你想要的系列：网络请求框架OkHttp3全解系列 - （四）拦截器详解2：连接、请求服务（重点）

//打开到目标服务的连接、处理下一个拦截器
public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;

  public ConnectInterceptor(OkHttpClient client) {
    this.client = client;
  }

  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    Transmitter transmitter = realChain.transmitter();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    Exchange exchange = transmitter.newExchange(chain, doExtensiveHealthChecks);

    return realChain.proceed(request, transmitter, exchange);
  }
}

  Exchange newExchange(Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
    synchronized (connectionPool) {
      if (noMoreExchanges) {
        throw new IllegalStateException("released");
      }
      if (exchange != null) {
        throw new IllegalStateException("cannot make a new request because the previous response "
            + "is still open: please call response.close()");
      }
    }

    ExchangeCodec codec = exchangeFinder.find(client, chain, doExtensiveHealthChecks);
    Exchange result = new Exchange(this, call, eventListener, exchangeFinder, codec);

    synchronized (connectionPool) {
      this.exchange = result;
      this.exchangeRequestDone = false;
      this.exchangeResponseDone = false;
      return result;
    }
  }

  public void prepareToConnect(Request request) {
    ...
    this.exchangeFinder = new ExchangeFinder(this, connectionPool, createAddress(request.url()),
        call, eventListener);
  }

  private Address createAddress(HttpUrl url) {
    SSLSocketFactory sslSocketFactory = null;
    HostnameVerifier hostnameVerifier = null;
    CertificatePinner certificatePinner = null;
    if (url.isHttps()) {
      sslSocketFactory = client.sslSocketFactory();
      hostnameVerifier = client.hostnameVerifier();
      certificatePinner = client.certificatePinner();
    }

    return new Address(url.host(), url.port(), client.dns(), client.socketFactory(),
        sslSocketFactory, hostnameVerifier, certificatePinner, client.proxyAuthenticator(),
        client.proxy(), client.protocols(), client.connectionSpecs(), client.proxySelector());
  }

  public ExchangeCodec find(
      OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
    int connectTimeout = chain.connectTimeoutMillis();
    int readTimeout = chain.readTimeoutMillis();
    int writeTimeout = chain.writeTimeoutMillis();
    int pingIntervalMillis = client.pingIntervalMillis();
    boolean connectionRetryEnabled = client.retryOnConnectionFailure();

    try {
      //找到一个健康的连接
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
      //利用连接实例化ExchangeCodec对象，如果是HTTP/2返回Http2ExchangeCodec，否则返回Http1ExchangeCodec
      return resultConnection.newCodec(client, chain);
    } catch (RouteException e) {
      trackFailure();
      throw e;
    } catch (IOException e) {
      trackFailure();
      throw new RouteException(e);
    }
  }

  private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
      int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
      boolean doExtensiveHealthChecks) throws IOException {
    while (true) {
      //找连接
      RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
          pingIntervalMillis, connectionRetryEnabled);

      // 是新连接 且不是HTTP2.0 就不用体检
      synchronized (connectionPool) {
        if (candidate.successCount == 0 && !candidate.isMultiplexed()) {
          return candidate;
        }
      }
      // 体检不健康，继续找
      if (!candidate.isHealthy(doExtensiveHealthChecks)) {
          //标记不可用
        candidate.noNewExchanges();
        continue;
      }

      return candidate;
    }
  }

  //为承载新的数据流 寻找 连接。寻找顺序是 已分配的连接、连接池、新建连接
  private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
      int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
    boolean foundPooledConnection = false;
    RealConnection result = null;
    Route selectedRoute = null;
    RealConnection releasedConnection;
    Socket toClose;
    synchronized (connectionPool) {
      //请求已被取消（Call的cancel方法->transmitter的cancel方法），抛异常
      if (transmitter.isCanceled()) throw new IOException("Canceled");
      hasStreamFailure = false; 

