整体设计

OkHttp设计较为复杂，但是层次还算清晰，我把主要的类关系画了个图，大概是这样。

OkHttp的类较多，这里只描述下重要的类关系，可以看出OkHttp的主要功能都集中在Interceptor中，通过Interceptor完成构建请求，建立Socket连接，建立SSLSocket连接，证书校验等步骤。

我们看下Interceptor的调用过程：

在这个拦截器设计中，我们可以对Request进行操作，同样也可以对Response进行操作。当然，我们添加的Interceprot是在所有Interceptor之前，意味着Request最先操作，Response最后操作。

RealCall类的getResponseWithInterceptorChain是创建Interceptor的方法，可以看出okHttp共添加了5个Interceptor，它们在网络请求的不同阶段起着不同的作用，下面将详细分析。

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Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    // 添加Http参数，处理Http返回
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    // 添加NetworkInterceptor
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));
 
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());
 
    return chain.proceed(originalRequest);
  }
 

OkHttpClient模块

OkHttpClient使用Builder进行构建，同时提供了一个newBuilder()的方法用来创建一个Builder，可以对原有OkHttpClient修改部分参数后创建一个新实例。

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  OkHttpClient(Builder builder) {
    this.dispatcher = builder.dispatcher;
    this.proxy = builder.proxy;
    this.protocols = builder.protocols;
    this.connectionSpecs = builder.connectionSpecs;
    this.interceptors = Util.immutableList(builder.interceptors);
    this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
    this.eventListenerFactory = builder.eventListenerFactory;
    this.proxySelector = builder.proxySelector;
    this.cookieJar = builder.cookieJar;
    this.cache = builder.cache;
    this.internalCache = builder.internalCache;
    this.socketFactory = builder.socketFactory;
 
    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 = Util.platformTrustManager();
      this.sslSocketFactory = newSslSocketFactory(trustManager);
      this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
    }
 
    if (sslSocketFactory != null) {
      Platform.get().configureSslSocketFactory(sslSocketFactory);
    }
 
    this.hostnameVerifier = builder.hostnameVerifier;
    this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
        certificateChainCleaner);
    this.proxyAuthenticator = builder.proxyAuthenticator;
    this.authenticator = builder.authenticator;
    this.connectionPool = builder.connectionPool;
    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;
    this.pingInterval = builder.pingInterval;
 
    if (interceptors.contains(null)) {
      throw new IllegalStateException("Null interceptor: " + interceptors);
    }
    if (networkInterceptors.contains(null)) {
      throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
    }
  }
 

OkHttpClient同时实现了CallFactory接口，这个是发起网络请求的关键类。 CallFactory

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interface Factory {
    Call newCall(Request request);
  }
 

OkHttpClient中

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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 */);
  }
  
 

RealCall中

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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);
    call.eventListener = client.eventListenerFactory().create(call);
    return call;
  }
 

Router路由选择模块

我们知道，发起网络请求首先涉及到DNS解析的过程，okHttp对这个DNS解析过程了优化，引入了Router的概念，对失效的IP地址进行缓存，一定程度上提高了DNS解析速度。

我们先看下OkHttp的DNS查询逻辑。

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public interface Dns {
  /**
   * A DNS that uses {@link InetAddress#getAllByName} to ask the underlying operating system to
   * lookup IP addresses. Most custom {@link Dns} implementations should delegate to this instance.
   */
  Dns SYSTEM = new Dns() {
    @Override public List<InetAddress> lookup(String hostname) throws UnknownHostException {
      if (hostname == null) throw new UnknownHostException("hostname == null");
      try {
        return Arrays.asList(InetAddress.getAllByName(hostname));
      } catch (NullPointerException e) {
        UnknownHostException unknownHostException =
            new UnknownHostException("Broken system behaviour for dns lookup of " + hostname);
        unknownHostException.initCause(e);
        throw unknownHostException;
      }
    }
  };
 
  /**
   * Returns the IP addresses of {@code hostname}, in the order they will be attempted by OkHttp. If
   * a connection to an address fails, OkHttp will retry the connection with the next address until
   * either a connection is made, the set of IP addresses is exhausted, or a limit is exceeded.
   */
  List<InetAddress> lookup(String hostname) throws UnknownHostException;
}
 
