<p>前言：对于 <em>OkHttp</em> 我接触的时间其实不太长，一直都是使用Retrofit + OkHttp 来做网络请求的，但是有同学说面试的时候可能会问框架源码，这样光是会用是不够的，于是便萌生了通一通OkHttp源码的念头。经过大约一周的时间，源码看了个大概（说来惭愧，也就知道里面的原理），这里变向大家介绍一下我的所得，希望对大家能有所帮助。这里推荐两篇博文： <a href="http://www.open-open.com/lib/view/open1462258981866.html" rel="nofollow,noindex">OkHttp 官方教程解析 - 彻底入门 OkHttp 使用</a> 和 <a href="/misc/goto?guid=4959677335833282607" rel="nofollow,noindex">拆轮子系列：拆 OkHttp</a> 前者能够让你入门OkHttp,后者能让你明白OkHttp的原理，我就是看的后者去看的源码，如果看我的不太懂，大家可以去看看上面的。同时，欢迎大家交流，提出意见，谢谢！</p>    <h2>总体流程</h2>    <p>下面的流程图是由上面的文章抄来的（自己画的图，用的visio）</p>    <p><em>整个流程是，通过 OkHttpClient 将构建的 Request 转换为Call，然后在RealCall中进行异步或同步任务，最后通过一些的拦截器 interceptor 发出网络请求和得到返回的 response 。 </em></p>    <p>将流程大概是这么个流程，大家可以有个大概的印象，继续向下看：</p>    <p><img src="https://simg.open-open.com/show/5e47411a9a996ae276342439cab19c3e.jpg"></p>    <p>OkHttp流程图.jpg</p>    <p>为了让大家有更深的印象，我准备追踪一个 GET 网络请求的具体流程，来介绍在源码中发生了什么。</p>    <h2>GET请求过程</h2>    <p>这是利用 OkHttp 写一个Get请求步骤，这里是一个同步的请求,异步的下面也会说：</p>    <pre>  <code class="language-java">//HTTP GET      public String get(String url) throws IOException {          //新建OKHttpClient客户端          OkHttpClient client = new OkHttpClient();          //新建一个Request对象          Request request = new Request.Builder()                  .url(url)                  .build();          //Response为OKHttp中的响应          Response response = client.newCall(request).execute();          if (response.isSuccessful()) {              return response.body().string();          }else{              throw new IOException("Unexpected code " + response);          }      }</code></pre>    <h2>OKHttpClient：流程的总控制者</h2>    <p><img src="https://simg.open-open.com/show/69bb681806bbce2bbe6000b45e444ef3.png"></p>    <p>OkHttpClient的类设计图</p>    <p>使用OkHttp的时候我们都会创建一个OkHttpClient对象:</p>    <p>OkHttpClient client = new OkHttpClient();</p>    <p>这是做什么的呢？看下builder里面的参数：</p>    <pre>  <code class="language-java">final Dispatcher dispatcher;  //分发器      final Proxy proxy;  //代理      final List<Protocol> protocols; //协议      final List<ConnectionSpec> connectionSpecs; //传输层版本和连接协议      final List<Interceptor> interceptors; //拦截器      final List<Interceptor> networkInterceptors; //网络拦截器      final ProxySelector proxySelector; //代理选择      final CookieJar cookieJar; //cookie      final Cache cache; //缓存      final InternalCache internalCache;  //内部缓存      final SocketFactory socketFactory;  //socket 工厂      final SSLSocketFactory sslSocketFactory; //安全套接层socket 工厂，用于HTTPS      final CertificateChainCleaner certificateChainCleaner; // 验证确认响应证书 适用 HTTPS 请求连接的主机名。      final HostnameVerifier hostnameVerifier;    //  主机名字确认      final CertificatePinner certificatePinner;  //  证书链      final Authenticator proxyAuthenticator;     //代理身份验证      final Authenticator authenticator;      // 本地身份验证      final ConnectionPool connectionPool;    //连接池,复用连接      final Dns dns;  //域名      final boolean followSslRedirects;  //安全套接层重定向      final boolean followRedirects;  //本地重定向      final boolean retryOnConnectionFailure; //重试连接失败      final int connectTimeout;    //连接超时      final int readTimeout; //read 超时      final int writeTimeout; //write 超时</code></pre>    <p>在这些声明的对象中可以看出来，几乎所有用到的类都和 OkHttpClient 有关系。事实上，你能够通过它来设置改变一些参数，因为他是通过 建造者模式 实现的，因此你可以通过 builder() 来设置。如果不进行设置，在 Builder 中就会使用默认的设置：</p>    <pre>  <code class="language-java">dispatcher = new Dispatcher();              protocols = DEFAULT_PROTOCOLS;              connectionSpecs = DEFAULT_CONNECTION_SPECS;              proxySelector = ProxySelector.getDefault();              cookieJar = CookieJar.NO_COOKIES;              socketFactory = SocketFactory.getDefault();              hostnameVerifier = OkHostnameVerifier.INSTANCE;              certificatePinner = CertificatePinner.DEFAULT;              proxyAuthenticator = Authenticator.NONE;              authenticator = Authenticator.NONE;              connectionPool = new ConnectionPool();              dns = Dns.SYSTEM;              followSslRedirects = true;              followRedirects = true;              retryOnConnectionFailure = true;              connectTimeout = 10_000;              readTimeout = 10_000;              writeTimeout = 10_000;</code></pre>    <p>看到这，如果你还不明白的话，也没关系，在 OkHttp 中只是设置用的的各个东西。