OKHttp源碼解析

rbr008 8年前發布 | 53K 次閱讀 OkHttp Android開發移動開發 HTTP Cache

前言：對于 OkHttp 我接觸的時間其實不太長，一直都是使用Retrofit + OkHttp 來做網絡請求的，但是有同學說面試的時候可能會問框架源碼，這樣光是會用是不夠的，于是便萌生了通一通OkHttp源碼的念頭。經過大約一周的時間，源碼看了個大概（說來慚愧，也就知道里面的原理），這里變向大家介紹一下我的所得，希望對大家能有所幫助。這里推薦兩篇博文： OkHttp 官方教程解析 - 徹底入門 OkHttp 使用和拆輪子系列：拆 OkHttp 前者能夠讓你入門OkHttp,后者能讓你明白OkHttp的原理，我就是看的后者去看的源碼，如果看我的不太懂，大家可以去看看上面的。同時，歡迎大家交流，提出意見，謝謝！

總體流程

下面的流程圖是由上面的文章抄來的（自己畫的圖，用的visio）

整個流程是，通過 OkHttpClient 將構建的 Request 轉換為Call，然后在RealCall中進行異步或同步任務，最后通過一些的攔截器 interceptor 發出網絡請求和得到返回的 response 。

將流程大概是這么個流程，大家可以有個大概的印象，繼續向下看：

OkHttp流程圖.jpg

為了讓大家有更深的印象，我準備追蹤一個 GET 網絡請求的具體流程，來介紹在源碼中發生了什么。

GET請求過程

這是利用 OkHttp 寫一個Get請求步驟，這里是一個同步的請求,異步的下面也會說：

//HTTP GET
    public String get(String url) throws IOException {
        //新建OKHttpClient客戶端
        OkHttpClient client = new OkHttpClient();
        //新建一個Request對象
        Request request = new Request.Builder()
                .url(url)
                .build();
        //Response為OKHttp中的響應
        Response response = client.newCall(request).execute();
        if (response.isSuccessful()) {
            return response.body().string();
        }else{
            throw new IOException("Unexpected code " + response);
        }
    }

OKHttpClient：流程的總控制者

OkHttpClient的類設計圖

使用OkHttp的時候我們都會創建一個OkHttpClient對象:

OkHttpClient client = new OkHttpClient();

這是做什么的呢？看下builder里面的參數：

final Dispatcher dispatcher;  //分發器
    final Proxy proxy;  //代理
    final List<Protocol> protocols; //協議
    final List<ConnectionSpec> connectionSpecs; //傳輸層版本和連接協議
    final List<Interceptor> interceptors; //攔截器
    final List<Interceptor> networkInterceptors; //網絡攔截器
    final ProxySelector proxySelector; //代理選擇
    final CookieJar cookieJar; //cookie
    final Cache cache; //緩存
    final InternalCache internalCache;  //內部緩存
    final SocketFactory socketFactory;  //socket 工廠
    final SSLSocketFactory sslSocketFactory; //安全套接層socket 工廠，用于HTTPS
    final CertificateChainCleaner certificateChainCleaner; // 驗證確認響應證書 適用 HTTPS 請求連接的主機名。
    final HostnameVerifier hostnameVerifier;    //  主機名字確認
    final CertificatePinner certificatePinner;  //  證書鏈
    final Authenticator proxyAuthenticator;     //代理身份驗證
    final Authenticator authenticator;      // 本地身份驗證
    final ConnectionPool connectionPool;    //連接池,復用連接
    final Dns dns;  //域名
    final boolean followSslRedirects;  //安全套接層重定向
    final boolean followRedirects;  //本地重定向
    final boolean retryOnConnectionFailure; //重試連接失敗
    final int connectTimeout;    //連接超時
    final int readTimeout; //read 超時
    final int writeTimeout; //write 超時

