接上篇抽丝剥茧okhttp（三） https://www.jianshu.com/p/cf59397dce1f
下面是极简版的okhttp请求网络的流程图，之前分析过了Response、Request这两个涉及http本身的协议的封装。那么如何宏观上看整个请求流程呢？
1 OkHttpClient负责把这些部件和配置装配起来
2 用Call对象发出请求，
3 在发送和接收过程中通过很多拦截器处理，
4 之后我们接得了response对象，整个网络请求流程完成。
一共四步，联想实际的网络请求不难理解。

image.png

那么这四步具体是如何实现的呢？我们一步一步的来抽丝剥茧。

OkhttpClient

OkhttpClient充当的是一个客户端的角色，负责收集请求，配置信息，创建用于实际发出请求的Call。下面根据我们实际项目顺序了解一下OkhttpClient的运行机制。

1 .OkhttpClient的创建

public OkHttpClient() {
    this(new Builder());
  }

  OkHttpClient(Builder builder) {
    this.dispatcher = builder.dispatcher;
    this.proxy = builder.proxy;
    this.protocols = builder.protocols;
    this.connectionSpecs = builder.connectionSpecs;
    this.interceptors = Util.immutableList(builder.interceptors);
    this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
    this.eventListenerFactory = builder.eventListenerFactory;
    this.proxySelector = builder.proxySelector;
    this.cookieJar = builder.cookieJar;
    this.cache = builder.cache;
    this.internalCache = builder.internalCache;
    this.socketFactory = builder.socketFactory;

    boolean isTLS = false;
    for (ConnectionSpec spec : connectionSpecs) {
      isTLS = isTLS || spec.isTls();
    }

    if (builder.sslSocketFactory != null || !isTLS) {
      this.sslSocketFactory = builder.sslSocketFactory;
      this.certificateChainCleaner = builder.certificateChainCleaner;
    } else {
      X509TrustManager trustManager = systemDefaultTrustManager();
      this.sslSocketFactory = systemDefaultSslSocketFactory(trustManager);
      this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
    }

    this.hostnameVerifier = builder.hostnameVerifier;
    this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
        certificateChainCleaner);
    this.proxyAuthenticator = builder.proxyAuthenticator;
    this.authenticator = builder.authenticator;
    this.connectionPool = builder.connectionPool;
    this.dns = builder.dns;
    this.followSslRedirects = builder.followSslRedirects;
    this.followRedirects = builder.followRedirects;
    this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
    this.connectTimeout = builder.connectTimeout;
    this.readTimeout = builder.readTimeout;
    this.writeTimeout = builder.writeTimeout;
    this.pingInterval = builder.pingInterval;

    if (interceptors.contains(null)) {
      throw new IllegalStateException("Null interceptor: " + interceptors);
    }
    if (networkInterceptors.contains(null)) {
      throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
    }
  }

OkhttpClient的创建也builder模式，但暴露给外部的是一new出来的，内部调用第二个采用builder构造方法。这样做是为了初始化builder中默认的重要参数，避免用户调用的时候这么多的参数用户很大可能上会配错一些东西。这样简化了操作。

我们如何发起请求,获得response呢

 Request request = new Request.Builder()
      .url(url)
      .build();

  Response response = client.newCall(request).execute();

OkhttpClient实现了Call.Factory的newCall()，这样为我们生产出来一个新的随时可以发起网络请求的Call对象；

/**
   * Prepares the {@code request} to be executed at some point in the future.
   */
@Override 
public Call newCall(Request request) {
    return RealCall.newRealCall(this, request, false /* for web socket */);
  }

可以看到他返回的是一个用RealCall从创建出来的而且是RealCall对象。

static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    // Safely publish the Call instance to the EventListener.
    RealCall call = new RealCall(client, originalRequest, forWebSocket);
    call.eventListener = client.eventListenerFactory().create(call);
    return call;
  }

这个RealCall对象正是对网络请求的真正发起者，起重大作用。RealCall实现了Call接口，我们看Call的定义。

public interface Call extends Cloneable {
//返回原始的Request
  Request request();

//立即执行网络请求
  Response execute() throws IOException;

 //在将来的某一时刻发起请求
  void enqueue(Callback responseCallback);

//判断是否被执行了
  boolean isExecuted();
//判断是否被取消了
  boolean isCanceled();

//创建出来一个新的Call对象
  Call clone();

  interface Factory {
    Call newCall(Request request);
  }
}

RealCall中复写的这些方法全权的执行了网络请求的工作。所以我们绕了这么大弯子终于看到在哪进行的网络请求了。

  @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this);
    }
  }
@Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

execute 和 enqueue中分别调用了
client.dispatcher().finished(this);;和 client.dispatcher().enqueue(new AsyncCall(responseCallback));
一个同步一个异步的。用dispatcher放到一个队列中进行分发运行，对于enqueue因为AsyncCall实现了Runnable接口，在dispatcher中的的线程池运行队列里的Runnable对象，执行executorService().execute(call);所以最后的网络请求还在这个AsyncCall对象对于runnable的实现中。他们共同的会调用

