本文主要以源码形式解读OkHttp内部实现,源码基于okhttp:3.10.0。

同步请求

异步请求的例子

先看一个异步请求的例子：

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//(1) builder模式配置参数构建request对象
 Request request = new Request.Builder()
                .url("http://baidu.com?key=values")
                .get()
                .build();
            //(2)builder构建OkHttpClient对象
        new OkHttpClient.Builder().build()
                .newCall(request)//(3)request入参返回RealCall
                .enqueue(new okhttp3.Callback() {//(4)请求回调
                    @Override
                    public void onFailure(Call call, IOException e) {
                        System.out.println(e.getMessage());
                    }

                    @Override
                    public void onResponse(Call call, Response response) throws IOException {
                        System.out.println("thread:"+Thread.currentThread());
                        if (response.isSuccessful()) {
                            ResponseBody responseBody = response.body();
                            data.setText(responseBody.string());
                        }

                    }
                });

上面是一个OkHttp异步请求的代码，先构建一个Request对象设置请求地址、请求方式、header以及非GET请求还可设置body，然后创建OkHttpClient对象调用newCall设置request对象得到RealCall，RealCall调用enqueue发起异步请求并设置请求回调完成了一个简单的异步请求，OkHttpClient在实际开发中需要单例，原因会在后面的内容中有答案。

（1）接下来先看下Request内部：

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/**
 * An HTTP request. Instances of this class are immutable if their {@link #body} is null or itself
 * immutable.
 */
public final class Request {
  final HttpUrl url;
  final String method;
  final Headers headers;
  final @Nullable RequestBody body;
  final Object tag;

  private volatile CacheControl cacheControl; // Lazily initialized.

  Request(Builder builder) {
    this.url = builder.url;
    this.method = builder.method;
    this.headers = builder.headers.build();
    this.body = builder.body;
    this.tag = builder.tag != null ? builder.tag : this;
  }
    /**
     * Attaches {@code tag} to the request. It can be used later to cancel the request. If the tag
     * is unspecified or null, the request is canceled by using the request itself as the tag.
     */
    public Builder tag(Object tag) {
      this.tag = tag;
      return this;
    }

    public Request build() {
      if (url == null) throw new IllegalStateException("url == null");
      return new Request(this);
    }
  }
}

Request包含请求的参数url请求地址、method请求方法、header请求头数据、请求body以及tag标签。

（2）OkHttpClient主要暴露给外部调用,OkHttpClient对象的创建也通过builder模式，这里主要关注它的构造方法：

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OkHttpClient(Builder builder) {
    this.dispatcher = builder.dispatcher;//（1）分发器
    this.proxy = builder.proxy;//（2）代理类
    this.protocols = builder.protocols;//（3）协议类
    this.connectionSpecs = builder.connectionSpecs;//（4）连接规模 确定TLS版本和密码套件
    this.interceptors = Util.immutableList(builder.interceptors);//（5）自定义应用拦截器
    this.networkInterceptors = Util.immutableList(builder.networkInterceptors);//（6）自定义网络拦截器
    this.eventListenerFactory = builder.eventListenerFactory;//（7）事件监听工厂
    this.proxySelector = builder.proxySelector;//（8）代理选择器
    this.cookieJar = builder.cookieJar;
    this.cache = builder.cache;//（9）缓冲类
    this.internalCache = builder.internalCache;//（10）
    this.socketFactory = builder.socketFactory;//（11）socket工厂
    
     boolean isTLS = false;
    for (ConnectionSpec spec : connectionSpecs) {
      isTLS = isTLS || spec.isTls();
    }

    if (builder.sslSocketFactory != null || !isTLS) {
      this.sslSocketFactory = builder.sslSocketFactory;
      this.certificateChainCleaner = builder.certificateChainCleaner;
    } else {
      X509TrustManager trustManager = systemDefaultTrustManager();
      this.sslSocketFactory = systemDefaultSslSocketFactory(trustManager);
      this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
    }

    this.hostnameVerifier = builder.hostnameVerifier;
    this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
        certificateChainCleaner);
    this.proxyAuthenticator = builder.proxyAuthenticator;
    this.authenticator = builder.authenticator;
    this.connectionPool = builder.connectionPool;//（12）连接池
    this.dns = builder.dns;
    this.followSslRedirects = builder.followSslRedirects;
    this.followRedirects = builder.followRedirects;
    this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
    this.connectTimeout = builder.connectTimeout;
    this.readTimeout = builder.readTimeout;
    this.writeTimeout = builder.writeTimeout;
  }

