okhttp-重温OKHTTP源码

本文基于OkHttp4.12.0源码分析 官方地址

概括

本篇主要是对okhttp开源库的一个详细解析，包含详细的请求流程分析、各大拦截器的解读等。

使用方法

同步请求：创建一个OKHttpClient对象，一个Request对象，然后利用它们创建一个Call对象，最后调用execute()方法来拿到Response

val client = OkHttpClient()
val request = Request.Builder()
                .url("http://www.baidu.com")
                .build()

val response = client.newCall(request).execute();

异步调用：异步调用不同的在于使用enqueue方法并传入一个Callback接口来获取返回的Response。

val client = OkHttpClient()
val request = Request.Builder()
                .url("http://www.baidu.com")
                .build()
client.newCall(request).enqueue(object : Callback {
            override fun onFailure(call: Call, e: IOException) {
                e.printStackTrace()
            }

            override fun onResponse(call: Call, response: Response) {
                Log.d("TAG", "response = " + response.message)
                Log.d("TAG", "response = " + response.body!!.toString())
            }
})

下面关于一些关键类的解析。

OKHttpClient

这是一个请求配置类，采用了建造者模式，方便用户配置一些请求参数，如配置callTimeout，cookie，interceptor等等。

open class OkHttpClient internal constructor(
  builder: Builder
) : Cloneable, Call.Factory, WebSocket.Factory {
@get:JvmName("dispatcher") val dispatcher: Dispatcher = builder.dispatcher

  @get:JvmName("connectionPool") val connectionPool: ConnectionPool = builder.connectionPool

  /**
   * Returns an immutable list of interceptors that observe the full span of each call: from before
   * the connection is established (if any) until after the response source is selected (either the
   * origin server, cache, or both).
   */
  @get:JvmName("interceptors") val interceptors: List<Interceptor> =
      builder.interceptors.toImmutableList()

....

  constructor() : this(Builder())

....

  /** Prepares the [request] to be executed at some point in the future. */
  override fun newCall(request: Request): Call = RealCall(this, request, forWebSocket = false)


class Builder constructor() {
    internal var dispatcher: Dispatcher = Dispatcher()
    internal var connectionPool: ConnectionPool = ConnectionPool()
    internal val interceptors: MutableList<Interceptor> = mutableListOf()
    internal val networkInterceptors: MutableList<Interceptor> = mutableListOf()
    internal var eventListenerFactory: EventListener.Factory = EventListener.NONE.asFactory()
    internal var retryOnConnectionFailure = true
    internal var authenticator: Authenticator = Authenticator.NONE
    internal var followRedirects = true
    internal var followSslRedirects = true
    internal var cookieJar: CookieJar = CookieJar.NO_COOKIES
    internal var cache: Cache? = null
    internal var dns: Dns = Dns.SYSTEM
    internal var proxy: Proxy? = null
    internal var proxySelector: ProxySelector? = null
    internal var proxyAuthenticator: Authenticator = Authenticator.NONE
    internal var socketFactory: SocketFactory = SocketFactory.getDefault()
    internal var sslSocketFactoryOrNull: SSLSocketFactory? = null
    internal var x509TrustManagerOrNull: X509TrustManager? = null
    internal var connectionSpecs: List<ConnectionSpec> = DEFAULT_CONNECTION_SPECS
    internal var protocols: List<Protocol> = DEFAULT_PROTOCOLS
    internal var hostnameVerifier: HostnameVerifier = OkHostnameVerifier
    internal var certificatePinner: CertificatePinner = CertificatePinner.DEFAULT
    internal var certificateChainCleaner: CertificateChainCleaner? = null
    internal var callTimeout = 0
    internal var connectTimeout = 10_000
    internal var readTimeout = 10_000
    internal var writeTimeout = 10_000
    internal var pingInterval = 0
    internal var minWebSocketMessageToCompress = RealWebSocket.DEFAULT_MINIMUM_DEFLATE_SIZE
    internal var routeDatabase: RouteDatabase? = null

....

Request

这里作为请求参数的配置类，本身也采用了建造者模式，但相比OKHttpClient，Request就比较简单了，只有四个参数，分别是url、method、headers、body。

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 {
    internal var url: HttpUrl? = null
    internal var method: String
    internal var headers: Headers.Builder
    internal var body: RequestBody? = null
...