      // 尝试使用 已给数据流分配的连接.（例如重定向请求时，可以复用上次请求的连接）
      releasedConnection = transmitter.connection;
      //有已分配的连接，但已经被限制承载新的数据流，就尝试释放掉（如果连接上已没有数据流），并返回待关闭的socket。
      toClose = transmitter.connection != null && transmitter.connection.noNewExchanges
          ? transmitter.releaseConnectionNoEvents()
          : null;

      if (transmitter.connection != null) {
        // 不为空，说明上面没有释放掉，那么此连接可用
        result = transmitter.connection;
        releasedConnection = null;
      }

      if (result == null) {
        // 没有已分配的可用连接，就尝试从连接池获取。（连接池稍后详细讲解）
        if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, null, false)) {
          foundPooledConnection = true;
          result = transmitter.connection;
        } else if (nextRouteToTry != null) {
          selectedRoute = nextRouteToTry;//有可尝试的路由
          nextRouteToTry = null;
        } else if (retryCurrentRoute()) {
          selectedRoute = transmitter.connection.route();
        }
      }
    }
    closeQuietly(toClose);//（如果有）关闭待关闭的socket

    if (releasedConnection != null) {
      eventListener.connectionReleased(call, releasedConnection);//（如果有）回调连接释放事件
    }
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);//(如果有）回调（从连接池）获取连接事件
    }
    if (result != null) {
      // 如果有已分配可用连接 或 从连接池获取到连接，结束！  没有 就走下面的新建连接过程。
      return result;
    }

    // 如果需要路由信息，就获取。是阻塞操作
    boolean newRouteSelection = false;
    if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
      newRouteSelection = true;
      routeSelection = routeSelector.next();
    }

    List<Route> routes = null;
    synchronized (connectionPool) {
      if (transmitter.isCanceled()) throw new IOException("Canceled");

      if (newRouteSelection) {
        //现在有了IP地址，再次尝试从连接池获取。可能会因为连接合并而匹配。（这里传入了routes，上面的传的null）
        routes = routeSelection.getAll();
        if (connectionPool.transmitterAcquirePooledConnection(
            address, transmitter, routes, false)) {
          foundPooledConnection = true;
          result = transmitter.connection;
        }
      }
      //第二次连接池也没找到，就新建连接
      if (!foundPooledConnection) {
        if (selectedRoute == null) {
          selectedRoute = routeSelection.next();
        }

        // Create a connection and assign it to this allocation immediately. This makes it possible
        // for an asynchronous cancel() to interrupt the handshake we're about to do.
        result = new RealConnection(connectionPool, selectedRoute);
        connectingConnection = result;
      }
    }

    // 如果第二次从连接池的尝试成功了，结束，因为连接池中的连接是已经和服务器建立连接的
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
      return result;
    }

    // 第二次没成功，就把新建的连接，进行TCP + TLS 握手，与服务端建立连接. 是阻塞操作
    result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
        connectionRetryEnabled, call, eventListener);
    connectionPool.routeDatabase.connected(result.route());//从失败名单中移除

    Socket socket = null;
    synchronized (connectionPool) {
      connectingConnection = null;
      // 最后一次尝试从连接池获取，注意最后一个参数为true，即要求 多路复用（http2.0）
      //意思是，如果本次是http2.0，那么为了保证 多路复用性，（因为上面的握手操作不是线程安全）会再次确认连接池中此时是否已有同样连接
      if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, routes, true)) {
        // 如果获取到，就关闭我们创建里的连接，返回获取的连接
        result.noNewExchanges = true;
        socket = result.socket();
        result = transmitter.connection;

        // 那么这个刚刚连接成功的路由 就可以 用作下次 尝试的路由
        nextRouteToTry = selectedRoute;
      } else {
        //最后一次尝试也没有的话，就把刚刚新建的连接存入连接池
        connectionPool.put(result);
        transmitter.acquireConnectionNoEvents(result);//把连接赋给transmitter
      }
    }
    closeQuietly(socket);//如果刚刚建立的连接没用到，就关闭

    eventListener.connectionAcquired(call, result);
    return result;
  }

public final class Route {
  final Address address;
  final Proxy proxy;//代理
  final InetSocketAddress inetSocketAddress;//连接目标地址

  public Route(Address address, Proxy proxy, InetSocketAddress inetSocketAddress) {
    ...
    this.address = address;
    this.proxy = proxy;
    this.inetSocketAddress = inetSocketAddress;
  }

