OkHttp源码走心解析（很细 很长）

private final OkHttpClient client = new OkHttpClient();
    Request request = new Request.Builder()
      .url("https://github.com/")
      .build();
    //同步请求
    Response response = client.newCall(request).execute();
    //todo handle response

    //异步请求
    client.newCall(request).enqueue(new Callback() {
      @Override
      public void onFailure(@NotNull Call call, @NotNull IOException e) {
      //todo handle request failed
      }

      @Override
      public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
          //todo handle Response
      }
    });

open class OkHttpClient internal constructor(
  builder: Builder
) : Cloneable, Call.Factory, WebSocket.Factory {

  constructor() : this(Builder())

  class Builder constructor() {
    //调度器
    internal var dispatcher: Dispatcher = Dispatcher()
    //连接池
    internal var connectionPool: ConnectionPool = ConnectionPool()
    //整体流程拦截器
    internal val interceptors: MutableList<Interceptor> = mutableListOf()
    //网络流程拦截器
    internal val networkInterceptors: MutableList<Interceptor> = mutableListOf()
    //流程监听器
    internal var eventListenerFactory: EventListener.Factory = EventListener.NONE.asFactory()
    //连接失败时是否重连
    internal var retryOnConnectionFailure = true
    //服务器认证设置
    internal var authenticator: Authenticator = Authenticator.NONE
    //是否重定向
    internal var followRedirects = true
    //是否从HTTP重定向到HTTPS
    internal var followSslRedirects = true
    //cookie设置
    internal var cookieJar: CookieJar = CookieJar.NO_COOKIES
    //缓存设置
    internal var cache: Cache? = null
    //DNS设置
    internal var dns: Dns = Dns.SYSTEM
    //代理设置
    internal var proxy: Proxy? = null
    //代理选择器设置
    internal var proxySelector: ProxySelector? = null
    //代理服务器认证设置
    internal var proxyAuthenticator: Authenticator = Authenticator.NONE
    //socket配置
    internal var socketFactory: SocketFactory = SocketFactory.getDefault()
    //https socket配置
    internal var sslSocketFactoryOrNull: SSLSocketFactory? = null
    internal var x509TrustManagerOrNull: X509TrustManager? = null
    internal var connectionSpecs: List<ConnectionSpec> = DEFAULT_CONNECTION_SPECS
    //协议
    internal var protocols: List<Protocol> = DEFAULT_PROTOCOLS
    //域名校验
    internal var hostnameVerifier: HostnameVerifier = OkHostnameVerifier
    internal var certificatePinner: CertificatePinner = CertificatePinner.DEFAULT
    internal var certificateChainCleaner: CertificateChainCleaner? = null
    //请求超时
    internal var callTimeout = 0
    //连接超时
    internal var connectTimeout = 10_000
    //读取超时
    internal var readTimeout = 10_000
    //写入超时
    internal var writeTimeout = 10_000
    internal var pingInterval = 0
    internal var minWebSocketMessageToCompress = RealWebSocket.DEFAULT_MINIMUM_DEFLATE_SIZE
    internal var routeDatabase: RouteDatabase? = null
    
···省略代码···

class Request internal constructor(
  @get:JvmName("url") val url: HttpUrl,
  @get:JvmName("method") val method: String,
  @get:JvmName("headers") val headers: Headers,
  @get:JvmName("body") val body: RequestBody?,
  internal val tags: Map<Class<*>, Any>
) {

  open class Builder {
    //请求的URL
    internal var url: HttpUrl? = null
    //请求方法，如：GET、POST..
    internal var method: String
    //请求头
    internal var headers: Headers.Builder
    //请求体
    internal var body: RequestBody? = null
  ···省略代码···

interface Call : Cloneable {
  /** 返回发起此调用的原始请求 */
  fun request(): Request

  /**
   * 同步请求，立即执行。
   * 
   * 抛出两种异常：
   * 1. 请求失败抛出IOException;
   * 2. 如果在执行过一回的前提下再次执行抛出IllegalStateException；*/
  @Throws(IOException::class)
  fun execute(): Response

  /**
   * 异步请求，将请求安排在将来的某个时间点执行。
   * 如果在执行过一回的前提下再次执行抛出IllegalStateException */
  fun enqueue(responseCallback: Callback)

