OkHttp3源碼分析

重點

本文打算從三點來剖析OkHttp3：
* 網絡請求的整理流程-會使用
* 攔截器模式-易擴展
* 緩存和連接池-高性能

整體流程

我們在閱讀某一類源碼之前，首先要學會怎麼使用，其次纔是去了解內部的實現原理，實現方案上有什麼技巧。

okhttp的一張流程圖：

官方用例

1. get使用方式

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();
}

1. post使用的方式

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();
}

OkHttp 官方網址

請求過程

從上面的用例和流程圖可以總結出OkHttp的請求過程爲：
1. 創建OkHttpClient
2. 創建Request
3. 同步或者異步發出請求，並經過Interceptors處理
4. 得到Response

然後一步一步來分析

1. 創建OkHttpClient

通過new OkHttpClient()來創建一個OkHttpClient對象，當然這個是簡單的寫法，通常情況下，我們需要設置緩存策略、超時時長、攔截器等等。看下源碼：

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;
  }

第一眼看到這麼多參數，是不是被嚇到了，源碼比較長，但是它本身沒有特定的功能，Builder作爲一個載體，記錄下各種初始化的信息。例如，緩存地址、超時的時長、自定義攔截器等等。

這裏使用了Builder設計模式，更加易讀，關於Builder設計模式，可以看看這篇文章

2. 創建Request

Request類是一個純粹的載體，封裝了一個請求的Header、Method、url和body。這裏源碼就不貼出來了。

3. 發起請求

Request request = new Request.Builder().url(url).build();
Response response = client.newCall(request).execute();
response.body().string();

OkHttpClient實現了Call.Factory，負責創建新的Call，Call是OkHttp抽象出一個滿足請求的模型。

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

上面是OkHttpClient創建Call的過程，可以看到，RealCall是真正請求網絡的類。
1. 同步請求網絡：execute()

 @Override 
 public Response execute() throws IOException {
    synchronized (this) {
    // 從這裏可以看到，一個call只能執行一次，如果需要多次執行，可以使用clone()方法。
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } finally {
      client.dispatcher().finished(this);
    }
  }

1. 異步請求網絡：enqueue(callBack)

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

上面的同步和異步的網絡請求做了四件事情：
1. 判斷是否執行過，每個call請求只能請求一次，如果需要重複執行可以使用clone()方法
2. 獲取分發器dispatcher，執行execute()或者enqueue(new AsyncCall(callback))
3. 調用getResponseWithInterceptorChain()方法，獲取網絡請求的結果，異步請求都在AsyncCall中執行了。
4. 最後通知dispatcher執行完畢。

上面的四個步驟中涉及到了Dispatcher和AsyncCall兩個類，這裏做個簡單的介紹：
* Dispatcher
Disaptcher是一個分發器，它持有線程池、異步任務隊列和同步任務隊列，會依照不同的策略執行。
* AsyncCall
，AsyncCall是RealCall的內部類，持有RealCall的引用，實質上是一個Runnable，對RealCall做了一層封裝。

攔截器

OkHttpClient中真正發出網絡請求，解析返回結果的步驟是getResponseWithInterceptorChain()這個方法.

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);
    return chain.proceed(originalRequest);
  }

Interceptor是OkHttp最核心的一個東西，採用了責任鏈的設計模式，不要誤以爲它只是負責攔截請求進行一些額外的處理（例如cookie），實際上它把時機的網絡請求、緩存、透明壓縮等功能都統一了起來，每一個功能只是一個Interceptor，它們再連接成爲Interceptor.Chain，環環相扣，最終圓滿完成一次網絡請求。interceptor的順序也很重要，比如負責網絡請求的Interceptor必須放在最後，負責緩存策略的Inteceptor需要放到前面。

/**
 * A concrete interceptor chain that carries the entire interceptor chain: all application
 * interceptors, the OkHttp core, all network interceptors, and finally the network caller.
 */
public final class RealInterceptorChain implements Interceptor.Chain {
  private final List<Interceptor> interceptors;
  private final StreamAllocation streamAllocation;
  private final HttpCodec httpCodec;
  private final RealConnection connection;
  private final int index;
  private final Request request;
  private int calls;

  public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
      HttpCodec httpCodec, RealConnection connection, int index, Request request) {
    this.interceptors = interceptors;
    this.connection = connection;
    this.streamAllocation = streamAllocation;
    this.httpCodec = httpCodec;
    this.index = index;
    this.request = request;
  }

  @Override public Connection connection() {
    return connection;
  }

  public StreamAllocation streamAllocation() {
    return streamAllocation;
  }

  public HttpCodec httpStream() {
    return httpCodec;
  }

  @Override public Request request() {
    return request;
  }

  @Override 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 {
    if (index >= interceptors.size()) throw new AssertionError();

    calls++;