  RouteSelector(Address address, RouteDatabase routeDatabase, Call call,
      EventListener eventListener) {
    this.address = address;
    this.routeDatabase = routeDatabase;//连接池中的路由黑名单（连接失败的路由）
    this.call = call;
    this.eventListener = eventListener;

    resetNextProxy(address.url(), address.proxy());
  }
  //收集代理服务器
  private void resetNextProxy(HttpUrl url, Proxy proxy) {
    if (proxy != null) {
      // 若指定了代理，那么就这一个。（就是初始化OkhttpClient时配置的）
      proxies = Collections.singletonList(proxy);
    } else {
      //没配置就使用ProxySelector获取代理（若初始化OkhttpClient时没有配置ProxySelector，会使用系统默认的） 
      List<Proxy> proxiesOrNull = address.proxySelector().select(url.uri());
      proxies = proxiesOrNull != null && !proxiesOrNull.isEmpty()
          ? Util.immutableList(proxiesOrNull)
          : Util.immutableList(Proxy.NO_PROXY);
    }
    nextProxyIndex = 0;
  }

  //收集代理的路由信息
  public Selection next() throws IOException {
    if (!hasNext()) {//还有下一个代理
      throw new NoSuchElementException();
    }

    List<Route> routes = new ArrayList<>();
    while (hasNextProxy()) {
      Proxy proxy = nextProxy();
      //遍历proxy经DNS后的所有IP地址，组装成Route
      for (int i = 0, size = inetSocketAddresses.size(); i < size; i++) {
        Route route = new Route(address, proxy, inetSocketAddresses.get(i));
        if (routeDatabase.shouldPostpone(route)) {//此路由在黑名单中，存起来最后尝试
          postponedRoutes.add(route);
        } else {
          routes.add(route);
        }
      }

      if (!routes.isEmpty()) {
        break;
      }
    }

    if (routes.isEmpty()) {
      // 若没有拿到路由，就尝试上面存的黑名单的路由
      routes.addAll(postponedRoutes);
      postponedRoutes.clear();
    }
    //routes包装成Selection返回
    return new Selection(routes);
  }

public final class ConnectionPool {
  final RealConnectionPool delegate;
  //最大空闲连接数5，最大空闲时间5分钟
  public ConnectionPool() {
    this(5, 5, TimeUnit.MINUTES);
  }

  public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
    this.delegate = new RealConnectionPool(maxIdleConnections, keepAliveDuration, timeUnit);
  }
  //返回空闲连接数
  public int idleConnectionCount() {
    return delegate.idleConnectionCount();
  }
  //返回池子中的连接数
  public int connectionCount() {
    return delegate.connectionCount();
  }
  //关闭并移除所以空闲连接
  public void evictAll() {
    delegate.evictAll();
  }
}

  private final Deque<RealConnection> connections = new ArrayDeque<>();

  private final Runnable cleanupRunnable = () -> {
    //循环清理
    while (true) {
      //清理
      long waitNanos = cleanup(System.nanoTime());
      if (waitNanos == -1) return;
      if (waitNanos > 0) {
        long waitMillis = waitNanos / 1000000L;
        waitNanos -= (waitMillis * 1000000L);
        synchronized (RealConnectionPool.this) {
          try {
            //下一次清理之前的等待
            RealConnectionPool.this.wait(waitMillis, (int) waitNanos);
          } catch (InterruptedException ignored) {
          }
        }
      }
    }
  };
  //存
  void put(RealConnection connection) {
    assert (Thread.holdsLock(this));
    if (!cleanupRunning) {
      cleanupRunning = true;
      executor.execute(cleanupRunnable);
    }
    connections.add(connection);
  }

  long cleanup(long now) {
    int inUseConnectionCount = 0;//正在使用的连接数
    int idleConnectionCount = 0;//空闲连接数
    RealConnection longestIdleConnection = null;//空闲时间最长的连接
    long longestIdleDurationNs = Long.MIN_VALUE;//最长的空闲时间