Call

请求调用接口，表示这个请求已经准备好可以执行，也可以取消，只能执行一次。

interface Call : Cloneable {
  /** Returns the original request that initiated this call. */
  fun request(): Request

  /**
   * Invokes the request immediately, and blocks until the response can be processed or is in error.
   *
   * To avoid leaking resources callers should close the [Response] which in turn will close the
   * underlying [ResponseBody].
   *
   * ```
   * // ensure the response (and underlying response body) is closed
   * try (Response response = client.newCall(request).execute()) {
   *   ...
   * }
   * ```
   *
   * The caller may read the response body with the response's [Response.body] method. To avoid
   * leaking resources callers must [close the response body][ResponseBody] or the response.
   *
   * Note that transport-layer success (receiving a HTTP response code, headers and body) does not
   * necessarily indicate application-layer success: `response` may still indicate an unhappy HTTP
   * response code like 404 or 500.
   *
   * @throws IOException if the request could not be executed due to cancellation, a connectivity
   *     problem or timeout. Because networks can fail during an exchange, it is possible that the
   *     remote server accepted the request before the failure.
   * @throws IllegalStateException when the call has already been executed.
   */
  @Throws(IOException::class)
  fun execute(): Response

  /**
   * Schedules the request to be executed at some point in the future.
   *
   * The [dispatcher][OkHttpClient.dispatcher] defines when the request will run: usually
   * immediately unless there are several other requests currently being executed.
   *
   * This client will later call back `responseCallback` with either an HTTP response or a failure
   * exception.
   *
   * @throws IllegalStateException when the call has already been executed.
   */
  fun enqueue(responseCallback: Callback)

  /** Cancels the request, if possible. Requests that are already complete cannot be canceled. */
  fun cancel()

  /**
   * Returns true if this call has been either [executed][execute] or [enqueued][enqueue]. It is an
   * error to execute a call more than once.
   */
  fun isExecuted(): Boolean

  fun isCanceled(): Boolean

  /**
   * Returns a timeout that spans the entire call: resolving DNS, connecting, writing the request
   * body, server processing, and reading the response body. If the call requires redirects or
   * retries all must complete within one timeout period.
   *
   * Configure the client's default timeout with [OkHttpClient.Builder.callTimeout].
   */
  fun timeout(): Timeout

  /**
   * Create a new, identical call to this one which can be enqueued or executed even if this call
   * has already been.
   */
  public override fun clone(): Call

  fun interface Factory {
    fun newCall(request: Request): Call
  }
}

RealCall

RealCall是Call接口的具体实现类，是应用端与网络层的连接器，输入应用端原始的请求与连接数据，以及网络层返回的response及其他数据流。通过使用方法可知，创建完RealCall对象后，就要调用同步或异步请求方法，所以它里面还包含同步请求execute()与异步请求enqueue()方法。

class RealCall(
  val client: OkHttpClient,
  /** The application's original request unadulterated by redirects or auth headers. */
  val originalRequest: Request,
  val forWebSocket: Boolean
) : Call {

....

override fun execute(): Response {
    check(executed.compareAndSet(false, true)) { "Already Executed" }

    timeout.enter()
    callStart()
    try {
      client.dispatcher.executed(this)
      return getResponseWithInterceptorChain()
    } finally {
      client.dispatcher.finished(this)
    }
  }

  override fun enqueue(responseCallback: Callback) {
    check(executed.compareAndSet(false, true)) { "Already Executed" }

    callStart()
    client.dispatcher.enqueue(AsyncCall(responseCallback))
  }
....

AsyncCall

异步请求调用，是RealCall的一个内部类，本身实现了Runnable接口，被调度器中的线程池所执行。

internal inner class AsyncCall(
    private val responseCallback: Callback
  ) : Runnable {

....

