okhttp-CallServerInterceptor解析

CallServerInterceptor为连接服务器的拦截器，此类中真正向服务器发送请求，读取响应

/** This is the last interceptor in the chain. It makes a network call to the server. */
public final class CallServerInterceptor implements Interceptor {
  private final boolean forWebSocket;
参数为是否为websocket，http不是websocket，okhttp也可以直接创建socket，此时为true
  public CallServerInterceptor(boolean forWebSocket) {
    this.forWebSocket = forWebSocket;
  }

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

    long sentRequestMillis = System.currentTimeMillis();
//依赖exchange写入请求头
    exchange.writeRequestHeaders(request);

    boolean responseHeadersStarted = false;
    Response.Builder responseBuilder = null;
如果请求方式为允许有请求体，并且请求体不为null的。
    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.
//http 100-continue用于客户端在发送POST数据给服务器前，征询服务器情况，看服务器是否处理POST的数据，
 如果不处理，客户端则不上传POST数据，如果处理，则POST上传数据。在现实应用中，通过在POST大数据时，
 才会使用100-continue协议。    
  if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        exchange.flushRequest();
        responseHeadersStarted = true;
        exchange.responseHeadersStart();
读取响应头
        responseBuilder = exchange.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
  针对HTTP2协议的双工请求写入请求头
        if (request.body().isDuplex()) {
          // 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 {
          // Write the request body if the "Expect: 100-continue" expectation was met.
http1写入请求头 与全双工请求区别在createRequestBody第二个参数不一样
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, false));
          request.body().writeTo(bufferedRequestBody);
          bufferedRequestBody.close();
        }
      } else {
没有请求体
 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 {
没有请求体
      exchange.noRequestBody();
    }

    if (request.body() == null || !request.body().isDuplex()) {
结束请求
      exchange.finishRequest();
    }

    if (!responseHeadersStarted) {
      exchange.responseHeadersStart();
    }

    if (responseBuilder == null) {
      responseBuilder = exchange.readResponseHeaders(false);
    }
正常情况的响应
    Response response = responseBuilder
        .request(request)
        .handshake(exchange.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
处理服务器返回100
      // server sent a 100-continue even though we did not request one.
      // try again to read the actual response
      response = exchange.readResponseHeaders(false)
          .request(request)
          .handshake(exchange.connection().handshake())
          .sentRequestAtMillis(sentRequestMillis)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();

      code = response.code();
    }

    exchange.responseHeadersEnd(response);
处理服务器返回101
    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();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      exchange.noNewExchangesOnConnection();
    }
处理服务器返回204 205
    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }
正常情况返回
    return response;
  }
}

从文中可知，写入读取请求和响应都是通过exchange类来处理的。接下来我们看看这个类