寫(xiě)在前面

作為一名android開(kāi)發(fā)者迫吐，要時(shí)時(shí)刻刻跟隨技術(shù)的發(fā)展潮流宰缤，OKHttp作為當(dāng)下最流行的網(wǎng)絡(luò)請(qǐng)求框架我們不得不重視痊剖，它的原理也幾乎是面試時(shí)必問(wèn)的問(wèn)題告丢，故而對(duì)其進(jìn)行學(xué)習(xí)并紀(jì)錄之枪蘑。

幾個(gè)類

• OKHttpClient
  okhttp3在項(xiàng)目中發(fā)起請(qǐng)求的代碼如下：
  okHttpClient.newCall(request).execute();
  OKHttpClient 類中組合了很多的類對(duì)象，并且繼承了Call.Factory岖免，提供的方法只有一個(gè)：newCall岳颇，返回的是一個(gè)Call對(duì)象(實(shí)際是RealCall是Call的實(shí)現(xiàn)類)。

@Override 
public Call newCall(Request request) {
    return new RealCall(this, request);
  }

使用okHttpClient最好創(chuàng)建一個(gè)單例颅湘，因?yàn)槊恳粋€(gè)client都有自己的一個(gè)連接池connection pool和線程池thread pools话侧。重用這些連接池和線程池可以減少延遲和節(jié)約內(nèi)存。
okHttpClient使用了builder模式

public static final class Builder {
    Dispatcher dispatcher;
    Proxy proxy;
    List<Protocol> protocols;
    List<ConnectionSpec> connectionSpecs;
    final List<Interceptor> interceptors = new ArrayList<>();
    final List<Interceptor> networkInterceptors = new ArrayList<>();
    ProxySelector proxySelector;
    CookieJar cookieJar;
    Cache cache;
    InternalCache internalCache;
    SocketFactory socketFactory;
    SSLSocketFactory sslSocketFactory;
    CertificateChainCleaner certificateChainCleaner;
    HostnameVerifier hostnameVerifier;
    CertificatePinner certificatePinner;
    Authenticator proxyAuthenticator;
    Authenticator authenticator;
    ConnectionPool connectionPool;
    Dns dns;
    boolean followSslRedirects;
    boolean followRedirects;
    boolean retryOnConnectionFailure;
    int connectTimeout;
    int readTimeout;
    int writeTimeout;
    ...
}

可以看到OKHttpClient中包含的所有字段闯参。

• Dispatcher
  Dispatcher我們可以理解為一個(gè)執(zhí)行策略（官方這樣說(shuō)的：Policy on when async requests are executed.）瞻鹏，當(dāng)我們調(diào)用newCall時(shí)悲立，它不斷的從RequestQueue中取出請(qǐng)求（Call），該引擎有同步和異步請(qǐng)求新博，同步請(qǐng)求通過(guò)Call.execute()直接返 回當(dāng)前的Response薪夕，而異步請(qǐng)求會(huì)把當(dāng)前的請(qǐng)求Call.enqueue添加（AsyncCall）到請(qǐng)求隊(duì)列中，并通過(guò)回調(diào)（Callback） 的方式來(lái)獲取最后結(jié)果赫悄。

synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }

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

注意到原献，這里enqueue方法中有一個(gè)判斷：如果當(dāng)前運(yùn)行的異步請(qǐng)求隊(duì)列長(zhǎng)度小于最大請(qǐng)求數(shù)，也就是64,并且主機(jī)的請(qǐng)求數(shù)小于每個(gè)主機(jī)的請(qǐng)求數(shù)也就是5,則把當(dāng)前請(qǐng)求添加到 運(yùn)行隊(duì)列埂淮，接著交給線程池ExecutorService處理姑隅，否則則放置到readAsyncCall進(jìn)行緩存，等待執(zhí)行倔撞。

• Call

public interface Call {

  Request request();

  Response execute() throws IOException;

  void enqueue(Callback responseCallback);

  void cancel();

  boolean isExecuted();

  boolean isCanceled();

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

Call是一個(gè)接口類讲仰，定義了Http請(qǐng)求的方法，并提供了一個(gè)內(nèi)部接口Factory痪蝇，OkHttpClient即實(shí)現(xiàn)了該接口叮盘。

• RealCall
  RealCall是Call的實(shí)現(xiàn)類，里面最主要的兩個(gè)方法是execute和enqueue霹俺。

final class RealCall implements Call {
   ...
   @Override
    public Response execute() throws IOException {
       synchronized (this) {
         if (executed) throw new IllegalStateException("Already Executed");
         executed = true;
       }
       try {
         client.dispatcher().executed(this);
         Response result = getResponseWithInterceptorChain();
         if (result == null) throw new IOException("Canceled");
         return result;
       } finally {
         client.dispatcher().finished(this);
       }
     }
   ...
   @Override 
   public void enqueue(Callback responseCallback) {
       synchronized (this) {
         if (executed) throw new IllegalStateException("Already Executed");
         executed = true;
       }
       client.dispatcher().enqueue(new AsyncCall(responseCallback));
     }
   ...
}