    //遍历连接：找到待清理的连接, 找到下一次要清理的时间（还未到最大空闲时间）
    synchronized (this) {
      for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
        RealConnection connection = i.next();

        //若连接正在使用，continue，正在使用连接数+1
        if (pruneAndGetAllocationCount(connection, now) > 0) {
          inUseConnectionCount++;
          continue;
        }
        //空闲连接数+1
        idleConnectionCount++;

        // 赋值最长的空闲时间和对应连接
        long idleDurationNs = now - connection.idleAtNanos;
        if (idleDurationNs > longestIdleDurationNs) {
          longestIdleDurationNs = idleDurationNs;
          longestIdleConnection = connection;
        }
      }
      //若最长的空闲时间大于5分钟 或 空闲数 大于5，就移除并关闭这个连接
      if (longestIdleDurationNs >= this.keepAliveDurationNs
          || idleConnectionCount > this.maxIdleConnections) {
        connections.remove(longestIdleConnection);
      } else if (idleConnectionCount > 0) {
        // else，就返回 还剩多久到达5分钟，然后wait这个时间再来清理
        return keepAliveDurationNs - longestIdleDurationNs;
      } else if (inUseConnectionCount > 0) {
        //连接没有空闲的，就5分钟后再尝试清理.
        return keepAliveDurationNs;
      } else {
        // 没有连接，不清理
        cleanupRunning = false;
        return -1;
      }
    }
    //关闭移除的连接
    closeQuietly(longestIdleConnection.socket());

    //关闭移除后 立刻 进行下一次的 尝试清理
    return 0;
  }

  private int pruneAndGetAllocationCount(RealConnection connection, long now) {
    //连接上的数据流，弱引用列表
    List<Reference<Transmitter>> references = connection.transmitters;
    for (int i = 0; i < references.size(); ) {
      Reference<Transmitter> reference = references.get(i);
      if (reference.get() != null) {
        i++;
        continue;
      }

      // 到这里，transmitter是泄漏的，要移除，且此连接不能再承载新的数据流（泄漏的原因就是下面的message）
      TransmitterReference transmitterRef = (TransmitterReference) reference;
      String message = "A connection to " + connection.route().address().url()
          + " was leaked. Did you forget to close a response body?";
      Platform.get().logCloseableLeak(message, transmitterRef.callStackTrace);
      references.remove(i);
      connection.noNewExchanges = true;

      //连接因为泄漏没有数据流了，那么可以立即移除了。所以设置 开始空闲时间 是5分钟前（厉害厉害！）
      if (references.isEmpty()) {
        connection.idleAtNanos = now - keepAliveDurationNs;
        return 0;
      }
    }
    //返回连接上的数据流数量，大于0说明正在使用。
    return references.size();
  }

  void acquireConnectionNoEvents(RealConnection connection) {
    assert (Thread.holdsLock(connectionPool));
    if (this.connection != null) throw new IllegalStateException();
    this.connection = connection;
    connection.transmitters.add(new TransmitterReference(this, callStackTrace));
  }

  //为transmitter 从连接池 获取 对应address的连接。若果routes不为空，可能会因为 连接合并（复用） 而获取到HTTP/2连接。
  boolean transmitterAcquirePooledConnection(Address address, Transmitter transmitter,
      @Nullable List<Route> routes, boolean requireMultiplexed) {
    assert (Thread.holdsLock(this));
    for (RealConnection connection : connections) {
      if (requireMultiplexed && !connection.isMultiplexed()) continue;
      if (!connection.isEligible(address, routes)) continue;
      transmitter.acquireConnectionNoEvents(connection);
      return true;
    }
    return false;
  }

