在OkHttp3中，其靈活性很大程度上體現在可以 intercept 其任意一個環節，而這個優勢便是okhttp3整個請求響應架構體系的精髓所在，先放出一張主框架請求流程圖，接著再分析原始碼。

Okhttp請求流程

String url = "http://wwww.baidu.com";
OkHttpClient okHttpClient = new OkHttpClient();
final Request request = new Request.Builder()
        .url(url)
        .build();
Call call = okHttpClient.newCall(request);
call.enqueue(new Callback() {
    @Override
    public void onFailure(Call call, IOException e) {
        Log.d(TAG, "onFailure: ");
    }

    @Override
    public void onResponse(Call call, Response response) throws IOException {
        Log.d(TAG, "onResponse: " + response.body().string());
    }
});
 

這大概是一個最簡單的一個例子了，在new OkHttpClient()內部使用構造器模式初始化了一些配置資訊：支援協議、任務分發器（其內部包含一個執行緒池，執行非同步請求）、連線池(其內部包含一個執行緒池，維護connection)、連線/讀/寫超時時長等資訊。

public Builder() {
    dispatcher = new Dispatcher(); //任務排程器
    protocols = DEFAULT_PROTOCOLS; //支援的協議
    connectionSpecs = DEFAULT_CONNECTION_SPECS;
    eventListenerFactory = EventListener.factory(EventListener.NONE);
    proxySelector = ProxySelector.getDefault();
    cookieJar = CookieJar.NO_COOKIES;
    socketFactory = SocketFactory.getDefault();
    hostnameVerifier = OkHostnameVerifier.INSTANCE;
    certificatePinner = CertificatePinner.DEFAULT;
    proxyAuthenticator = Authenticator.NONE;
    authenticator = Authenticator.NONE;
    connectionPool = new ConnectionPool(); //連線池
    dns = Dns.SYSTEM;
    followSslRedirects = true;
    followRedirects = true;
    retryOnConnectionFailure = true;
    connectTimeout = 10_000;//超時時間
    readTimeout = 10_000;
    writeTimeout = 10_000;
    pingInterval = 0;
}
 

第一行建立了一個Dispatcher任務排程器，它定義了三個雙向任務佇列，兩個非同步佇列：準備執行的請求佇列 readyAsyncCalls、正在執行的請求佇列 runningAsyncCalls；一個正在執行的同步請求佇列 runningSyncCalls；

public final class Dispatcher {
    private int maxRequests = 64; //最大請求數量
    private int maxRequestsPerHost = 5; //每臺主機最大的請求數量
    private @Nullable Runnable idleCallback;
    
    /** Executes calls. Created lazily. */
    private @Nullable ExecutorService executorService; //執行緒池
    
    /** Ready async calls in the order they'll be run. */
    private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
    
    /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
    private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
    
    /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
    private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
    
    /** 這個執行緒池沒有核心執行緒，執行緒數量沒有限制，空閒60s就會回收*/
    public synchronized ExecutorService executorService() {
        if (executorService == null) {
          executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
              new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
        }
        return executorService;
    }
}  
 

另外還有一個執行緒池 executorService ，這個執行緒池跟Android中的CachedThreadPool非常類似，這種型別的執行緒池，適用於大量的耗時較短的非同步任務。下一篇文章 將對OkHttp框架中的執行緒池做一個總結。

接下來接著看Request的構造，這個例子Request比較簡單，指定了請求方式 GET 和請求 url

  public static class Builder {
    HttpUrl url;
    String method;
    Headers.Builder headers;
    RequestBody body;
    Object tag;

    public Builder() {
      this.method = "GET";
      this.headers = new Headers.Builder();
    }
    
    public Builder url(HttpUrl url) {
      if (url == null) throw new NullPointerException("url == null");
      this.url = url;
      return this;
    }
    public Request build() {
        if (url == null) throw new IllegalStateException("url == null");
        return new Request(this);
    }
    ...
}    

緊接著通過 OkHttpClient 和 Request 構造一個 Call物件，它的實現是RealCall

public Call newCall(Request request) {
    return RealCall.newRealCall(this, request, false /* for web socket */);
}

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;
}

private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    this.client = client;
    this.originalRequest = originalRequest;
    this.forWebSocket = forWebSocket;
    this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
}

可以看到在 RealCall 的構造方法中建立了一個RetryAndFollowUpInterceptor，用於處理請求錯誤和重定向等，這是 Okhttp 框架的精髓 interceptor chain 中的一環，預設情況下也是第一個攔截器，除非呼叫 OkHttpClient.Builder#addInterceptor(Interceptor) 來新增全域性的攔截器。關於攔截器鏈的順序參見 RealCall#getResponseWithInterceptorChain() 方法。

