// 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;
}
}
}
首先通过Process.setThreadPriority(Process.THREAD\_PRIORITY\_BACKGROUND);设置线程优先级，然后通过mCache.initialize(); 初始化缓存块，其中mCache是由Volley.java中的`newRequestQueue(Context context, HttpStack stack)`方法中实例化传入的，其Cache接口的实现为new DiskBasedCache(cacheDir)，其中cacheDir默认在Volley.java中不设置为/data/data/app-package/cache/volley/。
接下来由while (true)可以发现缓存线程是一直在执行，其中通过mQuit标记进制是否结束线程的操作。mCacheQueue.take()从阻塞队列获取最前面的一个request，没有request就阻塞等待。接着通过mCache.get(request.getCacheKey());尝试从缓存中取出响应结果，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。在这个过程中调运了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。现在先来看下Request抽象基类的这部分代码：
/**
-
Subclasses must implement this to parse the raw network response
-
and return an appropriate response type. This method will be
-
called from a worker thread. The response will not be delivered
-
if you return null.
-
@param response Response from the network
-
@return The parsed response, or null in the case of an error
*/
abstract protected Response parseNetworkResponse(NetworkResponse response);
通过注释可以看到他就是一个解析模块的功能。
上面说了，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，其中一个CacheDispatcher是缓存线程，四个NetworkDispatcher是网络请求线程。CacheDispatcher的run方法刚才已经大致分析了，解析来看下NetworkDispatcher中是怎么处理网络请求队列的，具体代码如下所示：
@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);
}
}
}
和CacheDispatcher差不多，如上可以看见一个类似的while(true)循环，说明网络请求线程也是在不断运行的。
如上通过mNetwork.performRequest(request);代码来发送网络请求，而Network是一个接口，这里具体的实现之前已经分析是BasicNetwork，所以先看下它的performRequest()方法，如下所示：
NetWork接口的代码：
public interface Network {
/**
* Performs the specified request.
* @param request Request to process
* @return A {@link NetworkResponse} with data and caching metadata; will never be null
* @throws VolleyError on errors
*/
public NetworkResponse performRequest(Request<?> request) throws VolleyError;
}
上面说了，就是执行指定的请求。他的BasicNetwork实现子类如下：
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
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) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// A HTTP 304 response does not have all header fields. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// Some responses such as 204s do not have content. We must check.
if (httpResponse.getEntity() != null) {
responseContents = entityToBytes(httpResponse.getEntity());
} else {
// Add 0 byte response as a way of honestly representing a
// no-content request.
responseContents = new byte[0];
}
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}
return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
SystemClock.elapsedRealtime() - requestStart);
} 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, SystemClock.elapsedRealtime() - requestStart);
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);
}
}
}
}
这个方法是网络请求的具体实现，也是一个大while循环，其中mHttpStack.performRequest(request, headers);代码中的mHttpStack是Volley的newRequestQueue()方法中创建的实例，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，然后把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，同时将数据写入到缓存，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。
前面你可以看到在NetWorkDispatcher的run中最后执行了`mDelivery.postResponse(request, response);`，也就是说在解析完了NetworkResponse中的数据之后，又会调用ExecutorDelivery（ResponseDelivery接口的实现类）的postResponse()方法来回调解析出的数据，具体代码如下所示：
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
这里可以看见在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：
/**
-
A Runnable used for delivering network responses to a listener on the
-
main thread.