（12）OkHttpClient中创建了连接池，还维护了线程池（dispatcher中）和响应缓冲，所以在使用过程中要用单例。

（3）接着调用了OkHttpClient.newCall(req):

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/**
  * Prepares the {@code request} to be executed at some point in the future.
  */
 @Override public Call newCall(Request request) {
   return RealCall.newRealCall(this, request, false /* for web socket */);
 }

newCall方法内部调用RealCall.newRealCall方法并返回Call对象： 

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static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    // Safely publish the Call instance to the EventListener.
    RealCall call = new RealCall(client, originalRequest, forWebSocket);//(1)
    call.eventListener = client.eventListenerFactory().create(call);//(2)
    return call;
  }

(1)创建RealCall对象；RealCall对象持有OkHttpClient和Request，构造方法中还创建了重试/重定向拦截器RetryAndFollowInterceptor；
(2)从OkHttpClient得到evenListener对象，

（4）调用异步请求RealCall.enqueue方法:

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@Override public void enqueue(Callback responseCallback) {
   synchronized (this) {
     if (executed) throw new IllegalStateException("Already Executed");（1）
     executed = true;
   }
   captureCallStackTrace();（2）
   eventListener.callStart(this);（3）
   client.dispatcher().enqueue(new AsyncCall(responseCallback));//（4）
 }

（1）如果call已经被执行，抛出异常；
（2） 捕获RealCall类的栈轨迹；
（3）触发监听方法callStart(),表示请求开始；
（4）执行dispatcher分发器enqueue方法，创建了AsyncCall类，AsyncCall传入responseCallback。

下面查看dispatcher的enqueue方法： 

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synchronized void enqueue(AsyncCall call) {
   if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {//（1）
     runningAsyncCalls.add(call);//（2）
     executorService().execute(call);//（3）
   } else {
     readyAsyncCalls.add(call);//（4）
   }
 }

（1）把call添加到正在运行的队列的判断依据：如果正在执行的异步请求数小于最大请求数（默认64），并且同一个主机执行的异步请求小于单个主机运行的最大请求数（默认5）否则添加到准备队列；
（2）根据（1）把call添加到正在运行的队列；
（3）将call交线程池执行；
（4）不满足（1）把call添加到准备队列。
AsyncCall是Runnable实现类，execute方法完成请求和返回的执行。
AsyncCall. execute(): 

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@Override protected void execute() {
     boolean signalledCallback = false;
     try {
       Response response = getResponseWithInterceptorChain();//（1）
       if (retryAndFollowUpInterceptor.isCanceled()) {//（2）
         signalledCallback = true;
         responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
       } else {
         signalledCallback = true;
         responseCallback.onResponse(RealCall.this, response);//（3）
       }
     } catch (IOException e) {
       if (signalledCallback) {
         // Do not signal the callback twice!
         Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
       } else {
         eventListener.callFailed(RealCall.this, e);//（4）
         responseCallback.onFailure(RealCall.this, e);//（5）
       }
     } finally {
       client.dispatcher().finished(this);//（6）
     }
   }

（1）getResponseWithInterceptorChain方法得到response对象，getResponseWithInterceptorChain是核心实现，后边专门展开说明；
（2）请求如果取消返回，调用responseCallback.onFailure通知处理请求失败；
（3）否则正常请求返回，调用responseCallback.onResponse返回response对象，调用方就可以拿到请求的数据返回，做具体业务处理；
（4）（5）回调执行eventListener.callFailed和responseCallback.onFailure；
（6）dispatcher执行finished方法，finish内部会调用promoteCalls方法从readyAsyncCalls队列中取出call 添加到runningAsyncCalls中，executorService().execute(call)加入线程池中执行call。添加到runningAsyncCalls中的条件是小于运行runningAsyncCalls最大call数并且同一主机call数小于maxRequestsPerHost（即同一主机最大请求数）。

  • 接下来分析核心方法getResponseWithInterceptorChain(): 
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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 (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());//(6)
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));//(7)