 

核心是调用InetAddress.getAllByName(hostname)进行查询。

建立连接这个过程中是在ConnectInterceptor中完成的，我们看下ConnectInterceptor的代码。

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/** Opens a connection to the target server and proceeds to the next interceptor. */
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 {
    System.out.println("ConnectIntercept #intercept");
    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");
    // 发起网络请求，将数据放置在HttpCodec中
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();
 
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }
}
 

调用到StreamAllocation的newStream方法，StreamAllocation中就涉及到DNS解析->Router选择->建立Socket/SSLSocket连接的过程了。

下面看下Router模块的类关系。

StreamAllocation中通过RouteSelector获取所有IP地址，然后从ConnectionPool中查找可复用的连接，如果没有，则使用第一个Route创建RealConnection对象建立连接。

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private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
      int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
    boolean foundPooledConnection = false;
    RealConnection result = null;
    Route selectedRoute = null;
    Connection releasedConnection;
    Socket toClose;
    synchronized (connectionPool) {
      if (released) throw new IllegalStateException("released");
      if (codec != null) throw new IllegalStateException("codec != null");
      if (canceled) throw new IOException("Canceled");
 
      // Attempt to use an already-allocated connection. We need to be careful here because our
      // already-allocated connection may have been restricted from creating new streams.
      releasedConnection = this.connection;
      toClose = releaseIfNoNewStreams();
      if (this.connection != null) {
        // We had an already-allocated connection and it's good.
        result = this.connection;
        releasedConnection = null;
      }
      if (!reportedAcquired) {
        // If the connection was never reported acquired, don't report it as released!
        releasedConnection = null;
      }
 
      if (result == null) {
        // Attempt to get a connection from the pool.
        // 从 ConnectionPools里获取Connection
        Internal.instance.get(connectionPool, address, this, null);
        if (connection != null) {
          foundPooledConnection = true;
          result = connection;
        } else {
          selectedRoute = route;
        }
      }
    }
    closeQuietly(toClose);
 
    if (releasedConnection != null) {
      eventListener.connectionReleased(call, releasedConnection);
    }
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
    }
    if (result != null) {
      // If we found an already-allocated or pooled connection, we're done.
      return result;
    }
 
    // If we need a route selection, make one. This is a blocking operation.
    boolean newRouteSelection = false;
    if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
      newRouteSelection = true;
      routeSelection = routeSelector.next();
    }
 
    synchronized (connectionPool) {
      if (canceled) throw new IOException("Canceled");
 
      if (newRouteSelection) {
        // Now that we have a set of IP addresses, make another attempt at getting a connection from
        // the pool. This could match due to connection coalescing.
        List<Route> routes = routeSelection.getAll();
        for (int i = 0, size = routes.size(); i < size; i++) {
          Route route = routes.get(i);
          Internal.instance.get(connectionPool, address, this, route);
          if (connection != null) {
            foundPooledConnection = true;
            result = connection;
            this.route = route;
            break;
          }
        }
      }
 
      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.
        route = selectedRoute;
        refusedStreamCount = 0;
        // 创建新的Connection
        result = new RealConnection(connectionPool, selectedRoute);
        acquire(result, false);
      }
    }
 
    // If we found a pooled connection on the 2nd time around, we're done.
    // 找寻到之前建立的Http连接， 直接返回，不需要重新三次握手
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
      return result;
    }
 
    // Do TCP + TLS handshakes. This is a blocking operation.
    // 开始TCP和TLS握手
    result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
        connectionRetryEnabled, call, eventListener);
    routeDatabase().connected(result.route());
 
    Socket socket = null;
    synchronized (connectionPool) {
      reportedAcquired = true;
 
      // Pool the connection.，将Connection 缓存到ConnectionPools中
      Internal.instance.put(connectionPool, result);
 
