上一节我们介绍了OkHttp网络异步请求的整个流程。其中在流程的最后阶段，我们发现最终创建了HttpEngine对象，并分别调用的此对象的sendRequest和readResponse方法。这两个方法 分别有它相应的作用。这一节我们着重来分析sendRequest流程。

以下是sendRequest的整个方法中的内容:

public void sendRequest() throws RequestException, RouteException, IOException {
    if (cacheStrategy != null) return; // Already sent.
    if (httpStream != null) throw new IllegalStateException();

    Request request = networkRequest(userRequest);

    InternalCache responseCache = Internal.instance.internalCache(client);
    Response cacheCandidate = responseCache != null
        ? responseCache.get(request)
        : null;
    long now = System.currentTimeMillis();
    cacheStrategy = new CacheStrategy.Factory(now, request, cacheCandidate).get();
    networkRequest = cacheStrategy.networkRequest;
    cacheResponse = cacheStrategy.cacheResponse;

    if (responseCache != null) {
      responseCache.trackResponse(cacheStrategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn‘t applicable. Close it.
    }

    // If we‘re forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {
      userResponse = new Response.Builder()
          .request(userRequest)
          .priorResponse(stripBody(priorResponse))
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(EMPTY_BODY)
          .build();
      return;
    }

    // If we don‘t need the network, we‘re done.
    if (networkRequest == null) {
      userResponse = cacheResponse.newBuilder()
          .request(userRequest)
          .priorResponse(stripBody(priorResponse))
          .cacheResponse(stripBody(cacheResponse))
          .build();
      userResponse = unzip(userResponse);
      return;
    }

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean success = false;
    try {
      httpStream = connect();
      httpStream.setHttpEngine(this);

      if (writeRequestHeadersEagerly()) {
        long contentLength = OkHeaders.contentLength(request);
        if (bufferRequestBody) {
          if (contentLength > Integer.MAX_VALUE) {
            throw new IllegalStateException("Use setFixedLengthStreamingMode() or "
                + "setChunkedStreamingMode() for requests larger than 2 GiB.");
          }

          if (contentLength != -1) {
            // Buffer a request body of a known length.
            httpStream.writeRequestHeaders(networkRequest);
            requestBodyOut = new RetryableSink((int) contentLength);
          } else {
            // Buffer a request body of an unknown length. Don‘t write request headers until the
            // entire body is ready; otherwise we can‘t set the Content-Length header correctly.
            requestBodyOut = new RetryableSink();
          }
        } else {
          httpStream.writeRequestHeaders(networkRequest);
          requestBodyOut = httpStream.createRequestBody(networkRequest, contentLength);
        }
      }
      success = true;
    } finally {
      // If we‘re crashing on I/O or otherwise, don‘t leak the cache body.
      if (!success && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }
  }

可以看到sendRequest方法比较长，我对它进行分块解析。其中主要分以下两大块
1 先从 Cache 中判断当前请求是否可以从缓存中返回

2 如果没有Cache则连接网络

先来看下connect方法中是如何创建HttpStream对象的

调用SteamAllocation.newStream的方法创建HttpStream对象并返回。点进去代码如下所示：

从上图中可以看出，在newStream方法中先通过findHealthyConnection方法获取一个RealConnection对象，实际上就是查找可用的Socket对象。在OkHttp框架中有一个特点就是OkHttp可以使用一个Socket对象来维护拥有过个ip的Server端，对于Socket的实现后续再单独讲解，此处不再做介绍。
获取RealConnction对象之后，根据此对象再获取相应的HttpStream对象，我们一般返回的是Http1xStream对象，最后将resultStream赋值给全局变量stream。而这个全局变量会再下一节readResponse方法中再使用。

注意：本节主要对于sendRequest方法中比较核心的代码进行的跟踪分析，在此方法中还有对Request请求的Head和Body的添加操作并没有进行详细描述。感兴趣的同学可以自行研究。

下一节继续讲解HttpEngine.readResponse方法的流程