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Android网络编程（一）HTTP协议原理
Android网络编程（二）HttpClient与HttpURLConnection
Android网络编程（三）Volley用法全解析
Android网络编程（四）从源码解析volley
Android网络编程（五）OkHttp2.x用法全解析
Android网络编程（六）OkHttp3用法全解析
学会了OkHttp3的用法后，我们当然有必要来了解下OkHttp3的源码，当然现在网上的文章很多，我仍旧希望我这一系列文章篇是最简洁易懂的。
首先OKHttp3如何使用这里就不在赘述了，不明白的同学可以查看Android网络编程（五）OkHttp2.x用法全解析、
Android网络编程（六）OkHttp3用法全解析这两篇文章。当我们要请求网络的时候我们需要用OkHttpClient.newCall(request)进行execute或者enqueue操作，当我们调用newCall时：
@Override public Call newCall(Request request) {
return new RealCall(this, request);
}实际返回的是一个RealCall类，我们调用enqueue异步请求网络实际上是调用了RealCall的enqueue方法：
void enqueue(Callback responseCallback, boolean forWebSocket) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
client.dispatcher().enqueue(new AsyncCall(responseCallback, forWebSocket));
}可以看到最终的请求是dispatcher来完成的。
Dispatcher主要用于控制并发的请求，它主要维护了以下变量：
/** 最大并发请求数*/
private int maxRequests = 64;
/** 每个主机最大请求数*/
private int maxRequestsPerHost = 5;
/** 消费者线程池 */
private ExecutorService executorService;
/** 将要运行的异步请求队列 */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/**正在运行的异步请求队列 */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** 正在运行的同步请求队列 */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); public Dispatcher(ExecutorService executorService) {
this.executorService = executorService;
}
public Dispatcher() {
}
public synchronized ExecutorService executorService() {
if (executorService == null) {
executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
}
return executorService;
}Dispatcher有两个构造函数，可以使用自己设定线程池，如果没有设定线程池则会在请求网络前自己创建线程池，这个线程池类似于CachedThreadPool比较适合执行大量的耗时比较少的任务。不了解线程池的同学可以查看Android多线程(一)线程池这篇文章。其中用到了SynchronousQueue，不了解它的同学可以查看Java并发编程（六）阻塞队列这篇文章。
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}当正在运行的异步请求队列中的数量小于64并且正在运行的请求主机数小于5时则把请求加载到runningAsyncCalls中并在线程池中执行，否则就再入到readyAsyncCalls中进行缓存等待。
线程池中传进来的参数就是AsyncCall它是RealCall的内部类，内部也实现了execute方法：
@Override protected void execute() {
boolean signalledCallback = false;
try {
Response response = getResponseWithInterceptorChain(forWebSocket);
if (canceled) {
signalledCallback = true;
responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
} else {
signalledCallback = true;
responseCallback.onResponse(RealCall.this, response);
}
} catch (IOException e) {
if (signalledCallback) {
// Do not signal the callback twice!
logger.log(Level.INFO, "Callback failure for " + toLoggableString(), e);
} else {
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}首先我们来看看最后一行， 无论这个请求的结果如何都会执行client.dispatcher().finished(this)；
synchronized void finished(AsyncCall call) {
if (!runningAsyncCalls.remove(call)) throw new AssertionError("AsyncCall wasn‘t running!");
promoteCalls();
}finished方法将此次请求从runningAsyncCalls移除后还执行了promoteCalls方法：
private void promoteCalls() {
if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
AsyncCall call = i.next();
if (runningCallsForHost(call) < maxRequestsPerHost) {
i.remove();
runningAsyncCalls.add(call);
executorService().execute(call);
}
if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
}
}可以看到最关键的点就是会从readyAsyncCalls取出下一个请求，并加入runningAsyncCalls中并交由线程池处理。好了让我们再回到上面的AsyncCall的execute方法，我们会发getResponseWithInterceptorChain方法返回了Response，很明显这是在请求网络。
private Response getResponseWithInterceptorChain(boolean forWebSocket) throws IOException {
Interceptor.Chain chain = new ApplicationInterceptorChain(0, originalRequest, forWebSocket);
return chain.proceed(originalRequest);
}getResponseWithInterceptorChain方法，创建了ApplicationInterceptorChain，它是一个拦截器链，这个类也是RealCall的内部类，接下来执行了它的proceed方法：
@Override public Response proceed(Request request) throws IOException {
// If there‘s another interceptor in the chain, call that.
if (index < client.interceptors().size()) {
Interceptor.Chain chain = new ApplicationInterceptorChain(index + 1, request, forWebSocket);
//从拦截器列表取出拦截器
Interceptor interceptor = client.interceptors().get(index);
Response interceptedResponse = interceptor.intercept(chain);
if (interceptedResponse == null) {
throw new NullPointerException("application interceptor " + interceptor
+ " returned null");
}
return interceptedResponse;
}
// No more interceptors. Do HTTP.