  /** 取消请求。已经完成的请求不能被取消 */
  fun cancel()

  /** 是否已被执行  */
  fun isExecuted(): Boolean

  /** 是否被取消   */
  fun isCanceled(): Boolean

  /** 一个完整Call请求流程的超时时间配置，默认选自[OkHttpClient.Builder.callTimeout] */
  fun timeout(): Timeout

  /** 克隆这个call，创建一个新的相同的Call */
  public override fun clone(): Call

  /** 利用工厂模式来让 OkHttpClient 来创建 Call对象 */
  fun interface Factory {
    fun newCall(request: Request): Call
  }
}

OkHttpClient.kt

override fun newCall(request: Request): Call = RealCall(this, request, forWebSocket = false)

inner class AsyncCall(
    //用户传入的响应回调方法
    private val responseCallback: Callback
  ) : Runnable {
    //同一个域名的请求次数，volatile + AtomicInteger 保证在多线程下及时可见性与原子性
    @Volatile var callsPerHost = AtomicInteger(0)
      private set

    fun reuseCallsPerHostFrom(other: AsyncCall) {
      this.callsPerHost = other.callsPerHost
    }

···省略代码···

    fun executeOn(executorService: ExecutorService) {
      client.dispatcher.assertThreadDoesntHoldLock()

      var success = false
      try {
        //调用线程池执行
        executorService.execute(this)
        success = true
      } catch (e: RejectedExecutionException) {
        val ioException = InterruptedIOException("executor rejected")
        ioException.initCause(e)
        noMoreExchanges(ioException)
        //请求失败，调用 Callback.onFailure() 方法
        responseCallback.onFailure(this@RealCall, ioException)
      } finally {
        if (!success) {
          //请求失败，调用调度器finish方法
          client.dispatcher.finished(this) // This call is no longer running!
        }
      }
    }

    override fun run() {
      threadName("OkHttp ${redactedUrl()}") {
        var signalledCallback = false
        timeout.enter()
        try {
          //请求成功，获取到服务器返回的response
          val response = getResponseWithInterceptorChain()
          signalledCallback = true
          //调用 Callback.onResponse() 方法，将 response 传递出去
          responseCallback.onResponse(this@RealCall, response)
        } catch (e: IOException) {
          if (signalledCallback) {
            // Do not signal the callback twice!
            Platform.get().log("Callback failure for ${toLoggableString()}", Platform.INFO, e)
          } else {
            //请求失败，调用 Callback.onFailure() 方法
            responseCallback.onFailure(this@RealCall, e)
          }
        } catch (t: Throwable) {
          //请求出现异常，调用cancel方法来取消请求
          cancel()
          if (!signalledCallback) {
            val canceledException = IOException("canceled due to $t")
            canceledException.addSuppressed(t)
            //请求失败，调用 Callback.onFailure() 方法
            responseCallback.onFailure(this@RealCall, canceledException)
          }
          throw t
        } finally {
          //请求结束，调用调度器finish方法
          client.dispatcher.finished(this)
        }
      }
    }
  }

class Dispatcher constructor() {
  @get:Synchronized
  @get:JvmName("executorService") val executorService: ExecutorService
    get() {
      if (executorServiceOrNull == null) {
        //创建一个缓存线程池，来处理请求调用
        executorServiceOrNull = ThreadPoolExecutor(0, Int.MAX_VALUE, 60, TimeUnit.SECONDS,
            SynchronousQueue(), threadFactory("$okHttpName Dispatcher", false))
      }
      return executorServiceOrNull!!
    }

  /** 已准备好的异步请求队列 */
  @get:Synchronized
  private val readyAsyncCalls = ArrayDeque<AsyncCall>()

  /** 正在运行的异步请求队列, 包含取消但是还未finish的AsyncCall */
  private val runningAsyncCalls = ArrayDeque<AsyncCall>()

  /** 正在运行的同步请求队列, 包含取消但是还未finish的RealCall */
  private val runningSyncCalls = ArrayDeque<RealCall>()

···省略代码···
}

client.newCall(request).execute();

  RealCall.kt
  
  override fun execute(): Response {
    //CAS判断是否已经被执行了, 确保只能执行一次，如果已经执行过，则抛出异常
    check(executed.compareAndSet(false, true)) { "Already Executed" }

