OkHttp面试之--HttpEngine中的readResponse流程简介

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

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OkHttp面试之--HttpEngine中的readResponse流程简介

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