OkHttp面试之--HttpEngine中的readResponse流程简介

Posted 2020-08-20 Danny_姜

上一节主要大体看了一下sendRequest的流程，本节来看一下当请求发送出去之后，是如果读取请求体中的数据的，具体的代码都在HttpEngine.readResponse方法中，代码如下：

public void readResponse() throws IOException {
    if (userResponse != null) {
      return; // Already ready.
    }
    if (networkRequest == null && cacheResponse == null) {
      throw new IllegalStateException("call sendRequest() first!");
    }
    if (networkRequest == null) {
      return; // No network response to read.
    }

    Response networkResponse;

    if (forWebSocket) {
      httpStream.writeRequestHeaders(networkRequest);
      networkResponse = readNetworkResponse();
    } else if (!callerWritesRequestBody) {
      networkResponse = new NetworkInterceptorChain(0, networkRequest).proceed(networkRequest);
    } else {
      // Emit the request body's buffer so that everything is in requestBodyOut.
      if (bufferedRequestBody != null && bufferedRequestBody.buffer().size() > 0) {
        bufferedRequestBody.emit();
      }

      // Emit the request headers if we haven't yet. We might have just learned the Content-Length.
      if (sentRequestMillis == -1) {
        if (OkHeaders.contentLength(networkRequest) == -1
            && requestBodyOut instanceof RetryableSink) {
          long contentLength = ((RetryableSink) requestBodyOut).contentLength();
          networkRequest = networkRequest.newBuilder()
              .header("Content-Length", Long.toString(contentLength))
              .build();
        }
        httpStream.writeRequestHeaders(networkRequest);
      }

      // Write the request body to the socket.
      if (requestBodyOut != null) {
        if (bufferedRequestBody != null) {
          // This also closes the wrapped requestBodyOut.
          bufferedRequestBody.close();
        } else {
          requestBodyOut.close();
        }
        if (requestBodyOut instanceof RetryableSink) {
          httpStream.writeRequestBody((RetryableSink) requestBodyOut);
        }
      }

      networkResponse = readNetworkResponse();
    }

    receiveHeaders(networkResponse.headers());

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      if (validate(cacheResponse, networkResponse)) {
        userResponse = cacheResponse.newBuilder()
            .request(userRequest)
            .priorResponse(stripBody(priorResponse))
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();
        releaseStreamAllocation();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        InternalCache responseCache = Internal.instance.internalCache(client);
        responseCache.trackConditionalCacheHit();
        responseCache.update(cacheResponse, stripBody(userResponse));
        userResponse = unzip(userResponse);
        return;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    userResponse = networkResponse.newBuilder()
        .request(userRequest)
        .priorResponse(stripBody(priorResponse))
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (hasBody(userResponse)) {
      maybeCache();
      userResponse = unzip(cacheWritingResponse(storeRequest, userResponse));
    }
  }

从开始进行分析

此处的userResponse在第一次访问时是null的，因此不会进入此代码块，实际会进入如下判断语句块中

可以看到此处又调用了一个连接拦截器，对请求结果进行拦截，NetworkInterceptorChain也是实现了Intercept.Chain, 具体代码

class NetworkInterceptorChain implements Interceptor.Chain {
    private final int index;
    private final Request request;
    private int calls;

    NetworkInterceptorChain(int index, Request request) {
      this.index = index;
      this.request = request;
    }

    @Override public Connection connection() {
      return streamAllocation.connection();
    }

    @Override public Request request() {
      return request;
    }

    @Override public Response proceed(Request request) throws IOException {
      calls++;

      if (index > 0) {
        Interceptor caller = client.networkInterceptors().get(index - 1);
        Address address = connection().route().address();

        // Confirm that the interceptor uses the connection we've already prepared.
        if (!request.url().host().equals(address.url().host())
            || request.url().port() != address.url().port()) {
          throw new IllegalStateException("network interceptor " + caller
              + " must retain the same host and port");
        }

        // Confirm that this is the interceptor's first call to chain.proceed().
        if (calls > 1) {
          throw new IllegalStateException("network interceptor " + caller
              + " must call proceed() exactly once");
        }
      }

      if (index < client.networkInterceptors().size()) {
        // There's another interceptor in the chain. Call that.
        NetworkInterceptorChain chain = new NetworkInterceptorChain(index + 1, request);
        Interceptor interceptor = client.networkInterceptors().get(index);
        Response interceptedResponse = interceptor.intercept(chain);

        // Confirm that the interceptor made the required call to chain.proceed().
        if (chain.calls != 1) {
          throw new IllegalStateException("network interceptor " + interceptor
              + " must call proceed() exactly once");
        }
        if (interceptedResponse == null) {
          throw new NullPointerException("network interceptor " + interceptor
              + " returned null");
        }

        return interceptedResponse;
      }

      httpStream.writeRequestHeaders(request);

      //Update the networkRequest with the possibly updated interceptor request.
      networkRequest = request;

      if (permitsRequestBody(request) && request.body() != null) {
        Sink requestBodyOut = httpStream.createRequestBody(request, request.body().contentLength());
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
      }

      Response response = readNetworkResponse();

      int code = response.code();
      if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
        throw new ProtocolException(
            "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
      }

      return response;
    }
  }

主要来看下proceed方法

同样还是一个循环递归调用，因此代码其实在最后一层还是会调用一下代码

具体每一行代码的作用在图片中已经做了简单介绍。我们重点来看一下最重要的读取请求结果的方法–readNetworkResponse，代码如下：

到这我们就已经完成了从发送网络请求到读取请求结果的流程，简单总结一下在HttpEngine中两个方法的工作
sendRequest–查找合适的Socket对象并封装在HttpStream中
readResponse–通过HttpStream发送请求，并读取结果封装到Response对象中返回