    //请求超时开始计时
    timeout.enter()
    //开启请求监听
    callStart()
    try {
      //调用调度器中的 executed() 方法，调度器只是将 call 加入到了runningSyncCalls队列中
      client.dispatcher.executed(this)
      //调用getResponseWithInterceptorChain 方法拿到 response
      return getResponseWithInterceptorChain()
    } finally {
      //执行完毕，调度器将该 call 从 runningSyncCalls队列中移除
      client.dispatcher.finished(this)
    }
  }

  RealCall.kt

  override fun enqueue(responseCallback: Callback) {
    //CAS判断是否已经被执行了, 确保只能执行一次，如果已经执行过，则抛出异常
    check(executed.compareAndSet(false, true)) { "Already Executed" }
    //开启请求监听
    callStart()
    //新建一个AsyncCall对象，通过调度器enqueue方法加入到readyAsyncCalls队列中
    client.dispatcher.enqueue(AsyncCall(responseCallback))
  }

  Dispatcher.kt
  
  internal fun enqueue(call: AsyncCall) {
    //加锁，保证线程安全
    synchronized(this) {
      //将该请求调用加入到 readyAsyncCalls 队列中
      readyAsyncCalls.add(call)

      // Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
      // the same host.
      if (!call.call.forWebSocket) {
        //通过域名来查找有没有相同域名的请求，有则复用。
        val existingCall = findExistingCallWithHost(call.host)
        if (existingCall != null) call.reuseCallsPerHostFrom(existingCall)
      }
    }
    //执行请求
    promoteAndExecute()
  }


  private fun promoteAndExecute(): Boolean {
    this.assertThreadDoesntHoldLock()

    val executableCalls = mutableListOf<AsyncCall>()
    //判断是否有请求正在执行
    val isRunning: Boolean
    //加锁，保证线程安全
    synchronized(this) {
      //遍历 readyAsyncCalls 队列
      val i = readyAsyncCalls.iterator()
      while (i.hasNext()) {
        val asyncCall = i.next()
        //runningAsyncCalls 的数量不能大于最大并发请求数 64
        if (runningAsyncCalls.size >= this.maxRequests) break // Max capacity.
        //同域名最大请求数5，同一个域名最多允许5条线程同时执行请求
        if (asyncCall.callsPerHost.get() >= this.maxRequestsPerHost) continue // Host max capacity.

        //从 readyAsyncCalls 队列中移除，并加入到 executableCalls 及 runningAsyncCalls 队列中
        i.remove()
        asyncCall.callsPerHost.incrementAndGet()
        executableCalls.add(asyncCall)
        runningAsyncCalls.add(asyncCall)
      }
      //通过运行队列中的请求数量来判断是否有请求正在执行
      isRunning = runningCallsCount() > 0
    }

    //遍历可执行队列，调用线程池来执行AsyncCall
    for (i in 0 until executableCalls.size) {
      val asyncCall = executableCalls[i]
      asyncCall.executeOn(executorService)
    }

    return isRunning
  }

  Dispatcher.kt

  /** 异步请求调用结束方法 */
  internal fun finished(call: AsyncCall) {
    call.callsPerHost.decrementAndGet()
    finished(runningAsyncCalls, call)
  }

  /** 同步请求调用结束方法 */
  internal fun finished(call: RealCall) {
    finished(runningSyncCalls, call)
  }

  private fun <T> finished(calls: Deque<T>, call: T) {
    val idleCallback: Runnable?
    synchronized(this) {
      //将当前请求调用从 正在运行队列 中移除
      if (!calls.remove(call)) throw AssertionError("Call wasn't in-flight!")
      idleCallback = this.idleCallback
    }

    //继续执行剩余请求，将call从readyAsyncCalls中取出加入到runningAsyncCalls，然后执行
    val isRunning = promoteAndExecute()

    if (!isRunning && idleCallback != null) {
      //如果执行完了所有请求，处于闲置状态，调用闲置回调方法
      idleCallback.run()
    }
  }

  internal fun getResponseWithInterceptorChain(): Response {
    //拦截器列表
    val interceptors = mutableListOf<Interceptor>()
    interceptors += client.interceptors
    interceptors += RetryAndFollowUpInterceptor(client)
    interceptors += BridgeInterceptor(client.cookieJar)
    interceptors += CacheInterceptor(client.cache)
    interceptors += ConnectInterceptor
    if (!forWebSocket) {
      interceptors += client.networkInterceptors
    }
    interceptors += CallServerInterceptor(forWebSocket)