真正的流程要从里面的 newCall() 方法中说起：</p>    <pre>  <code class="language-java">/**          *  Prepares the {@code request} to be executed at some point in the future.          *  准备将要被执行的request          */          @Override          public Call newCall(Request request) {              return new RealCall(this, request);          }</code></pre>    <p>当通过 建造者模式 创建了 Request 之后（这个没什么好说），紧接着就通过下面的代码来获得 Response</p>    <p>大家还记得上面做 GET 请求时的这句代码吧：</p>    <p>Response response = client.newCall(request).execute(); 这就代码就开启了整个GET请求的流程：</p>    <h2>RealCall：真正的请求执行者。</h2>    <p>先看一下他的构造方法：</p>    <pre>  <code class="language-java">protected RealCall(OkHttpClient client, Request originalRequest) {        this.client = client;          this.originalRequest = originalRequest;          this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client);  }</code></pre>    <p>可以看到他传过来一个 OkHttpClient 对象和一个 originalRequest （我们创建的 Request ）。</p>    <p>接下来看它的 execute() 方法：</p>    <pre>  <code class="language-java">@Override      public Response execute() throws IOException {          synchronized (this) {              if (executed) throw new IllegalStateException("Already Executed"); //(1)              executed = true;          }          try {              client.dispatcher.executed(this);//(2)              Response result = getResponseWithInterceptorChain();//(3)              if (result == null) throw new IOException("Canceled");              return result;          }finally {              client.dispatcher.finished(this);//(4)          }      }</code></pre>    <ol>     <li>检查这个 call 是否已经被执行了，每个 call 只能被执行一次，如果想要一个完全一样的 call ，可以利用 all#clone 方法进行克隆。</li>     <li>利用 client.dispatcher().executed(this) 来进行实际执行， dispatcher 是刚才看到的 OkHttpClient.Builder 的成员之一，它的文档说自己是异步 HTTP 请求的执行策略，现在看来，同步请求它也有掺和。</li>     <li>调用 getResponseWithInterceptorChain() 函数获取 HTTP 返回结果，从函数名可以看出，这一步还会进行一系列“拦截”操作。</li>     <li> <p>最后还要通知 dispatcher 自己已经执行完毕。</p> <p>dispatcher 这里我们不过度关注，在同步执行的流程中，涉及到 dispatcher 的内容只不过是告知它我们的执行状态，比如开始执行了（调用 executed ），比如执行完毕了（调用 finished ），在异步执行流程中它会有更多的参与。</p> <p>真正发出网络请求，解析返回结果的，还是 getResponseWithInterceptorChain ：</p> <pre>  <code class="language-java">//拦截器的责任链。   private Response getResponseWithInterceptorChain() throws IOException {       // Build a full stack of interceptors.       List<Interceptor> interceptors = new ArrayList<>();       interceptors.addAll(client.interceptors());     //(1)       interceptors.add(retryAndFollowUpInterceptor);    //(2)       interceptors.add(new BridgeInterceptor(client.cookieJar()));    //(3)       interceptors.add(new CacheInterceptor(client.internalCache()));    //(4)       interceptors.add(new ConnectInterceptor(client));    //(5)       if (!retryAndFollowUpInterceptor.isForWebSocket()) {           interceptors.addAll(client.networkInterceptors());    //(6)       }       interceptors.add(new CallServerInterceptor(               retryAndFollowUpInterceptor.isForWebSocket()));     //(7)         Interceptor.Chain chain = new RealInterceptorChain(               interceptors, null, null, null, 0, originalRequest);       return chain.proceed(originalRequest); //  <<=========开始链式调用   }</code></pre> </li>    </ol>    <ol>     <li>在配置 OkHttpClient 时设置的 interceptors ；</li>     <li>负责失败重试以及重定向的 RetryAndFollowUpInterceptor ；</li>     <li>负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 BridgeInterceptor ；</li>     <li>负责读取缓存直接返回、更新缓存的 CacheInterceptor ；</li>     <li>负责和服务器建立连接的 ConnectInterceptor ；</li>     <li>配置 OkHttpClient 时设置的 networkInterceptors ；</li>     <li>负责向服务器发送请求数据、从服务器读取响应数据的 CallServerInterceptor 。</li>     <li>在 return chain.proceed(originalRequest); 中开启链式调用：</li>    </ol>    <h2>RealInterceptorChain</h2>    <pre>  <code class="language-java">public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,        Connection connection) throws IOException {      if (index >= interceptors.size()) throw new AssertionError();        calls++;        // If we already have a stream, confirm that the incoming request will use it.      //如果我们已经有一个stream。确定即将到来的request会使用它      if (this.httpCodec != null && !sameConnection(request.url())) {        throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)            + " must retain the same host and port");      }        // If we already have a stream, confirm that this is the only call to chain.proceed().      //如果我们已经有一个stream， 确定chain.proceed()唯一的call      if (this.httpCodec != null && calls > 1) {        throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)            + " must call proceed() exactly once");      }        // Call the next interceptor in the chain.      //调用链的下一个拦截器      RealInterceptorChain next = new RealInterceptorChain(          interceptors, streamAllocation, httpCodec, connection, index + 1, request);      Interceptor interceptor = interceptors.get(index);      Response response = interceptor.intercept(next);        // Confirm that the next interceptor made its required call to chain.proceed().      if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {        throw new IllegalStateException("network interceptor " + interceptor            + " must call proceed() exactly once");      }        // Confirm that the intercepted response isn't null.      if (response == null) {        throw new NullPointerException("interceptor " + interceptor + " returned null");      }        return response;    }  `</code></pre>    <p>代码很多，但是主要是进行一些判断，主要的代码在这：</p>    <pre>  <code class="language-java">// Call the next interceptor in the chain.      //调用链的下一个拦截器      RealInterceptorChain next = new RealInterceptorChain(          interceptors, streamAllocation, httpCodec, connection, index + 1, request);    //(1)      Interceptor interceptor = interceptors.get(index);     //(2)      Response response = interceptor.intercept(next);    //(3)</code></pre>    <ol>     <li>实例化下一个拦截器对应的 RealIterceptorChain 对象，这个对象会在传递给当前的拦截器</li>     <li>得到当前的拦截器： interceptors 是存放拦截器的 ArryList</li>     <li>调用当前拦截器的 intercept() 方法，并将下一个拦截器的 RealIterceptorChain 对象传递下去<br> 除了在client中自己设置的 interceptor ,第一个调用的就是 retryAndFollowUpInterceptor <h2>RetryAndFollowUpInterceptor:负责失败重试以及重定向</h2> 直接上代码</li>    </ol>    <pre>  <code class="language-java">@Override   public Response intercept(Chain chain) throws IOException {          Request request = chain.request();          streamAllocation = new StreamAllocation(                  client.connectionPool(), createAddress(request.url()));          int followUpCount = 0;          Response priorResponse = null;          while (true) {              if (canceled) {                  streamAllocation.release();                  throw new IOException("Canceled");              }                Response response = null;              boolean releaseConnection = true;              try {                  response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);    //(1)                  releaseConnection = false;              } catch (RouteException e) {                  // The attempt to connect via a route failed. The request will not have been sent.                  //通过路线连接失败，请求将不会再发送                  if (!recover(e.getLastConnectException(), true, request)) throw e.getLastConnectException();                  releaseConnection = false;                  continue;              } catch (IOException e) {                  // An attempt to communicate with a server failed. The request may have been sent.                  // 与服务器尝试通信失败，请求不会再发送。                  if (!recover(e, false, request)) throw e;                  releaseConnection = false;                  continue;              } finally {                  // We're throwing an unchecked exception. Release any resources.                  //抛出未检查的异常，释放资源                  if (releaseConnection) {                      streamAllocation.streamFailed(null);                      streamAllocation.release();                  }              }                // Attach the prior response if it exists. Such responses never have a body.              // 附加上先前存在的response。这样的response从来没有body              // TODO: 2016/8/23 这里没赋值，岂不是一直为空？              if (priorResponse != null) { //  (2)                  response = response.newBuilder()                          .priorResponse(priorResponse.newBuilder()                                  .body(null)                                  .build())                          .build();              }                Request followUp = followUpRequest(response); //判断状态码 (3)              if (followUp == null){                  if (!forWebSocket) {                      streamAllocation.release();                  }                  return response;              }                closeQuietly(response.body());                if (++followUpCount > MAX_FOLLOW_UPS) {                  streamAllocation.release();                  throw new ProtocolException("Too many follow-up requests: " + followUpCount);              }                if (followUp.body() instanceof UnrepeatableRequestBody) {                  throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());              }                if (!sameConnection(response, followUp.url())) {                  streamAllocation.release();                  streamAllocation = new StreamAllocation(                          client.connectionPool(), createAddress(followUp.url()));              } else if (streamAllocation.codec() != null) {                  throw new IllegalStateException("Closing the body of " + response                          + " didn't close its backing stream. Bad interceptor?");              }                request = followUp;              priorResponse = response;          }      }</code></pre>    <ol>     <li>这里是最关键的代码，可以看出在 response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null); 中直接调用了下一个拦截器，然后捕获可能的异常来进行操作</li>     <li>这里没看太懂，有点坑，以后补</li>     <li>这里对于返回的response的状态码进行判断，然后进行处理</li>    </ol>    <h2>BridgeInterceptor：</h2>    <p>负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 。</p>    <pre>  <code class="language-java">@Override   public Response intercept(Chain chain) throws IOException {      Request userRequest = chain.request();      Request.Builder requestBuilder = userRequest.newBuilder();        //检查request。将用户的request转换为发送到server的请求      RequestBody body = userRequest.body();     //(1)      if (body != null) {        MediaType contentType = body.contentType();        if (contentType != null) {          requestBuilder.header("Content-Type", contentType.toString());        }          long contentLength = body.contentLength();        if (contentLength != -1) {          requestBuilder.header("Content-Length", Long.toString(contentLength));          requestBuilder.removeHeader("Transfer-Encoding");        } else {          requestBuilder.header("Transfer-Encoding", "chunked");          requestBuilder.removeHeader("Content-Length");        }      }        if (userRequest.header("Host") == null) {        requestBuilder.header("Host", hostHeader(userRequest.url(), false));      }        if (userRequest.header("Connection") == null) {        requestBuilder.header("Connection", "Keep-Alive");      }        // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing      // the transfer stream.      //GZIP压缩      boolean transparentGzip = false;      if (userRequest.header("Accept-Encoding") == null) {        transparentGzip = true;        requestBuilder.header("Accept-Encoding", "gzip");      }        List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());      if (!cookies.isEmpty()) {        requestBuilder.header("Cookie", cookieHeader(cookies));      }        if (userRequest.header("User-Agent") == null) {        requestBuilder.header("User-Agent", Version.userAgent());      }        Response networkResponse = chain.proceed(requestBuilder.build());   //(2)        HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers()); //(3)        Response.Builder responseBuilder = networkResponse.newBuilder()          .request(userRequest);        if (transparentGzip          && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))          && HttpHeaders.hasBody(networkResponse)) {        GzipSource responseBody = new GzipSource(networkResponse.body().source());        Headers strippedHeaders = networkResponse.headers().newBuilder()            .removeAll("Content-Encoding")            .removeAll("Content-Length")            .build();        responseBuilder.headers(strippedHeaders);        responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));      }        return responseBuilder.build();    }</code></pre>    <ol>     <li>在（1）和（2）之间， BridgeInterceptor 对于 request 的格式进行检查，让构建了一个新的 request</li>     <li>调用下一个 interceptor 来得到response</li>     <li>（3）下面就是对得到的response进行一些判断操作，最后将结果返回。</li>    </ol>    <pre>  <code class="language-java">@Override   public Response intercept(Chain chain) throws IOException {      Response cacheCandidate = cache != null        //=============(1)          ? cache.get(chain.request()) //通过request得到缓存          : null;        long now = System.currentTimeMillis();        CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); //根据request来得到缓存策略===========(2)      Request networkRequest = strategy.networkRequest;      Response cacheResponse = strategy.cacheResponse;        if (cache != null) {        cache.trackResponse(strategy);      }        if (cacheCandidate != null && cacheResponse == null) { //存在缓存的response，但是不允许缓存        closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it. 缓存不适合，关闭      }        // If we're forbidden from using the network and the cache is insufficient, fail.        //如果我们禁止使用网络，且缓存为null，失败      if (networkRequest == null && cacheResponse == null) {        return new Response.Builder()            .request(chain.request())            .protocol(Protocol.HTTP_1_1)            .code(504)            .message("Unsatisfiable Request (only-if-cached)")            .body(EMPTY_BODY)            .sentRequestAtMillis(-1L)            .receivedResponseAtMillis(System.currentTimeMillis())            .build();      }        // If we don't need the network, we're done.      if (networkRequest == null) {  //没有网络请求，跳过网络，返回缓存        return cacheResponse.newBuilder()            .cacheResponse(stripBody(cacheResponse))            .build();      }        Response networkResponse = null;      try {        networkResponse = chain.proceed(networkRequest);//网络请求拦截器    //======(3)      } finally {        // If we're crashing on I/O or otherwise, don't leak the cache body.          //如果我们因为I/O或其他原因崩溃，不要泄漏缓存体        if (networkResponse == null && cacheCandidate != null) {          closeQuietly(cacheCandidate.body());        }      }        // If we have a cache response too, then we're doing a conditional get.========(4)        //如果我们有一个缓存的response，然后我们正在做一个条件GET      if (cacheResponse != null) {        if (validate(cacheResponse, networkResponse)) { //比较确定缓存response可用          Response response = cacheResponse.newBuilder()              .headers(combine(cacheResponse.headers(), networkResponse.headers()))              .cacheResponse(stripBody(cacheResponse))              .networkResponse(stripBody(networkResponse))              .build();          networkResponse.body().close();            // Update the cache after combining headers but before stripping the          // Content-Encoding header (as performed by initContentStream()).            //更新缓存，在剥离content-Encoding之前          cache.trackConditionalCacheHit();          cache.update(cacheResponse, response);          return response;        } else {          closeQuietly(cacheResponse.body());        }      }        Response response = networkResponse.newBuilder()          .cacheResponse(stripBody(cacheResponse))          .networkResponse(stripBody(networkResponse))          .build();        if (HttpHeaders.hasBody(response)) {    // =========(5)        CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);        response = cacheWritingResponse(cacheRequest, response);      }        return response;    }</code></pre>    <ol>     <li>首先，根据 request 来判断 cache 中是否有缓存的 response ，如果有，得到这个 response ，然后进行判断当前 response 是否有效，没有将 cacheCandate 赋值为空。