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);//(8)
  }

（1）构建全部拦截器list，先添加自定义应用层拦截器；
（2）添加重试/重定向拦截器；
（3）添加桥接拦截器；
（4）添加缓冲拦截器；
（5）添加连接拦截器；
（6）如果不是websocket，添加自定义网络拦截器；
（7）添加请求服务拦截器；
（8）传入拦截器list，请求，call对象，事件监听，连接超时时间以及读写超时时间生成Interceptor.Chain链对象，执行chain.proceed(originalRequest)。

chain.proceed(originalRequest)：

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public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection 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.
    if (this.httpCodec != null && !this.connection.supportsUrl(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().
    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.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);//(1)

    // 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");
    }

    if (response.body() == null) {
      throw new IllegalStateException(
          "interceptor " + interceptor + " returned a response with no body");
    }

    return response;
  }

(1)创建下一个RealInterceptorChain，将chain传入interceptor.intercept方法，intercept中会执行nextchain.proceed方法，然后再继续创建下一个RealInterceptorChain，intercept再执行下一个nextchain.proceed方法，这样循环调用所有拦截器，到最后一个拦截器CallServerInterceptor停止遍历，返回response，遍历循环流程如下图：

接下来阅读自带的拦截器代码：

  • RetryAndFollowUpInterceptor： 
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@Override public Response intercept(Chain chain) throws IOException {
  Request request = chain.request();
  RealInterceptorChain realChain = (RealInterceptorChain) chain;
  Call call = realChain.call();
  EventListener eventListener = realChain.eventListener();

  StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
      createAddress(request.url()), call, eventListener, callStackTrace);//（1）
  this.streamAllocation = streamAllocation;

  int followUpCount = 0;
  Response priorResponse = null;
  while (true) {
    if (canceled) {
      streamAllocation.release();//（2）
      throw new IOException("Canceled");
    }

    Response response;
    boolean releaseConnection = true;
    try {
      response = realChain.proceed(request, streamAllocation, null, null);//（3）
      releaseConnection = false;
    } catch (RouteException e) {
      // The attempt to connect via a route failed. The request will not have been sent.
      if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
        throw e.getLastConnectException();//（4）
      }
      releaseConnection = false;
      continue;
    } catch (IOException e) {
      // An attempt to communicate with a server failed. The request may have been sent.
      boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
      if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
      releaseConnection = false;//（5）
      continue;
    } finally {
      // We're throwing an unchecked exception. Release any resources.
      if (releaseConnection) {//（6）
        streamAllocation.streamFailed(null);
        streamAllocation.release();
      }
    }

    // Attach the prior response if it exists. Such responses never have a body.
    if (priorResponse != null) {//（7）
      response = response.newBuilder()
          .priorResponse(priorResponse.newBuilder()
                  .body(null)
                  .build())
          .build();
    }

    Request followUp = followUpRequest(response, streamAllocation.route());//（8）

    if (followUp == null) {//（9）
      if (!forWebSocket) {
        streamAllocation.release();
      }
      return response;
    }

    closeQuietly(response.body());//（10）

    if (++followUpCount > MAX_FOLLOW_UPS) {//（11）
      streamAllocation.release();
      throw new ProtocolException("Too many follow-up requests: " + followUpCount);
    }

    if (followUp.body() instanceof UnrepeatableRequestBody) {//（12）
      streamAllocation.release();
      throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
    }

    if (!sameConnection(response, followUp.url())) {//（13）
      streamAllocation.release();
      streamAllocation = new StreamAllocation(client.connectionPool(),
          createAddress(followUp.url()), call, eventListener, callStackTrace);
      this.streamAllocation = streamAllocation;
    } else if (streamAllocation.codec() != null) {//（14）
      throw new IllegalStateException("Closing the body of " + response
          + " didn't close its backing stream. Bad interceptor?");
    }

    request = followUp;//（15）
    priorResponse = response;//（16）
  }
}

（1）StreamAllocation用来协调连接（Connections）、流（Screams）和请求队列（Calls）；
（2）如果请求取消，调用screamAllocation.release()。release方法会关闭socket，并回调 eventListener.connectionReleased。
（3）执行realChain.proceed方法，proceed内部会创建下一个chain，再传入下一个拦截器，拦截器intercept继续执行负责的工作，并调用chain.proceed()。
（4）如果realChain.proceed方法抛出RouteException，那么调用recover方法，recover方法返回false则不会重试连接，抛出IOException异常，异常会在Call.execute方法中捕获执行eventListener.callFailed和onFailure方法，返回false的条件如下：

