死磕Android OkHttp3 原理探究

1. 前言

Okhttp3 儼然已成爲Android的主流網絡請求開源框架,它的設計很是巧妙,並且很是靈活,功能強大.它有以下默認特性:

• 支持HTTP/2，容許全部同一個主機地址的請求共享同一個Socket鏈接
• 鏈接池減小請求延時
• 透明的GZIP壓縮減小響應數據的大小
• 緩存響應內容，避免一些徹底重複的請求

如今的Android項目基本上都是以OkHttp來進行高效的網絡請求.固然,在使用的同時咱們須要去研究它的底層實現,從而讓咱們寫出更好的代碼.

2. 基本使用

這裏簡單介紹2種,GET和POST.推薦讓 OkHttpClient 保持單例，用同一個 OkHttpClient 實例來執行你的全部請求，由於每個 OkHttpClient 實例都擁有本身的鏈接池和線程池，重用這些資源能夠減小延時和節省資源，若是爲每一個請求建立一個 OkHttpClient實例，顯然就是一種資源的浪費。

1. 使用GET方式請求

public static final String URL = "http://www.baidu.com";
private OkHttpClient mOkHttpClient = new OkHttpClient();
private final Request mRequest = new Request.Builder().url(URL).build();

@Override
public void request() {
    mOkHttpClient.newCall(mRequest)
            //異步請求
            .enqueue(new Callback() {
                @Override
                public void onFailure(Call call, IOException e) {
                    e.printStackTrace();
                }

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

public static final String URL = "https://api.github.com/markdown/raw";
private OkHttpClient mOkHttpClient = new OkHttpClient.Builder()
        .build();
MediaType mMediaType = MediaType.parse("text/x-markdown; charset=utf-8");
String requestBody = "I am xfhy.";
private final Request mRequest = new Request.Builder()
        .url(URL)
        .post(RequestBody.create(mMediaType, requestBody))
        .build();

@Override
public void request() {
    //每個Call（其實現是RealCall）只能執行一次，不然會報異常
    mOkHttpClient.newCall(mRequest).enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {
            e.printStackTrace();
        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            Log.w(TAG, "onResponse: " + response.body().string());
        }
    });
}
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3. interceptor 攔截器-精髓

使用OkHttp3請求網絡仍是比較簡單,並且異步請求也比較輕鬆.

3.1 構建OkHttpClient

正如名字所描述的,OkHttpClient像是一個請求網絡的客戶端.它內部有不少不少的配置信息(支持協議、任務調度器、鏈接池、超時時間等),經過構造器模式初始化的這些配置信息.(這裏穿插一下,正如你所看到的這種一個類裏面不少不少屬性須要初始化的,通常就用構造器模式)

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

public Builder() {
  //任務調度器
  dispatcher = new Dispatcher();
  //支持的協議
  protocols = DEFAULT_PROTOCOLS;
  connectionSpecs = DEFAULT_CONNECTION_SPECS;
  eventListenerFactory = EventListener.factory(EventListener.NONE);
  proxySelector = ProxySelector.getDefault();
  if (proxySelector == null) {
    proxySelector = new NullProxySelector();
  }
  cookieJar = CookieJar.NO_COOKIES;
  socketFactory = SocketFactory.getDefault();
  hostnameVerifier = OkHostnameVerifier.INSTANCE;
  certificatePinner = CertificatePinner.DEFAULT;
  proxyAuthenticator = Authenticator.NONE;
  authenticator = Authenticator.NONE;
  //鏈接池
  connectionPool = new ConnectionPool();
  dns = Dns.SYSTEM;
  followSslRedirects = true;
  followRedirects = true;
  retryOnConnectionFailure = true;
  callTimeout = 0;
  //超時時間
  connectTimeout = 10_000;
  readTimeout = 10_000;
  writeTimeout = 10_000;
  pingInterval = 0;
}
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其中Dispatcher有一個線程池,用於執行異步的請求.而且內部還維護了3個雙向任務隊列,分別是:準備異步執行的任務隊列、正在異步執行的任務隊列、正在同步執行的任務隊列.