    // If we already have a stream, confirm that the incoming request will use it.
    if (this.httpCodec != null && !this.connection.supportsUrl(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().
    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.
    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;
  }
}

RealInterceptorChain的主要作用是傳遞參數和一個一個執行攔截器，並且設定一些條件，例如，每個攔截器只能調用一次chain.proceed這個方法。

OkHttp攔截器分析

先列出之前在getResponseWithInterceptorChain方法中添加的各Interceptor，概括一下它們分別負責什麼功能：
* client.interceptors()用戶自定義的Interceptor，能攔截到所有的請求
* RetryAndFollowUpInterceptor負責失敗重連和重定向相關
* BridgeInterceptor負責配置請求的頭信息，比如Keep-Alive、gzip、Cookie等可以優化請求
* CacheInterceptor負責緩存管理，使用DiskLruCache做本地緩存，CacheStrategy決定緩存策略
* ConnectInterceptor開始與目標服務器建立連接，獲得RealConnection
* client.networkInterceptors()用戶自定義的Interceptor，僅在生產網絡請求時生效
* CallServerInterceptor向服務器發出一次網絡請求的地方。

RetryAndFollowUpInterceptor

public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();

    streamAllocation = new StreamAllocation(
        client.connectionPool(), createAddress(request.url()), callStackTrace);

    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);
        releaseConnection = false;
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), 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, 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();
      }

      Request followUp = followUpRequest(response);

      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()), callStackTrace);
      } 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;
    }
  }

• streamAllocation對象是在這個Interceptor中創建的
• 在followUpRequest方法中判斷了是否需要重定向以及是否需要重連，需要重連時會返回一個request
• request爲null說明不需要重連，則直接返回response，否則輪訓重走網絡請求的流程。

BridgeInterceptor

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

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

    return responseBuilder.build();
  }

• BridgeInteceptor的Interceptor基本上都是添加請求的頭信息，例如啓動是否使用長連接Keep-Alive，設置Cookie，啓動壓縮與解壓gzip等。

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 {
      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;
  }

• 首先根據request從cache中取response
• 將request和respnse傳入CacheStrategy根據緩存策略（比如僅適用網絡加載，僅適用緩存，緩存時效等）得到有策略處理後的networkRequest和cacheResponse
• 若緩存策略要求僅從緩存中加載，且緩存未命中，則本次請求失敗
• 若緩存策略不要求僅從網絡獲取數據，則直接返回緩存內容
• 以上條件不滿足，則把獲得response的任務交給下一個Chain，開始執行網絡請求。
• 得到網絡請求結果後，如果已經有緩存了，則用最新的網絡數據更新緩存。
• 最後將本次請求的結果response根據cacheRequest寫入緩存

ConnectInterceptor

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);
  }

• 主要爲下一步最終進行網絡請求做鋪墊，這裏獲得了HttpCodec和RealConnection，然後將這些參數傳入下一個Chain。
• 在newStream方法中會去先嚐試從RealConnectionPool中尋找已經存在的連接，若未命中則創建一個連接並與服務器握手對接。
• 在完成連接後會將Socket對象通過Okio封裝成BufferedSource和BufferedSink，並將兩者傳入HttpCodec，在下一步網絡請求時會用到。

source = Okio.buffer(Okio.source(rawSocket));
sink = Okio.buffer(Okio.sink(rawSocket));

CallServerInterceptor

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();
    httpCodec.writeRequestHeaders(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();
        responseBuilder = httpCodec.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
      } 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) {
      responseBuilder = httpCodec.readResponseHeaders(false);
    }

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

    int code = response.code();
    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;
  }

• 可以發現具體的實現都交給了HttpCodec，它是對Http協議操作的一種抽象，針對HTTP/1.1與HTTP2有Http1Codec和Http2Codec兩種實現。
• 方法的命名都是read和write，因爲HttpCodec中最後的請求和響應是由上一步封裝的BufferedSource和BufferedSink來完成的，sink負責輸出流，將寫入的數據交由socket發出，source負責輸入流，從socket中讀取響應數據。
*

OkHttp緩存相關

• 緩存策略CacheStrategy
• 連接池RealConnectPool

緩存策略

緩存策略主要是是CacheStrategy這個類，依賴於本地緩存Cache和Http Header緩存配置。
Cache使用了DiskLruCache作爲緩存容器，以request.url作爲key來存儲和讀取response，這是一種很常見的緩存方式。
Http HEader使用Http協議中約定的Cache-Control、Expires、ETag、Last-Modified、Date`等字段和服務端交互，由這些字段信息決定是否使用緩存，關於這些字段的含義可以查看Http協議中的定義。

public final class CacheStrategy {
  /** The request to send on the network, or null if this call doesn't use the network. */
  public final @Nullable Request networkRequest;