1.应用层禁止重试 ；
2.定义了不可重复发送的请求body ；
3.捕获的异常严重等级属于致命 ；
4.没有更多的路由可重意重试；

如果上述的四种场景，请求会被发起重试。

（5）IOException,同样调用recover方法，按照（4）中逻辑判断是否重连；
（6）如果抛出没有catch的异常则执行StreamAllocation.screamFailed()和StreamAllocation.screamFailed；
（7）priorResponse是先前得到的响应数据，如果已经先前响应不为空，response会结合先前响应；
（8）根据响应码确认请求是否需要重定向，返回null表示不需要；
（9）不需要重定向就streamAllocation.release()释放连接并返回response，否则执行下面逻辑；
（10）释放response.body对象；
（11）当前重定向数大于最大可重定向数，则释放连接，抛出异常；
（12）请求不允许重复连接，则释放连接，抛出异常；
（13）检查是否是相同的连接，不是就释放当前连接，重新创建ScreamAllocation；
（14）codec为空抛出异常；
（15）重定向request赋值request，准备执行while循环；
（16）保存当前的response。

  • BridgeInterceptor： 
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@Override public Response intercept(Chain chain) throws IOException {
   Request userRequest = chain.request();
   Request.Builder requestBuilder = userRequest.newBuilder();

   RequestBody body = userRequest.body();
   if (body != null) {
     MediaType contentType = body.contentType();
     if (contentType != null) {
       requestBuilder.header("Content-Type", contentType.toString());//（1）
     }

     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.
   boolean transparentGzip = false;
   if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == 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());

   HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

   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);
     String contentType = networkResponse.header("Content-Type");
     responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
   }

   return responseBuilder.build();
 }

桥接拦截器主要功能：
1.将应用码转为网络码；
2.用户请求转为网络请求

  • CacheInterceptor
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@Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

//（1）
    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {//（2）
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    // If we're forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {//（3）
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {//（4）
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());//（5）
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {//（6）
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .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()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

//（7）
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);//（8）
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

    return response;
  }

（1）获取缓冲策略；
（2）缓冲策略不为空，缓冲响应为空时，关闭缓冲策略；
（3）网络被禁止，缓冲不存在时，返回失败；
（4）不需要网络，返回缓冲响应，缓冲生效；
（5）执行chain.proceed方法抛出异常时，关闭缓冲；
（6）有缓冲时根据条件使用缓冲响应；
（7）使用网络响应；
（8）给予本请求缓冲（添加到缓冲中）；

  • ConnectInterceptor
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@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, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();//（1）

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }

（1）获得RealConnection对象，调用下一个chain.proceed。

  • CallServerInterceptor
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 @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();

    realChain.eventListener().requestHeadersStart(realChain.call());
    httpCodec.writeRequestHeaders(request);//(1)
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);

    Response.Builder responseBuilder = null;
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
      // Continue" response before transmitting the request body. If we don't get that, return
      // what we did get (such as a 4xx response) without ever transmitting the request body.
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        httpCodec.flushRequest();
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

        request.body().writeTo(bufferedRequestBody);//（2）
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } else if (!connection.isMultiplexed()) {
        // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
        // from being reused. Otherwise we're still obligated to transmit the request body to
        // leave the connection in a consistent state.
        streamAllocation.noNewStreams();
      }
    }

    httpCodec.finishRequest();

    if (responseBuilder == null) {
      realChain.eventListener().responseHeadersStart(realChain.call());
      responseBuilder = httpCodec.readResponseHeaders(false);//（3）
    }

    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
      // server sent a 100-continue even though we did not request one.
      // try again to read the actual response
      responseBuilder = httpCodec.readResponseHeaders(false);

      response = responseBuilder
              .request(request)
              .handshake(streamAllocation.connection().handshake())
              .sentRequestAtMillis(sentRequestMillis)
              .receivedResponseAtMillis(System.currentTimeMillis())
              .build();

      code = response.code();
    }

    realChain.eventListener()
            .responseHeadersEnd(realChain.call(), response);

//（4）
    if (forWebSocket && code == 101) {
      // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      streamAllocation.noNewStreams();
    }

    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }

    return response;
  }

（1）写入请求头数据；
（2）写入请求body数据；
（3）读取响应头数据；
（4）读取响应body数据。

完整异步请求调用流程：