    //构建拦截器责任链
    val chain = RealInterceptorChain(
        call = this,
        interceptors = interceptors,
        index = 0,
        exchange = null,
        request = originalRequest,
        connectTimeoutMillis = client.connectTimeoutMillis,
        readTimeoutMillis = client.readTimeoutMillis,
        writeTimeoutMillis = client.writeTimeoutMillis
    )
    //如果call请求完成，那就意味着交互完成了，没有更多的东西来交换了
    var calledNoMoreExchanges = false
    try {
      //执行拦截器责任链来获取 response
      val response = chain.proceed(originalRequest)
      //如果被取消，关闭响应，抛出异常
      if (isCanceled()) {
        response.closeQuietly()
        throw IOException("Canceled")
      }
      return response
    } catch (e: IOException) {
      calledNoMoreExchanges = true
      throw noMoreExchanges(e) as Throwable
    } finally {
      if (!calledNoMoreExchanges) {
        noMoreExchanges(null)
      }
    }
  }

fun interface Interceptor {
  /** 拦截方法 */
  @Throws(IOException::class)
  fun intercept(chain: Chain): Response

  interface Chain {
    /** 原始请求数据 */
    fun request(): Request

    /** 核心方法，处理请求，获取response */
    @Throws(IOException::class)
    fun proceed(request: Request): Response
    
    fun connection(): Connection?

    fun call(): Call

    fun connectTimeoutMillis(): Int

    fun withConnectTimeout(timeout: Int, unit: TimeUnit): Chain

    fun readTimeoutMillis(): Int

    fun withReadTimeout(timeout: Int, unit: TimeUnit): Chain

    fun writeTimeoutMillis(): Int

    fun withWriteTimeout(timeout: Int, unit: TimeUnit): Chain
  }
}

class RealInterceptorChain(
  internal val call: RealCall,
  private val interceptors: List<Interceptor>,
  private val index: Int,
  internal val exchange: Exchange?,
  internal val request: Request,
  internal val connectTimeoutMillis: Int,
  internal val readTimeoutMillis: Int,
  internal val writeTimeoutMillis: Int
) : Interceptor.Chain {

···省略代码···
  private var calls: Int = 0
  override fun call(): Call = call
  override fun request(): Request = request

  @Throws(IOException::class)
  override fun proceed(request: Request): Response {
    check(index < interceptors.size)

    calls++

    if (exchange != null) {
      check(exchange.finder.sameHostAndPort(request.url)) {
        "network interceptor ${interceptors[index - 1]} must retain the same host and port"
      }
      check(calls == 1) {
        "network interceptor ${interceptors[index - 1]} must call proceed() exactly once"
      }
    }

    //index+1, 复制创建新的责任链，也就意味着调用责任链中的下一个处理者，也就是下一个拦截器
    val next = copy(index = index + 1, request = request)
    //取出当前拦截器
    val interceptor = interceptors[index]

    //执行当前拦截器的拦截方法
    @Suppress("USELESS_ELVIS")
    val response = interceptor.intercept(next) ?: throw NullPointerException(
        "interceptor $interceptor returned null")

    if (exchange != null) {
      check(index + 1 >= interceptors.size || next.calls == 1) {
        "network interceptor $interceptor must call proceed() exactly once"
      }
    }

    check(response.body != null) { "interceptor $interceptor returned a response with no body" }

    return response
  }
}

class HeaderInterceptor implements Interceptor {
    @Override
    public Response intercept(Chain chain) throws IOException {
        Request request = chain.request().newBuilder()
                .addHeader("device-android", "xxxxxxxxxxx")
                .addHeader("country-code", "ZH")
                .build();
        return chain.proceed(request);
    }
}

//然后在 OkHttpClient 中加入
OkHttpClient client = new OkHttpClient.Builder()
    .connectTimeout(60, TimeUnit.SECONDS)
    .readTimeout(15, TimeUnit.SECONDS)
    .writeTimeout(15, TimeUnit.SECONDS)
    .cookieJar(new MyCookieJar())
    .addInterceptor(new HeaderInterceptor())//添加自定义Header拦截器
    .build();

class RetryAndFollowUpInterceptor(private val client: OkHttpClient) : Interceptor {