/** Executes calls. Created lazily. */
//這個線程池是須要的時候纔會被初始化
private @Nullable ExecutorService executorService;

/** 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<>(); public synchronized ExecutorService executorService() { if (executorService == null) { //注意,該線程池沒有核心線程,線程數量能夠是Integer.MAX_VALUE個(至關於沒有限制),超過60秒沒幹事就要被回收 executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS, new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false)); } return executorService; } 複製代碼

3.2 構建Request

Request感受就是一個請求的封裝.它裏面封裝了url、method、header、body,該有的都有了.並且它也是用構造器模式來構建的,它默認的請求方式是GET

public final class Request {
  final HttpUrl url;
  final String method;
  final Headers headers;
  final @Nullable RequestBody body;
  final Map<Class<?>, Object> tags;


 public Builder() {
  this.method = "GET";
  this.headers = new Headers.Builder();
 }
    
  public static class Builder {
    @Nullable HttpUrl url;
    String method;
    Headers.Builder headers;
    @Nullable RequestBody body;

    /** A mutable map of tags, or an immutable empty map if we don't have any. */
    Map<Class<?>, Object> tags = Collections.emptyMap();

    public Builder() {
      this.method = "GET";
      this.headers = new Headers.Builder();
    }
}
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3.3 開始請求

咱們進入mOkHttpClient的newCall方法,它構造的是一個Call對象,其實是一個RealCall

/** * 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 */);
}
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RealCall#enqueue(Callback)

因此示例中的enqueue其實是RealCall中的方法

@Override public void enqueue(Callback responseCallback) {
    ......
    //將AsyncCall傳入任務調度器,
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
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將AsyncCall(這個咱們稍後再說)傳入任務調度器,任務任務調度器會將其存入待執行的請求隊列(上面提到的readyAsyncCalls)中,而後條件容許的話再加入到運行中的請求隊列(runningAsyncCalls)中,而後將這個請求放到任務調度器中的線程池中進行消費.下面是詳細代碼

----Dispatcher#enqueue(AsyncCall)
void enqueue(AsyncCall call) {
    synchronized (this) {
      readyAsyncCalls.add(call);

      // Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
      // the same host.
      if (!call.get().forWebSocket) {
        AsyncCall existingCall = findExistingCallWithHost(call.host());
        if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);
      }
    }
    promoteAndExecute();
  }

private boolean promoteAndExecute() {
    List<AsyncCall> executableCalls = new ArrayList<>();
    boolean isRunning;
    synchronized (this) {
      //從待執行隊列中取出來
      for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
        AsyncCall asyncCall = i.next();
        //若是正在執行的任務>=64  那麼就算了,先緩一緩
        if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
        if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.
    
        i.remove();
        asyncCall.callsPerHost().incrementAndGet();
        executableCalls.add(asyncCall);
        //加入到運行隊列中
        runningAsyncCalls.add(asyncCall);
      }
      isRunning = runningCallsCount() > 0;
    }
    
    for (int i = 0, size = executableCalls.size(); i < size; i++) {
      AsyncCall asyncCall = executableCalls.get(i);
      //一個個地開始執行    executorService方法是獲取線程池
      asyncCall.executeOn(executorService());
    }
    
    return isRunning;
}

//獲取線程池代碼
public synchronized ExecutorService executorService() {
    if (executorService == null) {
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

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上面咱們提到了不少次AsyncCall,它實際上是一個RealCall的非靜態內部類,因此能直接訪問到RealCall的屬性啥的,方便.同時,AsyncCall繼承自NamedRunnable,NamedRunnable實現了NamedRunnable.

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 {
      execute();
    } finally {
      Thread.currentThread().setName(oldName);
    }
  }

  protected abstract void execute();
}
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NamedRunnable中使用了模板方法模式,子類必須實現execute方法,而且將邏輯放在execute中.而且NamedRunnable中還設置了本身線程的名字,實屬方便管理.