  //用于判断 连接 是否 可以承载指向address的数据流
  boolean isEligible(Address address, @Nullable List<Route> routes) {
    //连接不再接受新的数据流，false
    if (transmitters.size() >= allocationLimit || noNewExchanges) return false;

    //匹配address中非host的部分
    if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;

    //匹配address的host，到这里也匹配的话，就return true
    if (address.url().host().equals(this.route().address().url().host())) {
      return true; // This connection is a perfect match.
    }

    //到这里hostname是没匹配的，但是还是有机会返回true：连接合并
    // 1. 连接须是 HTTP/2.
    if (http2Connection == null) return false;

    // 2. IP 地址匹配
    if (routes == null || !routeMatchesAny(routes)) return false;

    // 3. 证书匹配
    if (address.hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
    if (!supportsUrl(address.url())) return false;

    // 4. 证书 pinning 匹配.
    try {
      address.certificatePinner().check(address.url().host(), handshake().peerCertificates());
    } catch (SSLPeerUnverifiedException e) {
      return false;
    }

    return true; // The caller's address can be carried by this connection.
  }

  private boolean routeMatchesAny(List<Route> candidates) {
    for (int i = 0, size = candidates.size(); i < size; i++) {
      Route candidate = candidates.get(i);
      if (candidate.proxy().type() == Proxy.Type.DIRECT
          && route.proxy().type() == Proxy.Type.DIRECT
          && route.socketAddress().equals(candidate.socketAddress())) {
        return true;
      }
    }
    return false;
  }

  //移除关闭空闲连接
  public void evictAll() {
    List<RealConnection> evictedConnections = new ArrayList<>();
    synchronized (this) {
      for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
        RealConnection connection = i.next();
        if (connection.transmitters.isEmpty()) {
          connection.noNewExchanges = true;
          evictedConnections.add(connection);
          i.remove();
        }
      }
    }

    for (RealConnection connection : evictedConnections) {
      closeQuietly(connection.socket());
    }
  }

  //从连接上移除transmitter.
  @Nullable Socket releaseConnectionNoEvents() {
    assert (Thread.holdsLock(connectionPool));

    int index = -1;
    //遍历 此数据流依附的连接 上的所有数据流，找到index
    for (int i = 0, size = this.connection.transmitters.size(); i < size; i++) {
      Reference<Transmitter> reference = this.connection.transmitters.get(i);
      if (reference.get() == this) {
        index = i;
        break;
      }
    }

    if (index == -1) throw new IllegalStateException();
    //transmitters移除此数据流
    RealConnection released = this.connection;
    released.transmitters.remove(index);
    this.connection = null;
    //如果连接上没有有数据流了，就置为空闲（等待清理），并返回待关闭的socket
    if (released.transmitters.isEmpty()) {
      released.idleAtNanos = System.nanoTime();
      if (connectionPool.connectionBecameIdle(released)) {
        return released.socket();
      }
    }

    return null;
  }

  ExchangeCodec newCodec(OkHttpClient client, Interceptor.Chain chain) throws SocketException {
    if (http2Connection != null) {
      return new Http2ExchangeCodec(client, this, chain, http2Connection);
    } else {
      socket.setSoTimeout(chain.readTimeoutMillis());
      source.timeout().timeout(chain.readTimeoutMillis(), MILLISECONDS);
      sink.timeout().timeout(chain.writeTimeoutMillis(), MILLISECONDS);
      return new Http1ExchangeCodec(client, this, source, sink);
    }
  }

  private void establishProtocol(ConnectionSpecSelector connectionSpecSelector,
      int pingIntervalMillis, Call call, EventListener eventListener) throws IOException {
    //针对http请求，如果配置的协议包含Protocol.H2_PRIOR_KNOWLEDGE，则开启Http2连接
    if (route.address().sslSocketFactory() == null) {
      if (route.address().protocols().contains(Protocol.H2_PRIOR_KNOWLEDGE)) {
        socket = rawSocket;
        protocol = Protocol.H2_PRIOR_KNOWLEDGE;
        startHttp2(pingIntervalMillis);
        return;
      }