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    var request = chain.request
    val call = realChain.call
    var followUpCount = 0
    var priorResponse: Response? = null
    var newExchangeFinder = true
    var recoveredFailures = listOf<IOException>()
    while (true) {
      //这里会新建一个ExchangeFinder，ConnectInterceptor会使用到
      call.enterNetworkInterceptorExchange(request, newExchangeFinder)

      var response: Response
      var closeActiveExchange = true
      try {
        if (call.isCanceled()) {
          throw IOException("Canceled")
        }

        try {
          response = realChain.proceed(request)
          newExchangeFinder = true
        } catch (e: RouteException) {
          //尝试通过路由连接失败。该请求将不会被发送。
          if (!recover(e.lastConnectException, call, request, requestSendStarted = false)) {
            throw e.firstConnectException.withSuppressed(recoveredFailures)
          } else {
            recoveredFailures += e.firstConnectException
          }
          newExchangeFinder = false
          continue
        } catch (e: IOException) {
          //尝试与服务器通信失败。该请求可能已发送。
          if (!recover(e, call, request, requestSendStarted = e !is ConnectionShutdownException)) {
            throw e.withSuppressed(recoveredFailures)
          } else {
            recoveredFailures += e
          }
          newExchangeFinder = false
          continue
        }

        // Attach the prior response if it exists. Such responses never have a body.
        //尝试关联上一个response，注意：body是为null
        if (priorResponse != null) {
          response = response.newBuilder()
              .priorResponse(priorResponse.newBuilder()
                  .body(null)
                  .build())
              .build()
        }

        val exchange = call.interceptorScopedExchange
        //会根据 responseCode 来判断，构建一个新的request并返回来重试或者重定向
        val followUp = followUpRequest(response, exchange)

        if (followUp == null) {
          if (exchange != null && exchange.isDuplex) {
            call.timeoutEarlyExit()
          }
          closeActiveExchange = false
          return response
        }
        //如果请求体是一次性的，不需要再次重试
        val followUpBody = followUp.body
        if (followUpBody != null && followUpBody.isOneShot()) {
          closeActiveExchange = false
          return response
        }

        response.body?.closeQuietly()

        //最大重试次数，不同的浏览器是不同的，比如：Chrome为21，Safari则是16
        if (++followUpCount > MAX_FOLLOW_UPS) {
          throw ProtocolException("Too many follow-up requests: $followUpCount")
        }

        request = followUp
        priorResponse = response
      } finally {
        call.exitNetworkInterceptorExchange(closeActiveExchange)
      }
    }
  }

  /** 判断是否要进行重连，false->不尝试重连；true->尝试重连。*/
  private fun recover(
    e: IOException,
    call: RealCall,
    userRequest: Request,
    requestSendStarted: Boolean
  ): Boolean {
    //客户端禁止重试
    if (!client.retryOnConnectionFailure) return false

    //不能再次发送该请求体
    if (requestSendStarted && requestIsOneShot(e, userRequest)) return false

    //发生的异常是致命的，无法恢复，如:ProtocolException
    if (!isRecoverable(e, requestSendStarted)) return false

    //没有更多的路由来尝试重连
    if (!call.retryAfterFailure()) return false

    // 对于失败恢复，使用带有新连接的相同路由选择器
    return true
  }
···省略代码··· 

class BridgeInterceptor(private val cookieJar: CookieJar) : Interceptor {

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    //获取原始请求数据
    val userRequest = chain.request()
    val requestBuilder = userRequest.newBuilder()
    //重新构建请求头，请求体信息
    val body = userRequest.body

    val contentType = body.contentType()
    requestBuilder.header("Content-Type", contentType.toString())
    requestBuilder.header("Content-Length", contentLength.toString())
    requestBuilder.header("Transfer-Encoding", "chunked")
    requestBuilder.header("Host", userRequest.url.toHostHeader())
    requestBuilder.header("Connection", "Keep-Alive")

   ···省略代码···
   
    //添加cookie
    val cookies = cookieJar.loadForRequest(userRequest.url)
    if (cookies.isNotEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies))
    }
    //添加user-agent
    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", userAgent)
    }
    //重新构建一个Request，然后执行下一个拦截器来处理该请求
    val networkResponse = chain.proceed(requestBuilder.build())