*/
@SuppressWarnings(“rawtypes”)
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();
}
}
}
这段代码里的run方法中可以看到如下一部分细节：
// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
这段代码是最核心的，明显可以看到通过mRequest的deliverResponse或者deliverError将反馈发送到回调到UI线程。这也是你重写实现的接口方法。
[]( )再来整体看下
-------------------------------------------------------------------
现在是该回过头去看背景知识模块了，再看下那幅官方图，对比就明白咋回事了。结合上图和如上分析可以知道：
1. 当一个RequestQueue被成功申请后会开启一个CacheDispatcher和4个默认的NetworkDispatcher。
2. CacheDispatcher缓存调度器最为第一层缓冲，开始工作后阻塞的从缓存序列mCacheQueue中取得请求；对于已经取消的请求，标记为跳过并结束这个请求；新的或者过期的请求，直接放入mNetworkQueue中由N个NetworkDispatcher进行处理；已获得缓存信息（网络应答）却没有过期的请求，由Request的parseNetworkResponse进行解析，从而确定此应答是否成功。然后将请求和应答交由Delivery分发者进行处理，如果需要更新缓存那么该请求还会被放入mNetworkQueue中。
3. 将请求Request add到RequestQueue后对于不需要缓存的请求（需要额外设置，默认是需要缓存）直接丢入mNetworkQueue交给N个NetworkDispatcher处理；对于需要缓存的，新的请求加到mCacheQueue中给CacheDispatcher处理；需要缓存，但是缓存列表中已经存在了相同URL的请求，放在mWaitingQueue中做暂时处理，等待之前请求完毕后，再重新添加到mCacheQueue中。
4. 网络请求调度器NetworkDispatcher作为网络请求真实发生的地方，对消息交给BasicNetwork进行处理，同样的，请求和结果都交由Delivery分发者进行处理。
5. Delivery分发者实际上已经是对网络请求处理的最后一层了，在Delivery对请求处理之前，Request已经对网络应答进行过解析，此时应答成功与否已经设定；而后Delivery根据请求所获得的应答情况做不同处理；若应答成功，则触发deliverResponse方法，最终会触发开发者为Request设定的Listener；若应答失败，则触发deliverError方法，最终会触发开发者为Request设定的ErrorListener；处理完后，一个Request的生命周期就结束了，Delivery会调用Request的finish操作，将其从mRequestQueue中移除，与此同时，如果等待列表中存在相同URL的请求，则会将剩余的层级请求全部丢入mCacheQueue交由CacheDispatcher进行处理。
至此所有搞定。
PPPS一句：通过上面原理分析之后总结发现，推荐整个App全局持有一个RequestQueue的做法，这样会有相对比较高的性能效率。
文章转自：http://blog.youkuaiyun.com/yanbober/article/details/45307217
感谢博主的无私分享！
下面两篇文章也写得不错，也是剖析volley框架及源码
http://www.cnblogs.com/cpacm/p/4211719.html
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> **这份文档共498页，其中包括Java集合，并发编程，JVM，Dubbo,Redis，Spring全家桶，MySQL，Kafka等面试解析及知识点整理**
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句：通过上面原理分析之后总结发现，推荐整个App全局持有一个RequestQueue的做法，这样会有相对比较高的性能效率。
文章转自：http://blog.youkuaiyun.com/yanbober/article/details/45307217
感谢博主的无私分享！
下面两篇文章也写得不错，也是剖析volley框架及源码
http://www.cnblogs.com/cpacm/p/4211719.html
### 最后
**Java架构进阶面试及知识点文档笔记**
> **这份文档共498页，其中包括Java集合，并发编程，JVM，Dubbo,Redis，Spring全家桶，MySQL，Kafka等面试解析及知识点整理**
[外链图片转存中...(img-O4NKOVPK-1714504417948)]
**Java分布式高级面试问题解析文档**
**其中都是包括分布式的面试问题解析，内容有**分布式消息队列，Redis缓存，分库分表，微服务架构，分布式高可用，读写分离等等！
[外链图片转存中...(img-SUQpqLBH-1714504417949)]
**互联网Java程序员面试必备问题解析及文档学习笔记**
[外链图片转存中...(img-uy3A4OZb-1714504417949)]
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> **本文已被[CODING开源项目：【一线大厂Java面试题解析+核心总结学习笔记+最新讲解视频+实战项目源码】](https://bbs.youkuaiyun.com/topics/618154847)收录**
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