OkHttp源码分析之基本框架2

接上篇。
（主要从网络拉取响应分析）从sendRequest方法中可以看到

httpStream = connect();httpStream.setHttpEngine(this);

接下来，我们就看看connect()方法：
HttpEngine#connect

  private HttpStream connect() throws RouteException, RequestException, IOException {    boolean doExtensiveHealthChecks = !networkRequest.method().equals("GET");    return streamAllocation.newStream(client.connectTimeoutMillis(),        client.readTimeoutMillis(), client.writeTimeoutMillis(),        client.retryOnConnectionFailure(), doExtensiveHealthChecks);  }

继续跟踪
StreamAllocation#newStream

public HttpStream newStream(int connectTimeout, int readTimeout, int writeTimeout,      boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)      throws RouteException, IOException {    try {      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,          writeTimeout, connectionRetryEnabled, doExtensiveHealthChecks);      HttpStream resultStream;      if (resultConnection.framedConnection != null) {        resultStream = new Http2xStream(this, resultConnection.framedConnection);      } else {        resultConnection.socket().setSoTimeout(readTimeout);        resultConnection.source.timeout().timeout(readTimeout, MILLISECONDS);        resultConnection.sink.timeout().timeout(writeTimeout, MILLISECONDS);        resultStream = new Http1xStream(this, resultConnection.source, resultConnection.sink);      }      synchronized (connectionPool) {        stream = resultStream;        return resultStream;      }    } catch (IOException e) {      throw new RouteException(e);    }  }

可以看到这里创建了RealConnection resultConnection 对象。
具体创建的代码
StreamAllocation#findHealthyConnection

/**   * Finds a connection and returns it if it is healthy. If it is unhealthy the process is repeated   * until a healthy connection is found.   */  private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,      int writeTimeout, boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)      throws IOException, RouteException {    while (true) {      RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,          connectionRetryEnabled);      // If this is a brand new connection, we can skip the extensive health checks.      synchronized (connectionPool) {        if (candidate.successCount == 0) {          return candidate;        }      }      // Otherwise do a potentially-slow check to confirm that the pooled connection is still good.      if (candidate.isHealthy(doExtensiveHealthChecks)) {        return candidate;      }      connectionFailed(new IOException());    }  }

继续跟踪
StreamAllocation#findConnection

/**   * Returns a connection to host a new stream. This prefers the existing connection if it exists,   * then the pool, finally building a new connection.   */  private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,      boolean connectionRetryEnabled) throws IOException, RouteException {    Route selectedRoute;    synchronized (connectionPool) {      if (released) throw new IllegalStateException("released");      if (stream != null) throw new IllegalStateException("stream != null");      if (canceled) throw new IOException("Canceled");      RealConnection allocatedConnection = this.connection;      if (allocatedConnection != null && !allocatedConnection.noNewStreams) {        return allocatedConnection;      }      // Attempt to get a connection from the pool.      //如果连接池中已经存在连接，就从中取出(get)RealConnection      RealConnection pooledConnection = Internal.instance.get(connectionPool, address, this);      if (pooledConnection != null) {        this.connection = pooledConnection;        return pooledConnection;      }      selectedRoute = route;    }    if (selectedRoute == null) {      selectedRoute = routeSelector.next();      synchronized (connectionPool) {        route = selectedRoute;      }    }    RealConnection newConnection = new RealConnection(selectedRoute);    acquire(newConnection);//将建立成功的RealConnection放入(put)连接池缓存    synchronized (connectionPool) {      Internal.instance.put(connectionPool, newConnection);      this.connection = newConnection;      if (canceled) throw new IOException("Canceled");    }//根据选择的路线(Route)，调用Platform.get().connectSocket选择当前平台Runtime下最好的socket库进行握手    newConnection.connect(connectTimeout, readTimeout, writeTimeout, address.connectionSpecs(),        connectionRetryEnabled);    routeDatabase().connected(newConnection.route());    return newConnection;  }

到这里我们先介绍Socket管理(StreamAllocation)。

StreamAllocation

进行HTTP连接需要进行Socket握手，Socket握手的前提是根据域名或代理确定Socket的ip与端口。这个环节主要讲了http的握手过程与连接池的管理，分析的对象主要是StreamAllocation