 Response response = getResponseWithInterceptorChain();

获得了Response对象。那么getResponseWithInterceptorChain();又做了什么。

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }

这里添加了okhttp默认的几个拦截器，并进入chain.proceed(originalRequest);至此进入了一大堆拦截器各种拦截最后返回response的模式，关于拦截器我们以后再说，现在先过。经过层层拦截器我们终于拿到了最终的响应对象。如此推演我们可以推断出我们真正的请求应在最后一个拦截器中。我们看一下：就是那个CallServerInterceptor

/** 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;

  public CallServerInterceptor(boolean forWebSocket) {
    this.forWebSocket = forWebSocket;
  }

  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();

    realChain.eventListener().requestHeadersStart(realChain.call());
    httpCodec.writeRequestHeaders(request);
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);

    Response.Builder responseBuilder = null;
    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.
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        httpCodec.flushRequest();
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } else if (!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.
        streamAllocation.noNewStreams();
      }
    }

    httpCodec.finishRequest();

    if (responseBuilder == null) {
      realChain.eventListener().responseHeadersStart(realChain.call());
      responseBuilder = httpCodec.readResponseHeaders(false);
    }

    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
      // server sent a 100-continue even though we did not request one.
      // try again to read the actual response
      responseBuilder = httpCodec.readResponseHeaders(false);

      response = responseBuilder
              .request(request)
              .handshake(streamAllocation.connection().handshake())
              .sentRequestAtMillis(sentRequestMillis)
              .receivedResponseAtMillis(System.currentTimeMillis())
              .build();

      code = response.code();
    }

    realChain.eventListener()
            .responseHeadersEnd(realChain.call(), response);

    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(httpCodec.openResponseBody(response))
          .build();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      streamAllocation.noNewStreams();
    }

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

    return response;
  }

  static final class CountingSink extends ForwardingSink {
    long successfulCount;

    CountingSink(Sink delegate) {
      super(delegate);
    }

    @Override public void write(Buffer source, long byteCount) throws IOException {
      super.write(source, byteCount);
      successfulCount += byteCount;
    }
  }
}

果不其然 我们看眼注释就知道了：This is the last interceptor in the chain. It makes a network call to the server. */
也验证了我们之前的猜测。intercept方法中确确实实的执行了网络请求，但在okhttp3.internal包中的类涉及底层更多一些，本人目前精力有限，只能分析到这层，但目前看来对于轮子我们了解了差不多了，大件都拆了，小件拆了也不一定能用的到，所以就到这层。
这里返回的 response就是我最终可以使用的response了。

这个response我们一层一层返回上层 我们在哪能拿到还记得么？那就是execute()同步请求中直接返回，enqueue中的callback中

@Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this);
    }
  }


  @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }


public interface Callback {
  /**
   * Called when the request could not be executed due to cancellation, a connectivity problem or
   * timeout. Because networks can fail during an exchange, it is possible that the remote server
   * accepted the request before the failure.
   */
  void onFailure(Call call, IOException e);

  /**
   * Called when the HTTP response was successfully returned by the remote server. The callback may
   * proceed to read the response body with {@link Response#body}. The response is still live until
   * its response body is {@linkplain ResponseBody closed}. The recipient of the callback may
   * consume the response body on another thread.
   *
   * <p>Note that transport-layer success (receiving a HTTP response code, headers and body) does
   * not necessarily indicate application-layer success: {@code response} may still indicate an
   * unhappy HTTP response code like 404 or 500.
   */
  void onResponse(Call call, Response response) throws IOException;
}

这样开发者朋友们就拿到可请求之后的Response了。这样我们终于看到okhttp官网上的那几段示例代码都做了什么事情了；我顺便贴出来，顺便也回味一下：

Examples

GET A URL

This program downloads a URL and print its contents as a string. Full source.

OkHttpClient client = new OkHttpClient();

String run(String url) throws IOException {
  Request request = new Request.Builder()
      .url(url)
      .build();

  Response response = client.newCall(request).execute();
  return response.body().string();
}

POST TO A SERVER

This program posts data to a service. Full source.

public static final MediaType JSON
    = MediaType.parse("application/json; charset=utf-8");

OkHttpClient client = new OkHttpClient();

String post(String url, String json) throws IOException {
  RequestBody body = RequestBody.create(JSON, json);
  Request request = new Request.Builder()
      .url(url)
      .post(body)
      .build();
  Response response = client.newCall(request).execute();
  return response.body().string();
}

总结

总结一下，整个网络请求过程中okhttpclient的角色就是准备材料，Call对象负责发起，但发起的意义在于把请求放到队列里执行，在此之后一直到最后请求成功这中间又精力了若干的拦截器。正是这些拦截器实现了更多更具有拓展性的工作，默认添加进去的拦截器如下：

 List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

从字面意义上来看包含了众多功能，而且我们后期还可以自己的定制自己的interceptor添加到这里面实现各式各样的功能需求，所以interceptor的设计师okhttp的绝妙之笔。关于interceptor我们后续有机会再行解析。