return getResponse(request, forWebSocket);
}proceed方法每次从拦截器列表中取出拦截器，当存在多个拦截器时都会在第七行阻塞，并等待下一个拦截器的调用返回。下面分别以 拦截器链中有1个、2个拦截器的场景加以模拟：
拦截器主要用来观察，修改以及可能短路的请求输出和响应的回来。通常情况下拦截器用来添加，移除或者转换请求或者响应的头部信息。比如将域名替换为ip地址，将请求头中添加host属性，也可以添加我们应用中的一些公共参数，比如设备id、版本号等等。 不了解拦截器的可以查看Okhttp-wiki 之 Interceptors 拦截器这篇文章。
回到代码上来，我们看最后一行 return getResponse(request, forWebSocket)，如果没有更多的拦截器的话，就会执行网络请求，来看看getResponse方法做了些什么（RealCall.java）：
Response getResponse(Request request, boolean forWebSocket) throws IOException {
...省略
// Create the initial HTTP engine. Retries and redirects need new engine for each attempt.
engine = new HttpEngine(client, request, false, false, forWebSocket, null, null, null);
int followUpCount = 0;
while (true) {
if (canceled) {
engine.releaseStreamAllocation();
throw new IOException("Canceled");
}
boolean releaseConnection = true;
try {
engine.sendRequest();
engine.readResponse();
releaseConnection = false;
} catch (RequestException e) {
// The attempt to interpret the request failed. Give up.
throw e.getCause();
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
...省略
}
}getResponse方法比较长我省略了一些代码，可以看到创建了HttpEngine类并且调用HttpEngine的sendRequest方法和readResponse方法。
我们先来看看sendRequest方法：
public void sendRequest() throws RequestException, RouteException, IOException {
if (cacheStrategy != null) return; // Already sent.
if (httpStream != null) throw new IllegalStateException();
//请求头部添加
Request request = networkRequest(userRequest);
//获取client中的Cache,同时Cache在初始化的时候会去读取缓存目录中关于曾经请求过的所有信息。
InternalCache responseCache = Internal.instance.internalCache(client);
//cacheCandidate为上次与服务器交互缓存的Response
Response cacheCandidate = responseCache != null
? responseCache.get(request)
: null;
long now = System.currentTimeMillis();
//创建CacheStrategy.Factory对象，进行缓存配置
cacheStrategy = new CacheStrategy.Factory(now, request, cacheCandidate).get();
//网络请求
networkRequest = cacheStrategy.networkRequest;
//缓存的响应
cacheResponse = cacheStrategy.cacheResponse;
if (responseCache != null) {
//记录当前请求是网络发起还是缓存发起
responseCache.trackResponse(cacheStrategy);
}
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn‘t applicable. Close it.
}
//不进行网络请求并且缓存不存在或者过期则返回504错误
if (networkRequest == null && cacheResponse == null) {
userResponse = new Response.Builder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.protocol(Protocol.HTTP_1_1)
.code(504)
.message("Unsatisfiable Request (only-if-cached)")
.body(EMPTY_BODY)
.build();
return;
}
// 不进行网络请求，而且缓存可以使用，直接返回缓存
if (networkRequest == null) {
userResponse = cacheResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.cacheResponse(stripBody(cacheResponse))
.build();
userResponse = unzip(userResponse);
return;
}
//需要访问网络时
boolean success = false;
try {
httpStream = connect();
httpStream.setHttpEngine(this);
if (writeRequestHeadersEagerly()) {
long contentLength = OkHeaders.contentLength(request);
if (bufferRequestBody) {
if (contentLength > Integer.MAX_VALUE) {
throw new IllegalStateException("Use setFixedLengthStreamingMode() or "
+ "setChunkedStreamingMode() for requests larger than 2 GiB.");
}
if (contentLength != -1) {
// Buffer a request body of a known length.
httpStream.writeRequestHeaders(networkRequest);
requestBodyOut = new RetryableSink((int) contentLength);
} else {
// Buffer a request body of an unknown length. Don‘t write request headers until the
// entire body is ready; otherwise we can‘t set the Content-Length header correctly.
requestBodyOut = new RetryableSink();
}
} else {
httpStream.writeRequestHeaders(networkRequest);
requestBodyOut = httpStream.createRequestBody(networkRequest, contentLength);
}
}
success = true;
} finally {
// If we‘re crashing on I/O or otherwise, don‘t leak the cache body.
if (!success && cacheCandidate != null) {
closeQuietly(cacheCandidate.body());
}
}
}上面的代码显然是在发送请求，但是最主要的是做了缓存的策略。cacheCandidate是上次与服务器交互缓存的Response，这里的缓存都是基于Map，key是请求中url的md5，value是在文件中查询到的缓存，页面置换基于LRU算法，我们现在只需要知道它是一个可以读取缓存Header的Response即可。根据cacheStrategy的处理得到了networkRequest和cacheResponse这两个值，根据这两个值的数据是否为null来进行进一步的处理，当networkRequest和cacheResponse都为null的情况也就是不进行网络请求并且缓存不存在或者过期，这时候则返回504错误；当networkRequest 为null时也就是不进行网络请求，而且缓存可以使用时则直接返回缓存；其他的情况则请求网络。
接下来我们查看readResponse方法：
public void readResponse() throws IOException {
...省略
else{
//读取网络响应
networkResponse = readNetworkResponse();
}
//将响应头部存入Cookie中
receiveHeaders(networkResponse.headers());
// If we have a cache response too, then we‘re doing a conditional get.