      // If another multiplexed connection to the same address was created concurrently, then
      // release this connection and acquire that one.
      if (result.isMultiplexed()) {
        socket = Internal.instance.deduplicate(connectionPool, address, this);
        result = connection;
      }
    }
    closeQuietly(socket);
 
    eventListener.connectionAcquired(call, result);
    return result;
  }
 

RealConnection Http(s)网络请求模块

前面的StreamAllocation的分析中可以看出，如果在ConnectionPools中没有取出可以复用的Connection那么会重新创建一个Connection，即Socket连接，下面我们看下创建过程。

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// StreamAllocation 的findConnection中，
result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
        connectionRetryEnabled, call, eventListener);
 

建立连接需要两个步骤，建立Socket连接；如果是Https的话，还需要建立一个SSLSocket连接，我们先来看下建立Socket连接。

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private void connectSocket(int connectTimeout, int readTimeout, Call call,
      EventListener eventListener) throws IOException {
    Proxy proxy = route.proxy();
    Address address = route.address();
 
    rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP
        ? address.socketFactory().createSocket()
        : new Socket(proxy);
 
    eventListener.connectStart(call, route.socketAddress(), proxy);
    rawSocket.setSoTimeout(readTimeout);
    try {
      Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);
    } catch (ConnectException e) {
      ConnectException ce = new ConnectException("Failed to connect to " + route.socketAddress());
      ce.initCause(e);
      throw ce;
    }
 
    // The following try/catch block is a pseudo hacky way to get around a crash on Android 7.0
    // More details:
    // https://github.com/square/okhttp/issues/3245
    // https://android-review.googlesource.com/#/c/271775/
    try {
      // source会包装成Response返回给上层调用
      source = Okio.buffer(Okio.source(rawSocket));
      sink = Okio.buffer(Okio.sink(rawSocket));
    } catch (NullPointerException npe) {
      if (NPE_THROW_WITH_NULL.equals(npe.getMessage())) {
        throw new IOException(npe);
      }
    }
  }
 

下面看下建立SSLSocket连接

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  // TCP/Socket连接建立完毕后，进行一次TLS握手
  private void connectTls(ConnectionSpecSelector connectionSpecSelector) throws IOException {
    Address address = route.address();
    SSLSocketFactory sslSocketFactory = address.sslSocketFactory();
    boolean success = false;
    SSLSocket sslSocket = null;
    try {
      // Create the wrapper over the connected socket.
      sslSocket = (SSLSocket) sslSocketFactory.createSocket(
          rawSocket, address.url().host(), address.url().port(), true /* autoClose */);
 
      // Configure the socket's ciphers, TLS versions, and extensions.
      ConnectionSpec connectionSpec = connectionSpecSelector.configureSecureSocket(sslSocket);
      if (connectionSpec.supportsTlsExtensions()) {
        Platform.get().configureTlsExtensions(
            sslSocket, address.url().host(), address.protocols());
      }
 
      // 开始进行TLS握手
      // Force handshake. This can throw!
      sslSocket.startHandshake();
      // block for session establishment
      SSLSession sslSocketSession = sslSocket.getSession();
      Handshake unverifiedHandshake = Handshake.get(sslSocketSession);
 
 
      // TLS握手后，获取服务器端证书，然后进行HostnameVerifier校验
      // Verify that the socket's certificates are acceptable for the target host.
      if (!address.hostnameVerifier().verify(address.url().host(), sslSocketSession)) {
        X509Certificate cert = (X509Certificate) unverifiedHandshake.peerCertificates().get(0);
        throw new SSLPeerUnverifiedException("Hostname " + address.url().host() + " not verified:"
            + "\n    certificate: " + CertificatePinner.pin(cert)
            + "\n    DN: " + cert.getSubjectDN().getName()
            + "\n    subjectAltNames: " + OkHostnameVerifier.allSubjectAltNames(cert));
      }
 
      // Check that the certificate pinner is satisfied by the certificates presented.
      address.certificatePinner().check(address.url().host(),
          unverifiedHandshake.peerCertificates());
 