RealCall#enqueue(Callback)

public void enqueue(Callback responseCallback) {
    synchronized (this) {
        //每個請求只能之執行一次
        if (executed) throw new IllegalStateException("Already Executed");
        executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
}

可以看到，一個 Call 只能執行一次，否則會拋異常，這裡建立了一個 AsyncCall 並將Callback傳入，接著再交給任務分發器 Dispatcher 來進一步處理。

synchronized void enqueue(AsyncCall call) {
    //正在執行的任務數量小於最大值（64），並且此任務所屬主機的正在執行任務小於最大值（5）
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
        runningAsyncCalls.add(call);
        executorService().execute(call);
    } else {
        readyAsyncCalls.add(call);
    }
}

從 Dispatcher#enqueue()方法的策略可以看出，對於請求的入隊做了一些限制，若正在執行的請求數量小於最大值（預設64），並且此請求所屬主機的正在執行任務小於最大值（預設5），就加入正在執行的佇列並通過執行緒池來執行該任務，否則加入準備執行佇列中。

• 流程圖

現在回頭看看 AsyncCall ，它繼承自 NamedRunnable，而 NamedRunnable實現了 Runnable 介面，它的作用有2個： ①採用模板方法的設計模式，讓子類將具體的操作放在 execute()方法中; ②給執行緒指定一個名字，比如傳入模組名稱，方便監控執行緒的活動狀態；

public abstract class NamedRunnable implements Runnable {
  protected final String name;

  public NamedRunnable(String format, Object... args) {
    this.name = Util.format(format, args);
  }

  @Override public final void run() {
    String oldName = Thread.currentThread().getName();
    Thread.currentThread().setName(name);
    try {
      //採用模板方法讓子類將具體的操作放到此execute()方法
      execute();
    } finally {
      Thread.currentThread().setName(oldName);
    }
  }

  protected abstract void execute();
}

final class AsyncCall extends NamedRunnable {
    //省略...
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
        //呼叫 getResponseWithInterceptorChain()獲得響應內容
        Response response = getResponseWithInterceptorChain(); //①
        if (retryAndFollowUpInterceptor.isCanceled()) {
          //這個標記為主要是避免異常時2次回撥
          signalledCallback = true;
          //回撥Callback告知失敗
          responseCallback.onFailure(RealCall.this, new IOException("Canceled")); 
        } else {
          signalledCallback = true;
          //回撥Callback，將響應內容傳回去
          responseCallback.onResponse(RealCall.this, response);
        }
      } catch (IOException e) {
        if (signalledCallback) {
          // Do not signal the callback twice!
          Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
        } else {
          eventListener.callFailed(RealCall.this, e);
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        //不管請求成功與否，都進行finished()操作
        client.dispatcher().finished(this);//②
      }
    }
}

先看註釋②的行finally塊中執行的 client.dispatcher().finished(this)

void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
}

private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
        //從正在執行的佇列中將其移除
        if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
        if (promoteCalls) promoteCalls(); //推動下一個任務的執行
        runningCallsCount = runningCallsCount();//同步+非同步的正在執行任務數量
        idleCallback = this.idleCallback;
    }
    //如果沒有正在執行的任務，且idleCallback不為null，則回撥通知空閒了
    if (runningCallsCount == 0 && idleCallback != null) {
        idleCallback.run();
    }
}

其中promoteCalls()為推動下一個任務執行，其實它做的也很簡單，就是在條件滿足的情況下，將 readyAsyncCalls 中的任務移動到 runningAsyncCalls中，並交給執行緒池來執行，以下是它的實現。

private void promoteCalls() {
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
    
    //若條件允許，將readyAsyncCalls中的任務移動到runningAsyncCalls中，並交給執行緒池執行
    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();
    
      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        executorService().execute(call);
      }
      //當runningAsyncCalls滿了，直接退出迭代
      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
}

接下來就回到註釋①處的響應內容的獲取 getResponseWithInterceptorChain()

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>(); //這是一個List，是有序的
    interceptors.addAll(client.interceptors());//首先新增的是使用者新增的全域性攔截器
    interceptors.add(retryAndFollowUpInterceptor); //錯誤、重定向攔截器
   //橋接攔截器，橋接應用層與網路層，新增必要的頭、
    interceptors.add(new BridgeInterceptor(client.cookieJar())); 
    //快取處理，Last-Modified、ETag、DiskLruCache等
    interceptors.add(new CacheInterceptor(client.internalCache())); 
    //連線攔截器
    interceptors.add(new ConnectInterceptor(client));
    //從這就知道，通過okHttpClient.Builder#addNetworkInterceptor()傳進來的攔截器只對非網頁的請求生效
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    //真正訪問伺服器的攔截器
    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);
}

可以看這塊重點就是 interceptors 這個集合，首先將前面的 client.interceptors() 全部加入其中，還有在建立 RealCall時的 retryAndFollowUpInterceptor加入其中，接著還建立並添加了BridgeInterceptor、CacheInterceptor、ConnectInterceptor、CallServerInterceptor，最後通過RealInterceptorChain#proceed(Request)來執行整個 interceptor chain，可見把這個攔截器鏈搞清楚，整體流程也就明朗了。

RealInterceptorChain#proceed()

public Response proceed(Request request) throws IOException {
    return proceed(request, streamAllocation, httpCodec, connection);
}

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
  RealConnection connection) throws IOException {
    //省略異常處理...
    