上面的任務調度器中執行的AsyncCall,至關於就是執行的AsyncCall的execute的邏輯

@Override protected void execute() {
  boolean signalledCallback = false;
  transmitter.timeoutEnter();
  try {
    
    //-----------------------重點代碼 華麗的分割線圍起來---------------------------------
    //1. 經過攔截器鏈條,獲取最終的網絡請求結果
    Response response = getResponseWithInterceptorChain();
    //2. 標記已執行 不能再執行第二次了
    signalledCallback = true;
    //3. 將結果回調給調用處
    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);
  }
}
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開始了,開始了,重點來了,經過getResponseWithInterceptorChain方法這條攔截器鏈路能夠獲取到網絡請求的結果.而後咱們經過CallBack接口回調回調用處.

在開始以前,你們先看兩張圖,這張圖是整個攔截器的流程,也是OkHttp的精華,設計之巧妙.

從上面的代碼也能夠看到,getResponseWithInterceptorChain方法是獲取到了網絡請求的最終數據的.緊接着根據我畫了兩張圖,這兩張圖主要是描繪了從getResponseWithInterceptorChain進去以後發生的事,它內部會串行的執行一些特定的攔截器(interceptors),每一個攔截器負責一個特殊的職責.最後那個攔截器負責請求服務器,而後服務器返回了數據再根據這個攔截器的順序逆序返回回去,最終就獲得了網絡數據.

下面先簡單介紹一下這些攔截器,方便後面的源碼梳理

• RetryAndFollowUpInterceptor 負責請求的重定向操做，用於處理網絡請求中，請求失敗後的重試機制。
• BridgeInterceptor 主要是添加一些header
• CacheInterceptor 負責緩存
• ConnectInterceptor 打開與目標服務器的鏈接
• CallServerInterceptor 最後一個攔截器,負責請求網絡

3.4 進入攔截器調用鏈

有了上面的簡單介紹,咱們直接進入getResponseWithInterceptorChain方法一探究竟.

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    //用來盛放全部的攔截器的
    List<Interceptor> interceptors = new ArrayList<>();
    
    //1. 添加用戶定義的攔截器
    interceptors.addAll(client.interceptors());
    //2. 添加一些OkHttp自帶的攔截器
    interceptors.add(new RetryAndFollowUpInterceptor(client));
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    
    if (!forWebSocket) {
      //這裏還有一個網絡攔截器,也是能夠用戶自定義的
      interceptors.addAll(client.networkInterceptors());
    }
    
    //最終訪問服務器的攔截器
    interceptors.add(new CallServerInterceptor(forWebSocket));
    
    //3. 將攔截器,當前攔截器索引等傳入Interceptor.Chain
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
        originalRequest, this, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());
    
    boolean calledNoMoreExchanges = false;
    try {
      //4. 請求訪問下一個攔截器
      Response response = chain.proceed(originalRequest);
      if (transmitter.isCanceled()) {
        closeQuietly(response);
        throw new IOException("Canceled");
      }
      return response;
    } catch (IOException e) {
      calledNoMoreExchanges = true;
      throw transmitter.noMoreExchanges(e);
    } finally {
      if (!calledNoMoreExchanges) {
        transmitter.noMoreExchanges(null);
      }
    }
}
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能夠看到,OkHttp這個攔截器鏈的大致流程,最開始是用戶自定義的攔截器,而後纔是OkHttp本身默認的攔截器(須要注意的是,最後一個攔截器是CallServerInterceptor).而後將攔截器集合和當前攔截器的索引等數據傳入RealInterceptorChain,調用RealInterceptorChain對象的proceed,並最終獲得執行結果.看來邏輯在RealInterceptorChain的proceed方法內部

public final class RealInterceptorChain implements Interceptor.Chain {
    private final List<Interceptor> interceptors;
    private final Transmitter transmitter;
    private final @Nullable Exchange exchange;
    private final int index;
    private final Request request;
    private final Call call;
    private final int connectTimeout;
    private final int readTimeout;
    private final int writeTimeout;
    private int calls;
    