选择路线与自动重连(RouteSelector)

此步骤用于获取socket的ip与端口,各位请欣赏源码中next()的迷之缩进与递归，代码进行了如下事情：
如果Proxy为null:

1. 在构造函数中设置代理为Proxy.NO_PROXY
2. 如果缓存中的lastInetSocketAddress为空，就通过DNS（默认是Dns.SYSTEM，包装了jdk自带的lookup函数）查询，并保存结果，注意结果是数组，即一个域名有多个IP，这就是自动重连的来源
3. 如果还没有查询到就递归调用next查询，直到查到为止
4. 一切next都没有枚举到，抛出NoSuchElementException，退出(这个几乎见不到)

如果Proxy为HTTP:

1. 设置socket的ip为代理地址的ip
2. 设置socket的端口为代理地址的端口
3. 一切next都没有枚举到，抛出NoSuchElementException，退出

连接socket链路(RealConnection)

当地址，端口准备好了，就可以进行TCP连接了（也就是我们常说的TCP三次握手），步骤如下：

1. 如果连接池中已经存在连接，就从中取出(get)RealConnection，如果没有命中就进入下一步
2. 根据选择的路线(Route)，调用Platform.get().connectSocket选择当前平台Runtime下最好的socket库进行握手
3. 将建立成功的RealConnection放入(put)连接池缓存
4. 如果存在TLS，就根据SSL版本与证书进行安全握手
5. 构造HttpStream并维护刚刚的socket连接，管道建立完成

释放socket链路(release)

如果不再需要（比如通信完成，连接失败等）此链路后，释放连接（也就是TCP断开的握手）

1. 尝试从缓存的连接池中删除(remove)
2. 如果没有命中缓存,就直接调用jdk的socket关闭

经过上述分析，相信大家都有了一定的概念，对上面的那段源码也就看的很自然。如果连接池中已经存在连接，就从中取出(get)RealConnection，如果没有命就根据选择的路线(Route)，调用Platform.get().connectSocket选择当前平台Runtime下最好的socket库进行握手。
RealConnection#connect

  public void connect(int connectTimeout, int readTimeout, int writeTimeout,      List<ConnectionSpec> connectionSpecs, boolean connectionRetryEnabled) throws RouteException {    if (protocol != null) throw new IllegalStateException("already connected");    RouteException routeException = null;    ConnectionSpecSelector connectionSpecSelector = new ConnectionSpecSelector(connectionSpecs);    Proxy proxy = route.proxy();    Address address = route.address();    if (route.address().sslSocketFactory() == null        && !connectionSpecs.contains(ConnectionSpec.CLEARTEXT)) {      throw new RouteException(new UnknownServiceException(          "CLEARTEXT communication not supported: " + connectionSpecs));    }    while (protocol == null) {      try {        rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP            ? address.socketFactory().createSocket()            : new Socket(proxy);        connectSocket(connectTimeout, readTimeout, writeTimeout, connectionSpecSelector);      } catch (IOException e) {        closeQuietly(socket);        closeQuietly(rawSocket);        socket = null;        rawSocket = null;        source = null;        sink = null;        handshake = null;        protocol = null;        if (routeException == null) {          routeException = new RouteException(e);        } else {          routeException.addConnectException(e);        }        if (!connectionRetryEnabled || !connectionSpecSelector.connectionFailed(e)) {          throw routeException;        }      }    }  }