    // Call the next interceptor in the chain.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);
    
    //省略異常處理...
    return response;
}

從這段實現可以看出，是按照新增到 interceptors 集合的順序，逐個往下呼叫攔截器的intercept()方法，所以在前面的攔截器會先被呼叫。這個例子中自然就是 RetryAndFollowUpInterceptor 了。

public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();
    //建立一個StreamAllocation
    StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;

    //統計重定向次數，不能大於20
    int followUpCount = 0; 
    Response priorResponse = null;
    while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
        //呼叫下一個interceptor的來獲得響應內容
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
          throw e.getLastConnectException();
        }
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
        boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
        if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
        releaseConnection = false;
        continue;
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }
    
     //重定向處理    
      Request followUp = followUpRequest(response, streamAllocation.route());

      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

      closeQuietly(response.body());

      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }

      if (!sameConnection(response, followUp.url())) {
        streamAllocation.release();
        streamAllocation = new StreamAllocation(client.connectionPool(),
            createAddress(followUp.url()), call, eventListener, callStackTrace);
        this.streamAllocation = streamAllocation;
      } else if (streamAllocation.codec() != null) {
        throw new IllegalStateException("Closing the body of " + response
            + " didn't close its backing stream. Bad interceptor?");
      }

      request = followUp;
      priorResponse = response;
    }
}

這個攔截器就如同它的名字retry and followUp，主要負責錯誤處理和重定向等問題，比如路由錯誤、IO異常等。

接下來就到了BridgeInterceptor#intercept()，在這個攔截器中，添加了必要請求頭資訊，gzip處理等。

public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();
    
    //從這開始給請求添加了一些請求頭資訊
    RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }

      long contentLength = body.contentLength();
      if (contentLength != -1) {
        requestBuilder.header("Content-Length", Long.toString(contentLength));
        requestBuilder.removeHeader("Transfer-Encoding");
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked");
        requestBuilder.removeHeader("Content-Length");
      }
    }

    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }

    if (userRequest.header("Connection") == null) {
      requestBuilder.header("Connection", "Keep-Alive");
    }

    // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }

    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }

    Response networkResponse = chain.proceed(requestBuilder.build());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);

    if (transparentGzip
        && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
        && HttpHeaders.hasBody(networkResponse)) {
      GzipSource responseBody = new GzipSource(networkResponse.body().source());
      Headers strippedHeaders = networkResponse.headers().newBuilder()
          .removeAll("Content-Encoding")
          .removeAll("Content-Length")
          .build();
      responseBuilder.headers(strippedHeaders);
      String contentType = networkResponse.header("Content-Type");
      responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
}

這個攔截器處理請求資訊、cookie、gzip等，接著往下是 CacheInterceptor

public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    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) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      //呼叫下一個攔截器進行網路請求    
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

    return response;
}

這個攔截器主要工作是做做快取處理，如果有有快取並且快取可用，那就使用快取，否則進行呼叫下一個攔截器 ConnectionInterceptor 進行網路請求，並將響應內容快取。

public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    
    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();
    
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

這個攔截器主要是開啟一個到目標伺服器的 connection 並呼叫下一個攔截器 CallServerInterceptor，這是攔截器鏈最後一個攔截器，它向伺服器發起真正的網路請求。

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;
 }

從上面的請求流程圖可以看出，OkHttp的攔截器鏈可謂是其整個框架的精髓，使用者可傳入的 interceptor 分為兩類： ①一類是全域性的 interceptor，該類 interceptor 在整個攔截器鏈中最早被呼叫，通過 OkHttpClient.Builder#addInterceptor(Interceptor) 傳入； ②另外一類是非網頁請求的 interceptor ，這類攔截器只會在非網頁請求中被呼叫，並且是在組裝完請求之後，真正發起網路請求前被呼叫，所有的 interceptor 被儲存在 List<Interceptor> interceptors 集合中，按照新增順序來逐個呼叫，具體可參考 RealCall#getResponseWithInterceptorChain() 方法。通過 OkHttpClient.Builder#addNetworkInterceptor(Interceptor) 傳入；