request>();
//缓存队列
private final PriorityBlockingQueue<Request> mCacheQueue =
new PriorityBlockingQueue<Request>();
//网络队列 正在进入的
private final PriorityBlockingQueue<Request> mNetworkQueue =
new PriorityBlockingQueue<Request>();
含义如注释！那么我们接着看 add(Request request) 这个方法！首先还是先把任务加入 当前队列。之后request.shouldCache() 判断该任务需要被缓存。不需要的话 直接进入 网络队列！否则的话就加入缓存队列！！
分析完了怎么加入队列之后，我们要来分析下两外两个类了 CacheDispatcher 和 NetworkDispatcher 这里先说下这两个类的共同点：那就是都继承了 Thread 类！也就说他们都是线程类！可以被执行。这也就解释了 RequestQueue 类中的start() 方法中 mCacheDispatcher.start() 和 networkDispatcher.start() 这两句代码！
那么我们先来看一下 CacheDispatcher 这个类！我们直接该类最核心的方法 run() 方法！
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
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;
}
}
}
这段代码中有句代码非常抢眼，没错！那就是 while(true) 这句话了！有的童鞋可能已经想到了：这是个死循环(这不废话！)，另外肯定是在不同的从某个队列中取/存数据。没错，就是这样啊！其实很简单！我们接着细说：首先从缓存队列中去除队列，接着判断该请求是否已经取消 if (request.isCanceled()) 如果已经取消的话，就是不走下面的代码！继续从头循环！反之，从缓存中读取数据，如果没有的话就把该队列加入网络请求队列。如果有的但是缓存已经过期的话 也是加入网络请求队列（ if (entry == null) 和 if (entry.isExpired()) 这两个 if 下的语句就是处理上面两个功能的）。如果以上两个条件都不满足的话！就直接 request.parseNetworkResponse(new NetworkResponse(entry.data, entry.responseHeaders)) 解析缓存中的数据进行回调了！
下面我们看 NetworkDispatcher 类的代码，同样的我们直接看 核心代码:
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request request;
while (true) {
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;
}
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
// 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) {
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
mDelivery.postError(request, new VolleyError(e));
}
}
}
同样的是，先从网络请求队列中取出任务，接着在判断是否要取消，如果要则跳过下面的代码，重新取任务！调用 NetworkResponse networkResponse = mNetwork.performRequest(request); 这句代码，获取 网络返回结果！接着，代码和 CacheDispatcher 中差不多！唯一的区别就是：如果当前的请求需要加入缓存，则加入缓存！细心的同学可能发现了，CacheDispatcher 和 NetworkDispatcher 这两个类中有句相同的代码 Response<?> response = request.parseNetworkResponse(networkResponse); 就是这句！核心的就是parseNetworkResponse(networkResponse) 这个函数的实现我在上一篇 Volley 的使用以及自定义Request 中已经说过了！是有我们实现的！
这里的我们还得在注意一个类：那就是 BasicNetwork ！其实这个类没有什么可以细说的！他就是请求网络接着返回结果！我这里也把核心代码上一下：
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = new HashMap<String, String>();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
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) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
request.getCacheEntry().data, responseHeaders, true);
}
responseContents = entityToBytes(httpResponse.getEntity());
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {
throw new IOException();
}
return new NetworkResponse(statusCode, responseContents, responseHeaders, false);
} 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);
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);
}
}
}
}
这个类 我就真的不细讲啦！！
上面已经说了，在解析玩数据之后其实就分发数据了！让用户能够在 UI线程中调用了！现在我们就来看一下这个分发类：ExecutorDelivery。 我们还是先看这个类的构造函数：
public (final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}
从代码中我们可以看到，我们需要传一个Handller进入，此时我们在回想一下这个类在 RequestQueue 类中是怎么初始化的？
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
没错！大家可以看到，是传入了一个主线中的handler！
当在 CacheDispatcher 和 NetworkDispatcher 这两个类中调用了 mDelivery.postResponse(request, response); 这个方法的时，我们来看一下 ExecutorDelivery 这个类都做了什么！
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
我们直接执行了了个Runnable-->ResponseDeliveryRunnable。那他又做了什么呢？
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();
}
}
我们看最直接的这句代码 mRequest.deliverResponse(mResponse.result); 我们调用了 Request的中的一个方法（这个方法，依然是要我们自己实现！）。接着，我们在看 ExecutorDelivery 的构造函数时，我们就会豁然开朗！数据终于到了 UI线程了！