在這些聲明的對象中可以看出來，幾乎所有用到的類都和 OkHttpClient 有關系。事實上，你能夠通過它來設置改變一些參數，因為他是通過建造者模式實現的，因此你可以通過 builder() 來設置。如果不進行設置，在 Builder 中就會使用默認的設置：

dispatcher = new Dispatcher();
            protocols = DEFAULT_PROTOCOLS;
            connectionSpecs = DEFAULT_CONNECTION_SPECS;
            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();
            dns = Dns.SYSTEM;
            followSslRedirects = true;
            followRedirects = true;
            retryOnConnectionFailure = true;
            connectTimeout = 10_000;
            readTimeout = 10_000;
            writeTimeout = 10_000;

看到這，如果你還不明白的話，也沒關系，在 OkHttp 中只是設置用的的各個東西。真正的流程要從里面的 newCall() 方法中說起：

/**
        *  Prepares the {@code request} to be executed at some point in the future.
        *  準備將要被執行的request
        */
        @Override
        public Call newCall(Request request) {
            return new RealCall(this, request);
        }

當通過建造者模式創建了 Request 之后（這個沒什么好說），緊接著就通過下面的代碼來獲得 Response

大家還記得上面做 GET 請求時的這句代碼吧：

Response response = client.newCall(request).execute(); 這就代碼就開啟了整個GET請求的流程：

RealCall：真正的請求執行者。

先看一下他的構造方法：

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

可以看到他傳過來一個 OkHttpClient 對象和一個 originalRequest （我們創建的 Request ）。

接下來看它的 execute() 方法：

@Override
    public Response execute() throws IOException {
        synchronized (this) {
            if (executed) throw new IllegalStateException("Already Executed"); //(1)
            executed = true;
        }
        try {
            client.dispatcher.executed(this);//(2)
            Response result = getResponseWithInterceptorChain();//(3)
            if (result == null) throw new IOException("Canceled");
            return result;
        }finally {
            client.dispatcher.finished(this);//(4)
        }
    }

檢查這個 call 是否已經被執行了，每個 call 只能被執行一次，如果想要一個完全一樣的 call ，可以利用 all#clone 方法進行克隆。
利用 client.dispatcher().executed(this) 來進行實際執行， dispatcher 是剛才看到的 OkHttpClient.Builder 的成員之一，它的文檔說自己是異步 HTTP 請求的執行策略，現在看來，同步請求它也有摻和。
調用 getResponseWithInterceptorChain() 函數獲取 HTTP 返回結果，從函數名可以看出，這一步還會進行一系列“攔截”操作。

最后還要通知 dispatcher 自己已經執行完畢。

dispatcher 這里我們不過度關注，在同步執行的流程中，涉及到 dispatcher 的內容只不過是告知它我們的執行狀態，比如開始執行了（調用 executed ），比如執行完畢了（調用 finished ），在異步執行流程中它會有更多的參與。

真正發出網絡請求，解析返回結果的，還是 getResponseWithInterceptorChain ：

//攔截器的責任鏈。
 private Response getResponseWithInterceptorChain() throws IOException {
     // Build a full stack of interceptors.
     List<Interceptor> interceptors = new ArrayList<>();
     interceptors.addAll(client.interceptors());     //(1)
     interceptors.add(retryAndFollowUpInterceptor);    //(2)
     interceptors.add(new BridgeInterceptor(client.cookieJar()));    //(3)
     interceptors.add(new CacheInterceptor(client.internalCache()));    //(4)
     interceptors.add(new ConnectInterceptor(client));    //(5)
     if (!retryAndFollowUpInterceptor.isForWebSocket()) {
         interceptors.addAll(client.networkInterceptors());    //(6)
     }
     interceptors.add(new CallServerInterceptor(
             retryAndFollowUpInterceptor.isForWebSocket()));     //(7)

     Interceptor.Chain chain = new RealInterceptorChain(
             interceptors, null, null, null, 0, originalRequest);
     return chain.proceed(originalRequest); //  <<=========開始鏈式調用
 }