      socket = rawSocket;
      protocol = Protocol.HTTP_1_1;
      return;
    }
    //针对https请求，会在TLS握手后，根据平台获取协议（），如果协议是Protocol.HTTP_2，则开启Http2连接
    eventListener.secureConnectStart(call);
    connectTls(connectionSpecSelector);
    eventListener.secureConnectEnd(call, handshake);

    if (protocol == Protocol.HTTP_2) {
      startHttp2(pingIntervalMillis);
    }
  }

  private void startHttp2(int pingIntervalMillis) throws IOException {
    socket.setSoTimeout(0); // HTTP/2 connection timeouts are set per-stream.
    http2Connection = new Http2Connection.Builder(true)
        .socket(socket, route.address().url().host(), source, sink)
        .listener(this)
        .pingIntervalMillis(pingIntervalMillis)
        .build();
    http2Connection.start();
  }

public final class CallServerInterceptor implements Interceptor {
  private final boolean forWebSocket;

  public CallServerInterceptor(boolean forWebSocket) {
    this.forWebSocket = forWebSocket;
  }

  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Exchange exchange = realChain.exchange();//上个拦截器传入的exchange
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();
    //写请求头
    exchange.writeRequestHeaders(request);

    boolean responseHeadersStarted = false;
    Response.Builder responseBuilder = null;
    //含body的请求
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // 若请求头包含 "Expect: 100-continue" , 就会等服务端返回含有 "HTTP/1.1 100 Continue"的响应，然后再发送请求body. 
      //如果没有收到这个响应（例如收到的响应是4xx），那就不发送body了。
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        exchange.flushRequest();
        responseHeadersStarted = true;
        exchange.responseHeadersStart();
        responseBuilder = exchange.readResponseHeaders(true);
      }
      //responseBuilder为null说明服务端返回了100，也就是可以继续发送body了
      if (responseBuilder == null) {
        if (request.body().isDuplex()) {//默认是false不会进入
          // Prepare a duplex body so that the application can send a request body later.
          exchange.flushRequest();
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, true));
          request.body().writeTo(bufferedRequestBody);
        } else {
          // 满足了 "Expect: 100-continue" ，写请求body
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, false));
          request.body().writeTo(bufferedRequestBody);
          bufferedRequestBody.close();
        }
      } else {
       //没有满足 "Expect: 100-continue" ，请求发送结束
        exchange.noRequestBody();
        if (!exchange.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.
          exchange.noNewExchangesOnConnection();
        }
      }
    } else {
     //没有body，请求发送结束
      exchange.noRequestBody();
    }

    //请求发送结束
    if (request.body() == null || !request.body().isDuplex()) {
      exchange.finishRequest();
    }
    //回调 读响应头开始事件（如果上面没有）
    if (!responseHeadersStarted) {
      exchange.responseHeadersStart();
    }
    //读响应头（如果上面没有）
    if (responseBuilder == null) {
      responseBuilder = exchange.readResponseHeaders(false);
    }
    //构建response
    Response response = responseBuilder
        .request(request)
        .handshake(exchange.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
      //这里服务端又返回了个100，就再尝试获取真正的响应（）
      response = exchange.readResponseHeaders(false)
          .request(request)
          .handshake(exchange.connection().handshake())
          .sentRequestAtMillis(sentRequestMillis)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();

      code = response.code();
    }
    //回调读响应头结束
    exchange.responseHeadersEnd(response);
    //这里就是获取响应body了
    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(exchange.openResponseBody(response))
          .build();
    }
    //请求头中Connection是close，表示请求完成后要关闭连接
    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      exchange.noNewExchangesOnConnection();
    }
    //204（无内容）、205（充值内容），body应该是空
    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }

    return response;
  }
}