首先我們還是去看它的intercept方法

@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();

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

其中，StreamAllocation我們是在第一個RetryAndFollow攔截器裡建立的，接著，使用StreamAllocation建立一個HttpCodec，這個物件是用來編碼request和解碼response的。然後，通過StreamAllocation獲取一個RealConnection，RealConnection這個物件是用來進行實際的網路傳輸的，最後 ，繼續呼叫proceed方法，傳入下一個攔截器。

newStream解析

我們看原始碼

  public HttpCodec newStream(
      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 {
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
      HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);

      synchronized (connectionPool) {
        codec = resultCodec;
        return resultCodec;
      }
    } catch (IOException e) {
      throw new RouteException(e);
    }
  }
 

在try方法塊裡建立了RealConnection 和HttpCodec這兩個物件。先看一下findHealthyConnection這個方法是如何獲取connection的

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

      // If this is a brand new connection, we can skip the extensive health checks.
      synchronized (connectionPool) {
        if (candidate.successCount == 0) {
          return candidate;
        }
      }

      // Do a (potentially slow) check to confirm that the pooled connection is still good. If it
      // isn't, take it out of the pool and start again.
      if (!candidate.isHealthy(doExtensiveHealthChecks)) {
        noNewStreams();
        continue;
      }

      return candidate;
    }
  }
 

什麼是一個不健康的連結呢？比如socket連線沒有關閉，或者輸入輸出流沒有關閉等。我們來看findConnection這個方法

/**
   * 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,
      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.
        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;
        result = new RealConnection(connectionPool, selectedRoute);
        acquire(result, false);
      }
    }

    // If we found a pooled connection on the 2nd time around, we're done.
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
      return result;
    }

    // Do TCP + TLS handshakes. This is a blocking operation.
    result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
        connectionRetryEnabled, call, eventListener);
    routeDatabase().connected(result.route());

    Socket socket = null;
    synchronized (connectionPool) {
      reportedAcquired = true;

      // Pool the connection.
      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;
  }