  /** The cached response to return or validate; or null if this call doesn't use a cache. */
  public final @Nullable Response cacheResponse;

  CacheStrategy(Request networkRequest, Response cacheResponse) {
    this.networkRequest = networkRequest;
    this.cacheResponse = cacheResponse;
  }

  /** Returns true if {@code response} can be stored to later serve another request. */
  public static boolean isCacheable(Response response, Request request) {
    // Always go to network for uncacheable response codes (RFC 7231 section 6.1),
    // This implementation doesn't support caching partial content.
    switch (response.code()) {
      case HTTP_OK:
      case HTTP_NOT_AUTHORITATIVE:
      case HTTP_NO_CONTENT:
      case HTTP_MULT_CHOICE:
      case HTTP_MOVED_PERM:
      case HTTP_NOT_FOUND:
      case HTTP_BAD_METHOD:
      case HTTP_GONE:
      case HTTP_REQ_TOO_LONG:
      case HTTP_NOT_IMPLEMENTED:
      case StatusLine.HTTP_PERM_REDIRECT:
        // These codes can be cached unless headers forbid it.
        break;

      case HTTP_MOVED_TEMP:
      case StatusLine.HTTP_TEMP_REDIRECT:
        // These codes can only be cached with the right response headers.
        // http://tools.ietf.org/html/rfc7234#section-3
        // s-maxage is not checked because OkHttp is a private cache that should ignore s-maxage.
        if (response.header("Expires") != null
            || response.cacheControl().maxAgeSeconds() != -1
            || response.cacheControl().isPublic()
            || response.cacheControl().isPrivate()) {
          break;
        }
        // Fall-through.

      default:
        // All other codes cannot be cached.
        return false;
    }

    // A 'no-store' directive on request or response prevents the response from being cached.
    return !response.cacheControl().noStore() && !request.cacheControl().noStore();
  }

  public static class Factory {
    final long nowMillis;
    final Request request;
    final Response cacheResponse;

    /** The server's time when the cached response was served, if known. */
    private Date servedDate;
    private String servedDateString;

    /** The last modified date of the cached response, if known. */
    private Date lastModified;
    private String lastModifiedString;

    /**
     * The expiration date of the cached response, if known. If both this field and the max age are
     * set, the max age is preferred.
     */
    private Date expires;

    /**
     * Extension header set by OkHttp specifying the timestamp when the cached HTTP request was
     * first initiated.
     */
    private long sentRequestMillis;

    /**
     * Extension header set by OkHttp specifying the timestamp when the cached HTTP response was
     * first received.
     */
    private long receivedResponseMillis;

    /** Etag of the cached response. */
    private String etag;

    /** Age of the cached response. */
    private int ageSeconds = -1;

    public Factory(long nowMillis, Request request, Response cacheResponse) {
      this.nowMillis = nowMillis;
      this.request = request;
      this.cacheResponse = cacheResponse;

      if (cacheResponse != null) {
        this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
        this.receivedResponseMillis = cacheResponse.receivedResponseAtMillis();
        Headers headers = cacheResponse.headers();
        for (int i = 0, size = headers.size(); i < size; i++) {
          String fieldName = headers.name(i);
          String value = headers.value(i);
          if ("Date".equalsIgnoreCase(fieldName)) {
            servedDate = HttpDate.parse(value);
            servedDateString = value;
          } else if ("Expires".equalsIgnoreCase(fieldName)) {
            expires = HttpDate.parse(value);
          } else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
            lastModified = HttpDate.parse(value);
            lastModifiedString = value;
          } else if ("ETag".equalsIgnoreCase(fieldName)) {
            etag = value;
          } else if ("Age".equalsIgnoreCase(fieldName)) {
            ageSeconds = HttpHeaders.parseSeconds(value, -1);
          }
        }
      }
    }

    /**
     * Returns a strategy to satisfy {@code request} using the a cached response {@code response}.
     */
    public CacheStrategy get() {
      CacheStrategy candidate = getCandidate();

      if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
        // 禁止使用網絡請求，並且沒有渠道cache
        return new CacheStrategy(null, null);
      }

      return candidate;
    }

    /** 假設可以使用網絡，返回一個策略. */
    private CacheStrategy getCandidate() {
      // No cached response.
      if (cacheResponse == null) {
        return new CacheStrategy(request, null);
      }

      // 緩存結果缺少握手信息，直接走網絡請求流程.
      if (request.isHttps() && cacheResponse.handshake() == null) {
        return new CacheStrategy(request, null);
      }

      // 根據當前緩存Response的code值，http header等信息判斷本次緩存是否過期，是否可用，如果不可用則直接走網絡請求流程
      if (!isCacheable(cacheResponse, request)) {
        return new CacheStrategy(request, null);
      }
 //本次請求header包含`no-cache`,`If-Modified-Since`,`If-None-Match`等字段直接走網絡請求