在配置 OkHttpClient 時設置的 interceptors ；
負責失敗重試以及重定向的 RetryAndFollowUpInterceptor ；
負責把用戶構造的請求轉換為發送到服務器的請求、把服務器返回的響應轉換為用戶友好的響應的 BridgeInterceptor ；
負責讀取緩存直接返回、更新緩存的 CacheInterceptor ；
負責和服務器建立連接的 ConnectInterceptor ；
配置 OkHttpClient 時設置的 networkInterceptors ；
負責向服務器發送請求數據、從服務器讀取響應數據的 CallServerInterceptor 。
在 return chain.proceed(originalRequest); 中開啟鏈式調用：

RealInterceptorChain

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      Connection connection) throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();

    calls++;

    // If we already have a stream, confirm that the incoming request will use it.
    //如果我們已經有一個stream。確定即將到來的request會使用它
    if (this.httpCodec != null && !sameConnection(request.url())) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must retain the same host and port");
    }

    // If we already have a stream, confirm that this is the only call to chain.proceed().
    //如果我們已經有一個stream， 確定chain.proceed()唯一的call
    if (this.httpCodec != null && calls > 1) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must call proceed() exactly once");
    }

    // Call the next interceptor in the chain.
    //調用鏈的下一個攔截器
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

    // Confirm that the next interceptor made its required call to chain.proceed().
    if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
      throw new IllegalStateException("network interceptor " + interceptor
          + " must call proceed() exactly once");
    }

    // Confirm that the intercepted response isn't null.
    if (response == null) {
      throw new NullPointerException("interceptor " + interceptor + " returned null");
    }

    return response;
  }
`

代碼很多，但是主要是進行一些判斷，主要的代碼在這：

// Call the next interceptor in the chain.
    //調用鏈的下一個攔截器
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);    //(1)
    Interceptor interceptor = interceptors.get(index);     //(2)
    Response response = interceptor.intercept(next);    //(3)

實例化下一個攔截器對應的 RealIterceptorChain 對象，這個對象會在傳遞給當前的攔截器
得到當前的攔截器： interceptors 是存放攔截器的 ArryList
調用當前攔截器的 intercept() 方法，并將下一個攔截器的 RealIterceptorChain 對象傳遞下去
除了在client中自己設置的 interceptor ,第一個調用的就是 retryAndFollowUpInterceptor
RetryAndFollowUpInterceptor:負責失敗重試以及重定向
直接上代碼

@Override 
public Response intercept(Chain chain) throws IOException {
        Request request = chain.request();
        streamAllocation = new StreamAllocation(
                client.connectionPool(), createAddress(request.url()));
        int followUpCount = 0;
        Response priorResponse = null;
        while (true) {
            if (canceled) {
                streamAllocation.release();
                throw new IOException("Canceled");
            }

            Response response = null;
            boolean releaseConnection = true;
            try {
                response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);    //(1)
                releaseConnection = false;
            } catch (RouteException e) {
                // The attempt to connect via a route failed. The request will not have been sent.
                //通過路線連接失敗，請求將不會再發送
                if (!recover(e.getLastConnectException(), true, request)) throw e.getLastConnectException();
                releaseConnection = false;
                continue;
            } catch (IOException e) {
                // An attempt to communicate with a server failed. The request may have been sent.
                // 與服務器嘗試通信失敗，請求不會再發送。
                if (!recover(e, false, request)) throw e;
                releaseConnection = false;
                continue;
            } finally {
                // We're throwing an unchecked exception. Release any resources.
                //拋出未檢查的異常，釋放資源
                if (releaseConnection) {
                    streamAllocation.streamFailed(null);
                    streamAllocation.release();
                }
            }

            // Attach the prior response if it exists. Such responses never have a body.
            // 附加上先前存在的response。這樣的response從來沒有body
            // TODO: 2016/8/23 這里沒賦值，豈不是一直為空？
            if (priorResponse != null) { //  (2)
                response = response.newBuilder()
                        .priorResponse(priorResponse.newBuilder()
                                .body(null)
                                .build())
                        .build();
            }