if (cacheResponse != null) {
//检查缓存是否可用，如果可用。那么就用当前缓存的Response，关闭网络连接，释放连接。
if (validate(cacheResponse, networkResponse)) {
userResponse = cacheResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
networkResponse.body().close();
releaseStreamAllocation();
// Update the cache after combining headers but before stripping the
// Content-Encoding header (as performed by initContentStream()).
InternalCache responseCache = Internal.instance.internalCache(client);
responseCache.trackConditionalCacheHit();
// 更新缓存
responseCache.update(cacheResponse, stripBody(userResponse));
userResponse = unzip(userResponse);
return;
} else {
closeQuietly(cacheResponse.body());
}
}
userResponse = networkResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (hasBody(userResponse)) {
maybeCache();
userResponse = unzip(cacheWritingResponse(storeRequest, userResponse));
}
}这个方法发起刷新请求头部和请求体，解析HTTP响应头部。如果有缓存并且可用则用缓存的数据并更新缓存，否则就用网络请求返回的数据。
我们再来看看validate(cacheResponse, networkResponse)方法是如何判断缓存是否可用的：
private static boolean validate(Response cached, Response network) {
//如果服务器返回304则缓存有效
if (network.code() == HTTP_NOT_MODIFIED) {
return true;
}
//通过缓存和网络请求响应中的Last-Modified来计算是否是最新数据，如果是则缓存有效
Date lastModified = cached.headers().getDate("Last-Modified");
if (lastModified != null) {
Date networkLastModified = network.headers().getDate("Last-Modified");
if (networkLastModified != null
&& networkLastModified.getTime() < lastModified.getTime()) {
return true;
}
}
return false;
}如缓存果过期或者强制放弃缓存，在此情况下，缓存策略全部交给服务器判断，客户端只用发送条件get请求即可，如果缓存是有效的，则返回304 Not Modifiled，否则直接返回body。条件get请求有两种方式一种是Last-Modified-Date，一种是 ETag。这里采用了Last-Modified-Date，通过缓存和网络请求响应中的Last-Modified来计算是否是最新数据，如果是则缓存有效。
最后我们再回到RealCall的getResponse方法：
Response getResponse(Request request, boolean forWebSocket) throws IOException {
...省略
boolean releaseConnection = true;
try {
engine.sendRequest();
engine.readResponse();
releaseConnection = false;
} catch (RequestException e) {
// The attempt to interpret the request failed. Give up.
throw e.getCause();
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
HttpEngine retryEngine = engine.recover(e.getLastConnectException(), null);
if (retryEngine != null) {
releaseConnection = false;
engine = retryEngine;
continue;
}
// Give up; recovery is not possible.
throw e.getLastConnectException();
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
HttpEngine retryEngine = engine.recover(e, null);
if (retryEngine != null) {
releaseConnection = false;
engine = retryEngine;
continue;
}
// Give up; recovery is not possible.
throw e;
} finally {
// We‘re throwing an unchecked exception. Release any resources.
if (releaseConnection) {
StreamAllocation streamAllocation = engine.close();
streamAllocation.release();
}
}
...省略
engine = new HttpEngine(client, request, false, false, forWebSocket, streamAllocation, null,
response);
}
}查看代码第11行和21行当发生IOException或者RouteException时会执行HttpEngine的recover方法：
public HttpEngine recover(IOException e, Sink requestBodyOut) {
if (!streamAllocation.recover(e, requestBodyOut)) {
return null;
}
if (!client.retryOnConnectionFailure()) {
return null;
}
StreamAllocation streamAllocation = close();
// For failure recovery, use the same route selector with a new connection.
return new HttpEngine(client, userRequest, bufferRequestBody, callerWritesRequestBody,
forWebSocket, streamAllocation, (RetryableSink) requestBodyOut, priorResponse);
}最后一行可以看到就是重新创建了HttpEngine并返回，用来完成重连。
到这里OkHttp请求网络的流程基本上讲完了，下面是关于OKHttp的请求流程图：
参考资料：
http://www.jianshu.com/p/aad5aacd79bf
http://www.jianshu.com/p/64e256c1dbbf
http://www.cnblogs.com/LuLei1990/p/5534791.html
http://frodoking.github.io/2015/03/12/android-okhttp/
Android网络编程（七）源码解析OkHttp前篇[请求网络]
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原文地址：http://blog.csdn.net/itachi85/article/details/52085256