      CacheControl requestCaching = request.cacheControl();
      if (requestCaching.noCache() || hasConditions(request)) {
        return new CacheStrategy(request, null);
      }

      long ageMillis = cacheResponseAge();
      long freshMillis = computeFreshnessLifetime();

      if (requestCaching.maxAgeSeconds() != -1) {
        freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
      }

      long minFreshMillis = 0;
      if (requestCaching.minFreshSeconds() != -1) {
        minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
      }

      long maxStaleMillis = 0;
      CacheControl responseCaching = cacheResponse.cacheControl();
      if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
        maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
      }

      if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
        Response.Builder builder = cacheResponse.newBuilder();
        if (ageMillis + minFreshMillis >= freshMillis) {
          builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
        }
        long oneDayMillis = 24 * 60 * 60 * 1000L;
        if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
          builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");
        }
        return new CacheStrategy(null, builder.build());
      }

      // Find a condition to add to the request. If the condition is satisfied, the response body
      // will not be transmitted.
      String conditionName;
      String conditionValue;
      if (etag != null) {
        conditionName = "If-None-Match";
        conditionValue = etag;
      } else if (lastModified != null) {
        conditionName = "If-Modified-Since";
        conditionValue = lastModifiedString;
      } else if (servedDate != null) {
        conditionName = "If-Modified-Since";
        conditionValue = servedDateString;
      } else {
        return new CacheStrategy(request, null); // No condition! Make a regular request.
      }

      Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
      Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);

      Request conditionalRequest = request.newBuilder()
          .headers(conditionalRequestHeaders.build())
          .build();
      return new CacheStrategy(conditionalRequest, cacheResponse);
    }

    /**
     * Returns the number of milliseconds that the response was fresh for, starting from the served
     * date.
     */
    private long computeFreshnessLifetime() {
      CacheControl responseCaching = cacheResponse.cacheControl();
      if (responseCaching.maxAgeSeconds() != -1) {
        return SECONDS.toMillis(responseCaching.maxAgeSeconds());
      } else if (expires != null) {
        long servedMillis = servedDate != null
            ? servedDate.getTime()
            : receivedResponseMillis;
        long delta = expires.getTime() - servedMillis;
        return delta > 0 ? delta : 0;
      } else if (lastModified != null
          && cacheResponse.request().url().query() == null) {
        // As recommended by the HTTP RFC and implemented in Firefox, the
        // max age of a document should be defaulted to 10% of the
        // document's age at the time it was served. Default expiration
        // dates aren't used for URIs containing a query.
        long servedMillis = servedDate != null
            ? servedDate.getTime()
            : sentRequestMillis;
        long delta = servedMillis - lastModified.getTime();
        return delta > 0 ? (delta / 10) : 0;
      }
      return 0;
    }

    /**
     * Returns the current age of the response, in milliseconds. The calculation is specified by RFC
     * 2616, 13.2.3 Age Calculations.
     */
    private long cacheResponseAge() {
      long apparentReceivedAge = servedDate != null
          ? Math.max(0, receivedResponseMillis - servedDate.getTime())
          : 0;
      long receivedAge = ageSeconds != -1
          ? Math.max(apparentReceivedAge, SECONDS.toMillis(ageSeconds))
          : apparentReceivedAge;
      long responseDuration = receivedResponseMillis - sentRequestMillis;
      long residentDuration = nowMillis - receivedResponseMillis;
      return receivedAge + responseDuration + residentDuration;
    }

    /**
     * Returns true if computeFreshnessLifetime used a heuristic. If we used a heuristic to serve a
     * cached response older than 24 hours, we are required to attach a warning.
     */
    private boolean isFreshnessLifetimeHeuristic() {
      return cacheResponse.cacheControl().maxAgeSeconds() == -1 && expires == null;
    }

    /**
     * Returns true if the request contains conditions that save the server from sending a response
     * that the client has locally. When a request is enqueued with its own conditions, the built-in
     * response cache won't be used.
     */
    private static boolean hasConditions(Request request) {
      return request.header("If-Modified-Since") != null || request.header("If-None-Match") != null;
    }
  }
}

• 首先在Factory方法中獲取它所依賴的參數，就是我們之前提到過的Cache中渠道的緩存和Http Header配置信息。
• 然後進入比較核心的方法getCandidate，在這裏會根據之前拿到的依賴參數通過各種if判斷返回不同的CacheStrategy對象
• 本職上其實返回不同的networkRequest和cacheResponse，這樣上層只需要關注這兩個參數就知道下一步該如何做處理，複雜的判斷都封裝到了CacheStrategy對外透明，具體判斷過程在代碼中做了註釋。
*

連接池

okHttp利用連接池來複用連接，避免反覆握手建立連接，並且具備在合適的時候揮手連接的能力，這也是okhttp設計出彩的地方之一。