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

    //创建一个新的responseBuilder，目的是将原始请求数据构建到response中
    val responseBuilder = networkResponse.newBuilder()
        .request(userRequest)

    if (transparentGzip &&
        "gzip".equals(networkResponse.header("Content-Encoding"), ignoreCase = true) &&
        networkResponse.promisesBody()) {
      val responseBody = networkResponse.body
      if (responseBody != null) {
        val gzipSource = GzipSource(responseBody.source())
        val strippedHeaders = networkResponse.headers.newBuilder()
            .removeAll("Content-Encoding")
            .removeAll("Content-Length")
            .build()
        //修改response header信息，移除Content-Encoding，Content-Length信息
        responseBuilder.headers(strippedHeaders)
        val contentType = networkResponse.header("Content-Type")
        //修改response body信息
        responseBuilder.body(RealResponseBody(contentType, -1L, gzipSource.buffer()))
      }
    }
    
    return responseBuilder.build()
   
···省略代码···

class CacheInterceptor(internal val cache: Cache?) : Interceptor {

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val call = chain.call()
    //通过request从OkHttpClient.cache中获取缓存
    val cacheCandidate = cache?.get(chain.request())

    val now = System.currentTimeMillis()
    //创建一个缓存策略，用来确定怎么使用缓存
    val strategy = CacheStrategy.Factory(now, chain.request(), cacheCandidate).compute()
    //为空表示不使用网络，反之，则表示使用网络
    val networkRequest = strategy.networkRequest
    //为空表示不使用缓存，反之，则表示使用缓存
    val cacheResponse = strategy.cacheResponse
    //追踪网络与缓存的使用情况
    cache?.trackResponse(strategy)
    val listener = (call as? RealCall)?.eventListener ?: EventListener.NONE
    //有缓存但不适用，关闭它
    if (cacheCandidate != null && cacheResponse == null) {
      cacheCandidate.body?.closeQuietly()
    }

    //如果网络被禁止，但是缓存又是空的，构建一个code为504的response，并返回
    if (networkRequest == null && cacheResponse == null) {
      return Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(HTTP_GATEWAY_TIMEOUT)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build().also {
            listener.satisfactionFailure(call, it)
          }
    }

    //如果我们禁用了网络不使用网络，且有缓存，直接根据缓存内容构建并返回response
    if (networkRequest == null) {
      return cacheResponse!!.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build().also {
            listener.cacheHit(call, it)
          }
    }
    //为缓存添加监听
    if (cacheResponse != null) {
      listener.cacheConditionalHit(call, cacheResponse)
    } else if (cache != null) {
      listener.cacheMiss(call)
    }

    var networkResponse: Response? = null
    try {
      //责任链往下处理，从服务器返回response 赋值给 networkResponse
      networkResponse = chain.proceed(networkRequest)
    } finally {
      //捕获I/O或其他异常，请求失败，networkResponse为空，且有缓存的时候，不暴露缓存内容。
      if (networkResponse == null && cacheCandidate != null) {
        cacheCandidate.body?.closeQuietly()
      }
    }

    //如果有缓存
    if (cacheResponse != null) {
      //且网络返回response code为304的时候，使用缓存内容新构建一个Response返回。
      if (networkResponse?.code == HTTP_NOT_MODIFIED) {
        val 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.also {
          listener.cacheHit(call, it)
        }
      } else {
        //否则关闭缓存响应体
        cacheResponse.body?.closeQuietly()
      }
    }

    //构建网络请求的response
    val response = networkResponse!!.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build()

    //如果cache不为null，即用户在OkHttpClient中配置了缓存，则将上一步新构建的网络请求response存到cache中
    if (cache != null) {
      //根据response的code,header以及CacheControl.noStore来判断是否可以缓存
      if (response.promisesBody() && CacheStrategy.isCacheable(response, networkRequest)) {
        // 将该response存入缓存
        val cacheRequest = cache.put(response)
        return cacheWritingResponse(cacheRequest, response).also {
          if (cacheResponse != null) {
            listener.cacheMiss(call)
          }
        }
      }
      //根据请求方法来判断缓存是否有效，只对Get请求进行缓存，其它方法的请求则移除
      if (HttpMethod.invalidatesCache(networkRequest.method)) {
        try {
          //缓存无效，将该请求缓存从client缓存配置中移除
          cache.remove(networkRequest)
        } catch (_: IOException) {
          // The cache cannot be written.
        }
      }
    }

    return response
  }
  
···省略代码···  

object ConnectInterceptor : Interceptor {
  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    //初始化一个exchange对象
    val exchange = realChain.call.initExchange(chain)
    //根据这个exchange对象来复制创建一个新的连接责任链
    val connectedChain = realChain.copy(exchange = exchange)
    //执行该连接责任链
    return connectedChain.proceed(realChain.request)
  }
}