      // Success! Save the handshake and the ALPN protocol.
      String maybeProtocol = connectionSpec.supportsTlsExtensions()
          ? Platform.get().getSelectedProtocol(sslSocket)
          : null;
      socket = sslSocket;
      source = Okio.buffer(Okio.source(socket));
      sink = Okio.buffer(Okio.sink(socket));
      handshake = unverifiedHandshake;
      protocol = maybeProtocol != null
          ? Protocol.get(maybeProtocol)
          : Protocol.HTTP_1_1;
      success = true;
    } catch (AssertionError e) {
      if (Util.isAndroidGetsocknameError(e)) throw new IOException(e);
      throw e;
    } finally {
      if (sslSocket != null) {
        Platform.get().afterHandshake(sslSocket);
      }
      if (!success) {
        closeQuietly(sslSocket);
      }
    }
  }
 

socket会被包装成sslSocket，source，sink都会被重新生成。

HttpCodec/Response模块

前面介绍了建立Socket连接，路由选择等过程，但是实际上解析网络流数据是通过HttpCodec进行的，先看下HttpCodec的定义。

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public interface HttpCodec {
  /**
   * The timeout to use while discarding a stream of input data. Since this is used for connection
   * reuse, this timeout should be significantly less than the time it takes to establish a new
   * connection.
   */
  int DISCARD_STREAM_TIMEOUT_MILLIS = 100;
 
  /** Returns an output stream where the request body can be streamed. */
  Sink createRequestBody(Request request, long contentLength);
 
  /** This should update the HTTP engine's sentRequestMillis field. */
  void writeRequestHeaders(Request request) throws IOException;
 
  /** Flush the request to the underlying socket. */
  void flushRequest() throws IOException;
 
  /** Flush the request to the underlying socket and signal no more bytes will be transmitted. */
  void finishRequest() throws IOException;
 
  /**
   * Parses bytes of a response header from an HTTP transport.
   *
   * @param expectContinue true to return null if this is an intermediate response with a "100"
   *     response code. Otherwise this method never returns null.
   */
  Response.Builder readResponseHeaders(boolean expectContinue) throws IOException;
 
  /** Returns a stream that reads the response body. */
  ResponseBody openResponseBody(Response response) throws IOException;
 
  /**
   * Cancel this stream. Resources held by this stream will be cleaned up, though not synchronously.
   * That may happen later by the connection pool thread.
   */
  void cancel();
}
 
 

HttpCodec有两个实现分别为Http1Codec和Http2Codec。

HttpCodec实例是通过RealConnection中进行获取的，最上层的调用是在ConnectInterceptor中。

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  public HttpCodec newCodec(OkHttpClient client, Interceptor.Chain chain,
      StreamAllocation streamAllocation) throws SocketException {
    if (http2Connection != null) {
      return new Http2Codec(client, chain, streamAllocation, http2Connection);
    } else {
      socket.setSoTimeout(chain.readTimeoutMillis());
      source.timeout().timeout(chain.readTimeoutMillis(), MILLISECONDS);
      sink.timeout().timeout(chain.writeTimeoutMillis(), MILLISECONDS);
      return new Http1Codec(client, streamAllocation, source, sink);
    }
  }
 

我们先分析下Http1.1版本的HttpCodec实现，可以看出HttpCodec是将OkHttpClient，StreamAllocation，Source , Sink都封装到一起了。

然后ConnectIntercptor将HttpCodec实例传给Chain，让下一个拦截器记性处理。

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@Override public Response intercept(Chain chain) throws IOException {
    System.out.println("ConnectIntercept #intercept");
    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");
    // 发起网络请求，将数据放置在HttpCodec中
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();
 
    
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }
 

HttpCodec真正的调用是在CallServerInterceptor中，通过HttpCodec，将RequestBody写入输入流，同时解码网络传输的数据。

CallServerInterceptor的核心逻辑如下:

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  @Override public Response intercept(Chain chain) throws IOException {
    System.out.println("CallServerIntercept #intercept");
    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);
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);
 
    Response.Builder responseBuilder = null;
    // 是否允许携带Request body
    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);
        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);
    }
 
    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);
 
    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 {
      // 创建ResponseBody
      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;
  }
 