</li>     <li>根据request判断缓存的策略，是否要使用了网络，缓存 或两者都使用</li>     <li>调用下一个拦截器，决定从网络上来得到 response</li>     <li>如果本地已经存在 cacheResponse ，那么让它和网络得到的 networkResponse 做比较，决定是否来更新缓存的 cacheResponse</li>     <li> <p>缓存未经缓存过的 response</p> <h2>ConnectInterceptor:建立连接</h2> <pre>  <code class="language-java">@Override   public Response intercept(Chain chain) throws IOException {      RealInterceptorChain realChain = (RealInterceptorChain) chain;      Request request = realChain.request();      StreamAllocation streamAllocation = realChain.streamAllocation();        // We need the network to satisfy this request. Possibly for validating a conditional GET.      boolean doExtensiveHealthChecks = !request.method().equals("GET");      HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);      RealConnection connection = streamAllocation.connection();        return realChain.proceed(request, streamAllocation, httpCodec, connection);  }</code></pre> </li>    </ol>    <p>实际上建立连接就是创建了一个 HttpCodec 对象，它将在后面的步骤中被使用，那它又是何方神圣呢？它是对 HTTP 协议操作的抽象，有两个实现： Http1Codec 和 Http2Codec ，顾名思义，它们分别对应 HTTP/1.1 和 HTTP/2 版本的实现。</p>    <p>在Http1Codec中，它利用 <a href="/misc/goto?guid=4959677335926595225" rel="nofollow,noindex">Okio</a> 对Socket的读写操作进行封装， Okio 以后有机会再进行分析，现在让我们对它们保持一个简单地认识：它对java.io和java.nio进行了封装，让我们更便捷高效的进行 IO 操作。</p>    <p>而创建 HttpCodec 对象的过程涉及到 StreamAllocation、RealConnection ，代码较长，这里就不展开，这个过程概括来说，就是找到一个可用的 RealConnection ，再利用 RealConnection 的输入输出（ BufferedSource 和 BufferedSink ）创建 HttpCodec 对象，供后续步骤使用。</p>    <h2>NetworkInterceptors</h2>    <p>配置OkHttpClient时设置的 NetworkInterceptors。</p>    <h2>CallServerInterceptor：发送和接收数据</h2>    <pre>  <code class="language-java">@Override public Response intercept(Chain chain) throws IOException {      HttpCodec httpCodec = ((RealInterceptorChain) chain).httpStream();      StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();      Request request = chain.request();        long sentRequestMillis = System.currentTimeMillis();      httpCodec.writeRequestHeaders(request);        if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {   //===(1)        Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);        request.body().writeTo(bufferedRequestBody);        bufferedRequestBody.close();      }        httpCodec.finishRequest();        Response response = httpCodec.readResponseHeaders()     //====(2)          .request(request)          .handshake(streamAllocation.connection().handshake())          .sentRequestAtMillis(sentRequestMillis)          .receivedResponseAtMillis(System.currentTimeMillis())          .build();        if (!forWebSocket || response.code() != 101) {        response = response.newBuilder()            .body(httpCodec.openResponseBody(response))            .build();      }        if ("close".equalsIgnoreCase(response.request().header("Connection"))          || "close".equalsIgnoreCase(response.header("Connection"))) {        streamAllocation.noNewStreams();      }        int code = response.code();      if ((code == 204 || code == 205) && response.body().contentLength() > 0) {        throw new ProtocolException(            "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());      }        return response;    }</code></pre>    <ol>     <li>检查请求方法，用 Httpcodec 处理 request</li>     <li>进行网络请求得到 response</li>     <li>返回r esponse</li>    </ol>    <h3>总结</h3>    <p>前面说了拦截器用了 责任链设计模式 ,它将请求一层一层向下传，知道有一层能够得到Resposne就停止向下传递，然后将 response 向上面的拦截器传递，然后各个拦截器会对 respone 进行一些处理，最后会传到 RealCall 类中通过 execute 来得到 esponse 。