RealCall.kt

internal fun initExchange(chain: RealInterceptorChain): Exchange {
    ...省略代码...
    //这里的exchangeFinder就是在RetryAndFollowUpInterceptor中创建的
    val exchangeFinder = this.exchangeFinder!!
    //返回一个ExchangeCodec（是个编码器，为request编码以及为response解码）
    val codec = exchangeFinder.find(client, chain)
    //根据exchangeFinder与codec新构建一个Exchange对象，并返回
    val result = Exchange(this, eventListener, exchangeFinder, codec)
  ...省略代码...
    return result
  }

ExchangeFinder.kt

fun find(
    client: OkHttpClient,
    chain: RealInterceptorChain
  ): ExchangeCodec {
    try {
      //查找合格可用的连接，返回一个 RealConnection 对象
      val resultConnection = findHealthyConnection(
          connectTimeout = chain.connectTimeoutMillis,
          readTimeout = chain.readTimeoutMillis,
          writeTimeout = chain.writeTimeoutMillis,
          pingIntervalMillis = client.pingIntervalMillis,
          connectionRetryEnabled = client.retryOnConnectionFailure,
          doExtensiveHealthChecks = chain.request.method != "GET"
      )
      //根据连接，创建并返回一个请求响应编码器：Http1ExchangeCodec 或者 Http2ExchangeCodec，分别对应Http1协议与Http2协议
      return resultConnection.newCodec(client, chain)
    } catch (e: RouteException) {
      trackFailure(e.lastConnectException)
      throw e
    } catch (e: IOException) {
      trackFailure(e)
      throw RouteException(e)
    }
  }

ExchangeFinder.kt

  private fun findHealthyConnection(
    connectTimeout: Int,
    readTimeout: Int,
    writeTimeout: Int,
    pingIntervalMillis: Int,
    connectionRetryEnabled: Boolean,
    doExtensiveHealthChecks: Boolean
  ): RealConnection {
    while (true) {
      //重点：查找连接
      val candidate = findConnection(
          connectTimeout = connectTimeout,
          readTimeout = readTimeout,
          writeTimeout = writeTimeout,
          pingIntervalMillis = pingIntervalMillis,
          connectionRetryEnabled = connectionRetryEnabled
      )
      //检查该连接是否合格可用，合格则直接返回该连接
      if (candidate.isHealthy(doExtensiveHealthChecks)) {
        return candidate
      }
      //如果该连接不合格，标记为不可用，从连接池中移除
      candidate.noNewExchanges()
    ...省略代码...
    }
  }

private fun findConnection(
    connectTimeout: Int, 
    readTimeout: Int,
    writeTimeout: Int,
    pingIntervalMillis: Int,
    connectionRetryEnabled: Boolean
  ): RealConnection {
    if (call.isCanceled()) throw IOException("Canceled")

    //第一次，尝试重连 call 中的 connection，不需要去重新获取连接
    val callConnection = call.connection // This may be mutated by releaseConnectionNoEvents()!
    if (callConnection != null) {
      var toClose: Socket? = null
      synchronized(callConnection) {
        if (callConnection.noNewExchanges || !sameHostAndPort(callConnection.route().address.url)) {
          toClose = call.releaseConnectionNoEvents()
        }
      }

      //如果 call 中的 connection 还没有释放，就重用它。
      if (call.connection != null) {
        check(toClose == null)
        return callConnection
      }

      //如果 call 中的 connection 已经被释放，关闭Socket.
      toClose?.closeQuietly()
      eventListener.connectionReleased(call, callConnection)
    }

    //需要一个新的连接，所以重置一些状态
    refusedStreamCount = 0
    connectionShutdownCount = 0
    otherFailureCount = 0

    //第二次，尝试从连接池中获取一个连接，不带路由，不带多路复用
    if (connectionPool.callAcquirePooledConnection(address, call, null, false)) {
      val result = call.connection!!
      eventListener.connectionAcquired(call, result)
      return result
    }

    //连接池中是空的，准备下次尝试连接的路由
    val routes: List<Route>?
    val route: Route
    
    ...省略代码...