上面逻辑中，通过HttpCodec的openResponseBody获取ResponseBody实例，这个ResponseBody会返回给上层调用者。

看下Http1Codec的openResponseBody实现

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@Override public ResponseBody openResponseBody(Response response) throws IOException {
    streamAllocation.eventListener.responseBodyStart(streamAllocation.call);
    String contentType = response.header("Content-Type");
 
    // 返回FixedLengthSource
    if (!HttpHeaders.hasBody(response)) {
      Source source = newFixedLengthSource(0);
      return new RealResponseBody(contentType, 0, Okio.buffer(source));
    }
 
    // 返回ChunkedSource
    if ("chunked".equalsIgnoreCase(response.header("Transfer-Encoding"))) {
      Source source = newChunkedSource(response.request().url());
      return new RealResponseBody(contentType, -1L, Okio.buffer(source));
    }
 
    long contentLength = HttpHeaders.contentLength(response);
    if (contentLength != -1) {
      Source source = newFixedLengthSource(contentLength);
      return new RealResponseBody(contentType, contentLength, Okio.buffer(source));
    }
 
    return new RealResponseBody(contentType, -1L, Okio.buffer(newUnknownLengthSource()));
  }
 

这个openReponseBody的作用是将HttpCodec的 Source进行包装，然后返回给上层调用。

这个时候ResponseBody的Source类型实际是对Socket的多重包装，所以，当我们不再需要这个Response的数据时，需要将source关闭，否则这个socket可能就泄漏了。

Socket也算一个FD类型，如果FD泄漏达到上限，会触发Crash。

ConnectionPool 连接池模块

ConnectionPool的作用是将Socket连接进行缓存，如果两次请求的同一个Host的话，可以复用之前的连接，减少TCP握手和TLS握手。

在StreamAllocation的findConnection中，会尝试通过ConnecitonPool获取已经缓存的Connection，获取之前会有个比较逻辑，具体是Connection的isEligible方法。

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  /**
   * Returns true if this connection can carry a stream allocation to {@code address}. If non-null
   * {@code route} is the resolved route for a connection.
   */
  public boolean isEligible(Address address, @Nullable Route route) {
    // If this connection is not accepting new streams, we're done.
    // 该连接是否正在使用
    if (allocations.size() >= allocationLimit || noNewStreams) return false;
 
    // If the non-host fields of the address don't overlap, we're done.
    if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;
 
    // HOST相同，直接使用之前的连接
    // If the host exactly matches, we're done: this connection can carry the address.
    if (address.url().host().equals(this.route().address().url().host())) {
      return true; // This connection is a perfect match.
    }
 
    // At this point we don't have a hostname match. But we still be able to carry the request if
    // our connection coalescing requirements are met. See also:
    // https://hpbn.co/optimizing-application-delivery/#eliminate-domain-sharding
    // https://daniel.haxx.se/blog/2016/08/18/http2-connection-coalescing/
 
    // 1. This connection must be HTTP/2.
    if (http2Connection == null) return false;
 
    // 2. The routes must share an IP address. This requires us to have a DNS address for both
    // hosts, which only happens after route planning. We can't coalesce connections that use a
    // proxy, since proxies don't tell us the origin server's IP address.
    if (route == null) return false;
    if (route.proxy().type() != Proxy.Type.DIRECT) return false;
    if (this.route.proxy().type() != Proxy.Type.DIRECT) return false;
    if (!this.route.socketAddress().equals(route.socketAddress())) return false;
 
    // 3. This connection's server certificate's must cover the new host.
    if (route.address().hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
    if (!supportsUrl(address.url())) return false;
 
    // 4. Certificate pinning must match the host.
    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.
  }
 

可以看出，Http2的规则和Http1的有所不同。 针对Http1.1来说，满足两个条件即可复用Connection。

  1. 没有正在使用的Http连接
  2. Host相同

这样的话，其实是忽略了Keep-Alive字段了，如果Http(s)连接满足上面的条件，都可以拿来复用。

ConnectionPools不会无限缓存连接，会有时间限制和个数限制，并且超过时间会自动清理连接池中的连接。

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