可以看到該類中的execute和enqueue都調(diào)用了dispatcher中的executed和enqueue（代碼前面已貼）柔吼。
上面的代碼中也可以解釋我們前面所說(shuō)的：“同步請(qǐng)求通過(guò)Call.execute()直接返 回當(dāng)前的Response，而異步請(qǐng)求會(huì)把當(dāng)前的請(qǐng)求Call.enqueue添加（AsyncCall）到請(qǐng)求隊(duì)列中丙唧，并通過(guò)回調(diào)（Callback） 的方式來(lái)獲取最后結(jié)果愈魏。”
execute通過(guò)getResponseWithInterceptorChain獲取返回Response想际。
同步方法上面的代碼已經(jīng)足夠了培漏， 這里重點(diǎn)說(shuō)一下異步請(qǐng)求如何獲得返回的結(jié)果：
再回到Dispatcher類，enqueue異步方法中執(zhí)行了executorService().execute(call)胡本，executorService()代碼如下：

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;
  }

可以看到上面的代碼中用到了線程池牌柄，這也就意味著后半句的executor(call)必然涉及到了多線程的問(wèn)題，我們來(lái)看代碼：

public interface Executor {
    void execute(Runnable command);
}

可以看到execute的參數(shù)是一個(gè)Runnable侧甫，這也意味著AsyncCall必然是一個(gè)Runnable的子類珊佣。下面來(lái)看AsyncCall的源碼：

• AsyncCall

final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;

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

從上面的代碼中我們可以看到AysncCall繼承自NamedRunnable抽象類，并且會(huì)執(zhí)行execute方法披粟，而execute方法中我們又看到了熟悉的代碼：
Response response = getResponseWithInterceptorChain();
可見(jiàn)返回結(jié)果也是通過(guò)調(diào)用這個(gè)方法得到的咒锻，只不過(guò)與同步相比中間增加了一些過(guò)程，并且請(qǐng)求結(jié)果是通過(guò)responseCallBack返回守屉。
我們來(lái)看一下這個(gè)方法的代碼：

private 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 (!retryAndFollowUpInterceptor.isForWebSocket()) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(
        retryAndFollowUpInterceptor.isForWebSocket()));

    Interceptor.Chain chain = new RealInterceptorChain(
        interceptors, null, null, null, 0, originalRequest);
    return chain.proceed(originalRequest);
  }

可以看到惑艇，這個(gè)方法中主要是用到了Interceptor類。

• Interceptor

public interface Interceptor {
  Response intercept(Chain chain) throws IOException;

  interface Chain {
    Request request();

    Response proceed(Request request) throws IOException;

    Connection connection();
  }
}

Interceptor接口代碼很少，但是滨巴；卻是okhttp中非常核心的類思灌，它把實(shí)際的網(wǎng)絡(luò)請(qǐng)求、緩存恭取、透明壓縮等功能都統(tǒng)一了起來(lái)泰偿，每一個(gè)功能都是一個(gè)Interceptor，它們?cè)龠B成一個(gè)Interceptor.Chain秽荤，環(huán)環(huán)相扣，完成一次網(wǎng)絡(luò)請(qǐng)求柠横。
分析getResponseWithInterceptorChain()方法中用到的interceptor：
1窃款，通過(guò)client設(shè)置的interceptors（即builder.addInterceptor()）
2，RetryAndFollowUpInterceptor牍氛，負(fù)責(zé)重試和重定向
3晨继，BridgeInterceptor，首先將應(yīng)用層的數(shù)據(jù)類型轉(zhuǎn)換為網(wǎng)絡(luò)調(diào)用層的數(shù)據(jù)類型搬俊，然后將網(wǎng)絡(luò)層返回的數(shù)據(jù)類型轉(zhuǎn)換為應(yīng)用層的數(shù)據(jù)類型
4紊扬，CacheInterceptor，負(fù)責(zé)讀取緩存唉擂，更新緩存
5餐屎，ConnectInterceptor，負(fù)責(zé)和服務(wù)器建立起鏈接
6玩祟，networkInterceptors腹缩，client設(shè)置的networkInterceptor
7，CallServerInterceptor空扎，負(fù)責(zé)向服務(wù)器發(fā)送請(qǐng)求數(shù)據(jù)藏鹊、從服務(wù)器讀取響應(yīng)數(shù)據(jù)
最后一個(gè)interceptor是負(fù)責(zé)跟服務(wù)器通訊的，其他的interceptor配置都要在此之前转锈。

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

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

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

    httpStream.finishRequest();

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

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

Http請(qǐng)求流程圖

okhttp_full_process.png