      //第三次，再次尝试从连接池中获取一个连接，带路由，不带多路复用
      if (connectionPool.callAcquirePooledConnection(address, call, routes, false)) {
        val result = call.connection!!
        eventListener.connectionAcquired(call, result)
        return result
      }

      route = localRouteSelection.next()
    }

    //第四次，手动创建一个新连接
    val newConnection = RealConnection(connectionPool, route)
    call.connectionToCancel = newConnection
    try {
      newConnection.connect(
          connectTimeout,
          readTimeout,
          writeTimeout,
          pingIntervalMillis,
          connectionRetryEnabled,
          call,
          eventListener
      )
    } finally {
      call.connectionToCancel = null
    }
    call.client.routeDatabase.connected(newConnection.route())

    //第五次，再次尝试从连接池中获取一个连接，带路由，带多路复用。
    //这一步主要是为了校验一下，比如已经有了一条连接了，就可以直接复用，而不用使用手动创建的新连接。
    if (connectionPool.callAcquirePooledConnection(address, call, routes, true)) {
      val result = call.connection!!
      nextRouteToTry = route
      newConnection.socket().closeQuietly()
      eventListener.connectionAcquired(call, result)
      return result
    }

    synchronized(newConnection) {
      //将手动创建的新连接放入连接池
      connectionPool.put(newConnection)
      call.acquireConnectionNoEvents(newConnection)
    }

    eventListener.connectionAcquired(call, newConnection)
    return newConnection
  }

class CallServerInterceptor(private val forWebSocket: Boolean) : Interceptor {

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    val exchange = realChain.exchange!!
    val request = realChain.request
    val requestBody = request.body
    var invokeStartEvent = true
    var responseBuilder: Response.Builder? = null
    try {
      //写入请求头
      exchange.writeRequestHeaders(request)
      //如果不是GET请求，并且请求体不为空
      if (HttpMethod.permitsRequestBody(request.method) && requestBody != null) {
        //当请求头为"Expect: 100-continue"时，在发送请求体之前需要等待服务器返回"HTTP/1.1 100 Continue" 的response，如果没有等到该response，就不发送请求体。
        //POST请求，先发送请求头，在获取到100继续状态后继续发送请求体
        if ("100-continue".equals(request.header("Expect"), ignoreCase = true)) {
          //刷新请求，即发送请求头
          exchange.flushRequest()
          //解析响应头
          responseBuilder = exchange.readResponseHeaders(expectContinue = true)
          exchange.responseHeadersStart()
          invokeStartEvent = false
        }
        //写入请求体
        if (responseBuilder == null) {
          if (requestBody.isDuplex()) {
            //如果请求体是双公体，就先发送请求头，稍后在发送请求体
            exchange.flushRequest()
            val bufferedRequestBody = exchange.createRequestBody(request, true).buffer()
            //写入请求体
            requestBody.writeTo(bufferedRequestBody)
          } else {
            //如果获取到了"Expect: 100-continue"响应，写入请求体
            val bufferedRequestBody = exchange.createRequestBody(request, false).buffer()
            requestBody.writeTo(bufferedRequestBody)
            bufferedRequestBody.close()
          }
       ···省略代码···
        //请求结束，发送请求体
        exchange.finishRequest()
    ···省略代码···

    try {
      if (responseBuilder == null) {
        //读取响应头
        responseBuilder = exchange.readResponseHeaders(expectContinue = false)!!
        ···省略代码···
      //构建一个response
      var response = responseBuilder
          .request(request)
          .handshake(exchange.connection.handshake())
          .sentRequestAtMillis(sentRequestMillis)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build()
      var code = response.code
      ···省略代码···
      return response

···省略代码···

inner class AsyncCall(
    private val responseCallback: Callback
  ) : Runnable {
    //同一个域名的请求次数，volatile + AtomicInteger 保证在多线程下及时可见性与原子性
    @Volatile var callsPerHost = AtomicInteger(0)
      private set
    ...省略代码...