            Request followUp = followUpRequest(response); //判斷狀態碼 (3)
            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) {
                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()));
            } 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;
        }
    }

這里是最關鍵的代碼，可以看出在 response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null); 中直接調用了下一個攔截器，然后捕獲可能的異常來進行操作
這里沒看太懂，有點坑，以后補
這里對于返回的response的狀態碼進行判斷，然后進行處理

BridgeInterceptor：

負責把用戶構造的請求轉換為發送到服務器的請求、把服務器返回的響應轉換為用戶友好的響應的。

@Override 
public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();

    //檢查request。將用戶的request轉換為發送到server的請求
    RequestBody body = userRequest.body();     //(1)
    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.
    //GZIP壓縮
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == 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());   //(2)

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers()); //(3)

    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);
      responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
  }

在（1）和（2）之間， BridgeInterceptor 對于 request 的格式進行檢查，讓構建了一個新的 request
調用下一個 interceptor 來得到response
（3）下面就是對得到的response進行一些判斷操作，最后將結果返回。

@Override 
public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null        //=============(1)
        ? cache.get(chain.request()) //通過request得到緩存
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); //根據request來得到緩存策略===========(2)
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

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

    if (cacheCandidate != null && cacheResponse == null) { //存在緩存的response，但是不允許緩存
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it. 緩存不適合，關閉
    }

    // If we're forbidden from using the network and the cache is insufficient, fail.
      //如果我們禁止使用網絡，且緩存為null，失敗
    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(EMPTY_BODY)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {  //沒有網絡請求，跳過網絡，返回緩存
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);//網絡請求攔截器    //======(3)
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
        //如果我們因為I/O或其他原因崩潰，不要泄漏緩存體
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // If we have a cache response too, then we're doing a conditional get.========(4)
      //如果我們有一個緩存的response，然后我們正在做一個條件GET
    if (cacheResponse != null) {
      if (validate(cacheResponse, networkResponse)) { //比較確定緩存response可用
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .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()).
          //更新緩存，在剝離content-Encoding之前
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

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

    if (HttpHeaders.hasBody(response)) {    // =========(5)
      CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);
      response = cacheWritingResponse(cacheRequest, response);
    }

    return response;
  }

首先，根據 request 來判斷 cache 中是否有緩存的 response ，如果有，得到這個 response ，然后進行判斷當前 response 是否有效，沒有將 cacheCandate 賦值為空。
根據request判斷緩存的策略，是否要使用了網絡，緩存或兩者都使用
調用下一個攔截器，決定從網絡上來得到 response
如果本地已經存在 cacheResponse ，那么讓它和網絡得到的 networkResponse 做比較，決定是否來更新緩存的 cacheResponse

緩存未經緩存過的 response

ConnectInterceptor:建立連接

@Override 
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, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

實際上建立連接就是創建了一個 HttpCodec 對象，它將在后面的步驟中被使用，那它又是何方神圣呢？它是對 HTTP 協議操作的抽象，有兩個實現： Http1Codec 和 Http2Codec ，顧名思義，它們分別對應 HTTP/1.1 和 HTTP/2 版本的實現。

在Http1Codec中，它利用 Okio 對Socket的讀寫操作進行封裝， Okio 以后有機會再進行分析，現在讓我們對它們保持一個簡單地認識：它對java.io和java.nio進行了封裝，讓我們更便捷高效的進行 IO 操作。

而創建 HttpCodec 對象的過程涉及到 StreamAllocation、RealConnection ，代碼較長，這里就不展開，這個過程概括來說，就是找到一個可用的 RealConnection ，再利用 RealConnection 的輸入輸出（ BufferedSource 和 BufferedSink ）創建 HttpCodec 對象，供后續步驟使用。

NetworkInterceptors

配置OkHttpClient時設置的 NetworkInterceptors。

CallServerInterceptor：發送和接收數據

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

    long sentRequestMillis = System.currentTimeMillis();
    httpCodec.writeRequestHeaders(request);