    public RealInterceptorChain(List<Interceptor> interceptors, Transmitter transmitter, @Nullable Exchange exchange, int index, Request request, Call call, int connectTimeout, int readTimeout, int writeTimeout) {
        this.interceptors = interceptors;
        this.transmitter = transmitter;
        this.exchange = exchange;
        this.index = index;
        this.request = request;
        this.call = call;
        this.connectTimeout = connectTimeout;
        this.readTimeout = readTimeout;
        this.writeTimeout = writeTimeout;
    }

    @Override 
    public Response proceed(Request request) throws IOException {
        return proceed(request, transmitter, exchange);
    }
    
    public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange) throws IOException {
        calls++;
        
        // Call the next interceptor in the chain.
        //調用下一個interceptor.注意到,這裏的index索引+1了的,因此是下一個interceptor
        RealInterceptorChain next = new RealInterceptorChain(interceptors, transmitter, exchange,
            index + 1, request, call, connectTimeout, readTimeout, writeTimeout);
        //當前interceptor
        Interceptor interceptor = interceptors.get(index);
        //調用interceptor的intercept方法
        Response response = interceptor.intercept(next);
        
        return response;
    }
}
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在proceed方法裏面主要是將下一個攔截器的RealInterceptorChain構建出來,而後傳入當前攔截器的intercept方法裏面,方便在intercept方法裏面執行下一個RealInterceptorChain的proceed方法.intercept方法返回的是獲取數據以後的Response.

下面進入intercept方法內部,Interceptor實際上是一個接口,而後全部的攔截器都實現了這個接口Interceptor.若是沒有用戶自定義的攔截器,那麼第一個攔截器就是RetryAndFollowUpInterceptor

RetryAndFollowUpInterceptor#intercept

@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Transmitter transmitter = realChain.transmitter();

    int followUpCount = 0;
    Response priorResponse = null;
    
    //死循環 直到達到重定向的最大次數
    while (true) {
      //準備一個流來承載request,若是存在則複用
      transmitter.prepareToConnect(request);

      if (transmitter.isCanceled()) {
        throw new IOException("Canceled");
      }

      Response response;
      boolean success = false;
      try {
        //調用下一個攔截器 
        response = realChain.proceed(request, transmitter, null);
        success = true;
      } catch (RouteException e) {
      
        //下面是一些失敗,而後又從新請求的代碼
      
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), transmitter, false, request)) {
          throw e.getFirstConnectException();
        }
        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, transmitter, requestSendStarted, request)) throw e;
        continue;
      } finally {
        // The network call threw an exception. Release any resources.
        if (!success) {
          transmitter.exchangeDoneDueToException();
        }
      }

      // 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();
      }

      Exchange exchange = Internal.instance.exchange(response);
      Route route = exchange != null ? exchange.connection().route() : null;
      Request followUp = followUpRequest(response, route);

      if (followUp == null) {
        if (exchange != null && exchange.isDuplex()) {
          transmitter.timeoutEarlyExit();
        }
        return response;
      }

      RequestBody followUpBody = followUp.body();
      if (followUpBody != null && followUpBody.isOneShot()) {
        return response;
      }

      closeQuietly(response.body());
      if (transmitter.hasExchange()) {
        exchange.detachWithViolence();
      }

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

      request = followUp;
      priorResponse = response;
    }
  }
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RetryAndFollowUpInterceptor主要是負責錯誤處理,以及重定向.固然重定向是有最大次數的,OkHttp規定是20次.