</p>    <h2>异步请求的流程：</h2>    <p>异步get请求示例如下：</p>    <pre>  <code class="language-java">private final OkHttpClient client = new OkHttpClient();      public void run() throws Exception {      Request request = new Request.Builder()          .url("http://publicobject.com/helloworld.txt")          .build();        client.newCall(request).enqueue(new Callback() {        @Override         public void onFailure(Call call, IOException e) {          e.printStackTrace();        }          @Override         public void onResponse(Call call, Response response) throws IOException {          if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);            Headers responseHeaders = response.headers();          for (int i = 0, size = responseHeaders.size(); i < size; i++) {            System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));          }            System.out.println(response.body().string());        }      });    }</code></pre>    <p>由代码中 client.newCall(request).enqueue(Callback) ，开始我们知道 client.newCall(request) 方法返回的是 RealCall 对象，接下来继续向下看 enqueue() 方法:</p>    <pre>  <code class="language-java">//异步任务使用      @Override       public void enqueue(Callback responseCallback) {          synchronized (this) {              if (executed) throw new IllegalStateException("Already Executed");              executed = true;          }          client.dispatcher().enqueue(new AsyncCall(responseCallback));      }</code></pre>    <p>调用了上面我们没有详细说的 Dispatcher 类中的 enqueue(Call ) 方法.接着继续看：</p>    <pre>  <code class="language-java">synchronized void enqueue(AsyncCall call) {          if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {              runningAsyncCalls.add(call);              executorService().execute(call);          } else {              readyAsyncCalls.add(call);          }      }</code></pre>    <p>如果中的 runningAsynCalls 不满，且 call 占用的 host 小于最大数量，则将 call 加入到 runningAsyncCalls 中执行，同时利用线程池执行 call ；否者将 call 加入到 readyAsyncCalls 中。 runningAsyncCalls 和 readyAsyncCalls 是什么呢？看下面：</p>    <pre>  <code class="language-java">/** Ready async calls in the order they'll be run. */  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>(); //正在准备中的异步请求队列    /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>(); //运行中的异步请求    /** Running synchronous calls. Includes canceled calls that haven't finished yet. */  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); //同步请求</code></pre>    <p>call 加入到线程池中执行了。现在再看 AsynCall的 代码，它是 RealCall 中的内部类:</p>    <pre>  <code class="language-java">//异步请求      final class AsyncCall extends NamedRunnable {          private final Callback responseCallback;            private AsyncCall(Callback responseCallback) {              super("OkHttp %s", redactedUrl());              this.responseCallback = responseCallback;          }            String host() {              return originalRequest.url().host();          }            Request request() {              return originalRequest;          }            RealCall get() {              return RealCall.this;          }            @Override protected void execute() {              boolean signalledCallback = false;              try {                  Response response = getResponseWithInterceptorChain();                  if (retryAndFollowUpInterceptor.isCanceled()) {                      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!                      Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);                  } else {                      responseCallback.onFailure(RealCall.this, e);                  }              } finally {                  client.dispatcher().finished(this);              }          }      }</code></pre>    <p>AysncCall 中的 execute() 中的方法，同样是通过 Response response = getResponseWithInterceptorChain(); 来获得response，这样异步任务也同样通过了interceptor，剩下的流程就和上面一样了。</p>    <h3>结语：</h3>    <p>看到这，不知道你是否明白了OkHttp的请求过程，如果有什么问题或意见，欢迎私信。</p>    <p>参考</p>    <ol>     <li><a href="http://www.open-open.com/lib/view/open1462258981866.html" rel="nofollow,noindex">OkHttp 官方教程解析 - 彻底入门 OkHttp 使用</a></li>     <li><a href="/misc/goto?guid=4959677335833282607" rel="nofollow,noindex">拆轮子系列：拆 OkHttp</a></li>    </ol>    <p> </p>    <p>来自：http://www.jianshu.com/p/27c1554b7fee</p>    <p> </p>