如果存在TLS，就根据SSL版本与证书进行安全握手
RealConnection#connectSocket

  /** Does all the work necessary to build a full HTTP or HTTPS connection on a raw socket. */  private void connectSocket(int connectTimeout, int readTimeout, int writeTimeout,      ConnectionSpecSelector connectionSpecSelector) throws IOException {    rawSocket.setSoTimeout(readTimeout);    try {      Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);    } catch (ConnectException e) {      throw new ConnectException("Failed to connect to " + route.socketAddress());    }    source = Okio.buffer(Okio.source(rawSocket));    sink = Okio.buffer(Okio.sink(rawSocket));    if (route.address().sslSocketFactory() != null) {      connectTls(readTimeout, writeTimeout, connectionSpecSelector);    } else {      protocol = Protocol.HTTP_1_1;      socket = rawSocket;    }    if (protocol == Protocol.SPDY_3 || protocol == Protocol.HTTP_2) {      socket.setSoTimeout(0); // Framed connection timeouts are set per-stream.      FramedConnection framedConnection = new FramedConnection.Builder(true)          .socket(socket, route.address().url().host(), source, sink)          .protocol(protocol)          .listener(this)          .build();      framedConnection.sendConnectionPreface();      // Only assign the framed connection once the preface has been sent successfully.      this.allocationLimit = framedConnection.maxConcurrentStreams();      this.framedConnection = framedConnection;    } else {      this.allocationLimit = 1;    }  }

上面这段代码写的就是根据选择的路线(Route)，调用Platform.get().connectSocket选择当前平台Runtime下最好的socket库进行握手。

这里又引入了一概念。HTTP请求序列化/反序列化
下面我们就来分析分析。

HTTP请求序列化/反序列化

分析的对象是HttpStream接口，在HTTP/1.1下是Http1xStream实现的。

获得HTTP流（httpStream）

以下为无缓存，无多次302跳转，网络良好，HTTP/1.1下的GET访问实例分析。
我们已经在上文的RealConnection通过connectSocket()构造HttpStream对象并建立套接字连接（完成三次握手）
在connect()有非常重要的一步，它通过okio库与远程socket建立了I/O连接，为了更好的理解，我们可以把它看成管道

//source 用于获取responsesource = Okio.buffer(Okio.source(rawSocket));//sink 用于write buffer 到serversink = Okio.buffer(Okio.sink(rawSocket));

拼装Raw请求与Headers(writeRequestHeaders)

我们通过Request.Builder构建了简陋的请求后，可能需要进行一些修饰，这时需要使用Interceptors对Request进行进一步的拼装了。
拦截器是okhttp中强大的流程装置，它可以用来监控log，修改请求，修改结果，甚至是对用户透明的GZIP压缩。类似于函数式编程中的flatmap操作。在okhttp中，内部维护了一个Interceptors的List，通过InterceptorChain进行多次拦截修改操作。

源代码中是自增递归(recursive)调用Chain.process()，直到interceptors().size()中的拦截器全部调用完。主要做了两件事：

1. 递归调用Interceptors，依次入栈对response进行处理
2. 当全部递归出栈完成后，移交给网络模块(getResponse)

 if (index < client.interceptors().size()) {    Interceptor.Chain chain = new ApplicationInterceptorChain(index + 1, request, forWebSocket);    Interceptor interceptor = client.interceptors().get(index);    //递归调用Chain.process()    Response interceptedResponse = interceptor.intercept(chain);    if (interceptedResponse == null) {      throw new NullPointerException("application interceptor " + interceptor          + " returned null");    }    return interceptedResponse;  }  // No more interceptors. Do HTTP.  return getResponse(request, forWebSocket);}

接下来是正式的网络请求getResponse()，此步骤通过http协议规范将对象中的数据信息序列化为Raw文本：

1. 在okhttp中，通过RequestLine，Requst，HttpEngine，Header等参数进行序列化操作，也就是拼装参数为socketRaw数据。拼装方法也比较暴力，直接按照RFC协议要求的格式进行concat输出就实现了
2. 通过sink写入write到socket连接。

获得响应(readResponseHeaders/Body)

此步骤根据获取到的Socket纯文本，解析为Response对象，我们可以看成是一个反序列化（通过http协议将Raw文本转成对象）的过程：
拦截器的设计:

1. 自定义网络拦截器请求进行递归入栈
2. 在自定义网络拦截器的intercept中，调用NetworkInterceptorChain的proceed(request),进行真正的网络请求(readNetworkResponse)
3. 接自定义请求递归出栈

网络读取(readNetworkResponse)分析:

1. 读取Raw的第一行，并反序列化为StatusLine对象
2. 以Transfer-Encoding: chunked的模式传输并组装Body

接下来进行释放socket连接，上文已经介绍过了。现在我们就获得到response对象，可以进行进一步的Gson等操作了。

到目前，基本框架也算是介绍完了。还是那句话，共同学习与进步。