RetryAndFollowUpInterceptor執行proceed方法是來到了BridgeInterceptor,它是一個鏈接橋.添加了不少header

@Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();
    
    //進行header的包裝
    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");
    }

    //添加Accept-Encoding：gzip
    // 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");
    }

    //建立OkhttpClient配置的cookieJar
    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());
    }
    
    //執行下一個Interceptor
    Response networkResponse = chain.proceed(requestBuilder.build());

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

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

    //先判斷服務器是否支持gzip壓縮,支持則交給Okio處理
    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)));
    }
    
    //最後將結果返回
    return responseBuilder.build();
  }
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BridgeInterceptor就跟它的名字那樣，它是一個鏈接橋.它負責把用戶構造的請求轉換成發送給服務器的請求,就是添加了很多的header,其中還有gzip等.

BridgeInterceptor的下一個攔截器是CacheInterceptor

@Override public Response intercept(Chain chain) throws IOException {
    ////若是配置了緩存：優先從緩存中讀取Response
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();
    
    //緩存策略，該策略經過某種規則來判斷緩存是否有效
    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.
    //若是根據緩存策略strategy禁止使用網絡，而且緩存無效，直接返回空的Response
    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.
    //若是根據緩存策略strategy禁止使用網絡，且有緩存則直接使用緩存
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    //須要網絡
    Response networkResponse = null;
    try {
      //執行下一個攔截器,發起網路請求
      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());
      }
    }

    //本地有緩存，
    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
        //而且服務器返回304狀態碼（說明緩存還沒過時或服務器資源沒修改）
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        //使用緩存數據
        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());
      }
    }

    //若是網絡資源已經修改：使用網絡響應返回的最新數據
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    //將最新的數據緩存起來
    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.
        }
      }
    }
    
    //返回最新的數據
    return response;
  }
複製代碼

CacheInterceptor是進行一些緩存上面的處理,接下來是ConnectInterceptor

@Override 
public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    Transmitter transmitter = realChain.transmitter();
    
    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    //判斷請求是否是GET方法, 不是的狀況下,須要進行有效監測
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    Exchange exchange = transmitter.newExchange(chain, doExtensiveHealthChecks);
    
    //執行下一個攔截器
    return realChain.proceed(request, transmitter, exchange);
}
複製代碼

ConnectInterceptor的下一個攔截器就是最好一個攔截器CallServerInterceptor了.

@Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Exchange exchange = realChain.exchange();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();
    //整理請求頭並寫入
    exchange.writeRequestHeaders(request);

    boolean responseHeadersStarted = false;
    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"))) {
        exchange.flushRequest();
        responseHeadersStarted = true;
        exchange.responseHeadersStart();
        responseBuilder = exchange.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        if (request.body().isDuplex()) {
          // Prepare a duplex body so that the application can send a request body later.
          exchange.flushRequest();
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, true));
          request.body().writeTo(bufferedRequestBody);
        } else {
          // Write the request body if the "Expect: 100-continue" expectation was met.
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, false));
          request.body().writeTo(bufferedRequestBody);
          bufferedRequestBody.close();
        }
      } else {
        exchange.noRequestBody();
        if (!exchange.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.
          exchange.noNewExchangesOnConnection();
        }
      }
    } else {
      exchange.noRequestBody();
    }

    if (request.body() == null || !request.body().isDuplex()) {
      //發送最終的請求
      exchange.finishRequest();
    }

    if (!responseHeadersStarted) {
      exchange.responseHeadersStart();
    }

    if (responseBuilder == null) {
      //響應頭
      responseBuilder = exchange.readResponseHeaders(false);
    }

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

      code = response.code();
    }

    exchange.responseHeadersEnd(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(exchange.openResponseBody(response))
          .build();
    }

    //斷開鏈接
    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      exchange.noNewExchangesOnConnection();
    }

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

    return response;
  }
複製代碼

這是鏈中最後一個攔截器，它向 服務器 發起了一次網絡訪問.負責向服務器發送請求數據、從服務器讀取響應數據.拿到數據以後再沿着鏈返回.

4. 總結

OkHttp的攔截器鏈設計得很是巧妙,是典型的責任鏈模式.並最終由最後一個鏈處理了網絡請求,並拿到結果.本文主要是對OkHttp主流程進行了梳理,經過本文能對OkHttp有一個總體的瞭解.