    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {   //===(1)
      Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
      BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
      request.body().writeTo(bufferedRequestBody);
      bufferedRequestBody.close();
    }

    httpCodec.finishRequest();

    Response response = httpCodec.readResponseHeaders()     //====(2)
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    if (!forWebSocket || response.code() != 101) {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

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

    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;
  }

檢查請求方法，用 Httpcodec 處理 request
進行網絡請求得到 response
返回r esponse

總結

前面說了攔截器用了責任鏈設計模式 ,它將請求一層一層向下傳，知道有一層能夠得到Resposne就停止向下傳遞，然后將 response 向上面的攔截器傳遞，然后各個攔截器會對 respone 進行一些處理，最后會傳到 RealCall 類中通過 execute 來得到 esponse 。

異步請求的流程：

異步get請求示例如下：

private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://publicobject.com/helloworld.txt")
        .build();

    client.newCall(request).enqueue(new Callback() {
      @Override 
      public void onFailure(Call call, IOException e) {
        e.printStackTrace();
      }

      @Override 
      public void onResponse(Call call, Response response) throws IOException {
        if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

        Headers responseHeaders = response.headers();
        for (int i = 0, size = responseHeaders.size(); i < size; i++) {
          System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
        }

        System.out.println(response.body().string());
      }
    });
  }

由代碼中 client.newCall(request).enqueue(Callback) ，開始我們知道 client.newCall(request) 方法返回的是 RealCall 對象，接下來繼續向下看 enqueue() 方法:

//異步任務使用
    @Override 
    public void enqueue(Callback responseCallback) {
        synchronized (this) {
            if (executed) throw new IllegalStateException("Already Executed");
            executed = true;
        }
        client.dispatcher().enqueue(new AsyncCall(responseCallback));
    }

調用了上面我們沒有詳細說的 Dispatcher 類中的 enqueue(Call ) 方法.接著繼續看：

synchronized void enqueue(AsyncCall call) {
        if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
            runningAsyncCalls.add(call);
            executorService().execute(call);
        } else {
            readyAsyncCalls.add(call);
        }
    }

如果中的 runningAsynCalls 不滿，且 call 占用的 host 小于最大數量，則將 call 加入到 runningAsyncCalls 中執行，同時利用線程池執行 call ；否者將 call 加入到 readyAsyncCalls 中。 runningAsyncCalls 和 readyAsyncCalls 是什么呢？看下面：

/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>(); //正在準備中的異步請求隊列

/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>(); //運行中的異步請求

/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); //同步請求

call 加入到線程池中執行了。現在再看 AsynCall的代碼，它是 RealCall 中的內部類:

//異步請求
    final class AsyncCall extends NamedRunnable {
        private final Callback responseCallback;

        private AsyncCall(Callback responseCallback) {
            super("OkHttp %s", redactedUrl());
            this.responseCallback = responseCallback;
        }

        String host() {
            return originalRequest.url().host();
        }

        Request request() {
            return originalRequest;
        }

        RealCall get() {
            return RealCall.this;
        }

        @Override protected void execute() {
            boolean signalledCallback = false;
            try {
                Response response = getResponseWithInterceptorChain();
                if (retryAndFollowUpInterceptor.isCanceled()) {
                    signalledCallback = true;
                    responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
                } else {
                    signalledCallback = true;
                    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 {
                    responseCallback.onFailure(RealCall.this, e);
                }
            } finally {
                client.dispatcher().finished(this);
            }
        }
    }

AysncCall 中的 execute() 中的方法，同樣是通過 Response response = getResponseWithInterceptorChain(); 來獲得response，這樣異步任務也同樣通過了interceptor，剩下的流程就和上面一樣了。

結語：

看到這，不知道你是否明白了OkHttp的請求過程，如果有什么問題或意見，歡迎私信。

參考

來自：http://www.jianshu.com/p/27c1554b7fee

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