Dispatcher

调度器，用来调度Call对象，同时包含线程池和异步请求队列，用来存放与执行AsyncCall对象。

流程分析

同步请求

val response = client.newCall(request).execute();

newCall方法就是创建一个RealCall对象，然后执行其execute方法

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

调度器调用executed犯法，将当前的RealCall对象加入到runningSyncCalls队列中，然后调用getResponseWithInterceptorChain方法拿到response。

异步请求

client.newCall(request).enqueue(object : Callback {
    override fun onFailure(call: Call, e: IOException) {
        e.printStackTrace()
    }

    override fun onResponse(call: Call, response: Response) {
        
    }
})


RealCall.kt
override fun enqueue(responseCallback: Callback) {
    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) {
    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.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>()
    val isRunning: Boolean
    synchronized(this) {
      //遍历readyAsyncCalls队列
      val i = readyAsyncCalls.iterator()
      while (i.hasNext()) {
        val asyncCall = i.next()
        //检查runningAsyncCalls的数量
        if (runningAsyncCalls.size >= this.maxRequests) break // Max capacity.
        //同域名最大请求数
        if (asyncCall.callsPerHost.get() >= this.maxRequestsPerHost) continue // Host max capacity.

        i.remove()
        asyncCall.callsPerHost.incrementAndGet()
        executableCalls.add(asyncCall)
        runningAsyncCalls.add(asyncCall)
      }
      isRunning = runningCallsCount() > 0
    }

    for (i in 0 until executableCalls.size) {
      val asyncCall = executableCalls[i]
      asyncCall.executeOn(executorService)
    }

    return isRunning
  }


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

      var success = false
      try {
        //执行AsyncCall这个runnable
        executorService.execute(this)
        success = true
      } catch (e: RejectedExecutionException) {
        val ioException = InterruptedIOException("executor rejected")
        ioException.initCause(e)
        noMoreExchanges(ioException)
        responseCallback.onFailure(this@RealCall, ioException)
      } finally {
        if (!success) {
          client.dispatcher.finished(this) // This call is no longer running!
        }
      }
    }


override fun run() {
      threadName("OkHttp ${redactedUrl()}") {
        var signalledCallback = false
        timeout.enter()
        try {
          //获取返回数据
          val response = getResponseWithInterceptorChain()
          signalledCallback = true
          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 {
            responseCallback.onFailure(this@RealCall, e)
          }
        } catch (t: Throwable) {
          cancel()
          if (!signalledCallback) {
            val canceledException = IOException("canceled due to $t")
            canceledException.addSuppressed(t)
            responseCallback.onFailure(this@RealCall, canceledException)
          }
          throw t
        } finally {
          client.dispatcher.finished(this)
        }
      }
    }
  }

获取Response

@Throws(IOException::class)
  internal fun getResponseWithInterceptorChain(): Response {
    // Build a full stack of interceptors.
    //拦截器
    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)
    //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
    )
    
    var calledNoMoreExchanges = false
    try {
      //根据拦截器责任链来获取response
      val response = chain.proceed(originalRequest)
      if (isCanceled()) {
        response.closeQuietly()
        throw IOException("Canceled")
      }
      return response
    } catch (e: IOException) {
      calledNoMoreExchanges = true
      throw noMoreExchanges(e) as Throwable
    } finally {
      if (!calledNoMoreExchanges) {
        noMoreExchanges(null)
      }
    }
  }

RealInterceptorChain.kt

@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"
      }
    }

    // Call the next interceptor in the chain.
    //拷贝下一级责任链
    val next = copy(index = index + 1, request = request)
    val interceptor = interceptors[index]

    @Suppress("USELESS_ELVIS")
    //执行拦截器拦截方法
    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
  }

Interceptor

该接口只声明了一个拦截器方法，同时，在其内部，还有一个Chain接口，核心方法是proceed(request)处理请求来获取response

各类拦截器

client.interceptors：由开发者自定义的拦截器，会在所有拦截器处理之前进行最早的拦截处理，可用于添加一些公共参数，如自定义header、自定义log等等。

class HeadInterceptor : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response {
        val request = chain.request().newBuilder()
            .addHeader("device-serial", "********8")
            .build()
        return chain.proceed(request)
    }
}

fun test() {
    val client = OkHttpClient.Builder()
        .connectTimeout(60, TimeUnit.MILLISECONDS)
        .readTimeout(15, TimeUnit.MILLISECONDS)
        .writeTimeout(15, TimeUnit.MILLISECONDS)
        .addInterceptor(HeadInterceptor())
        .build()
}

RetryAndFollowUpInterceptor：这里会对连接做一些初始化的工作，以及请求失败的重试工作，重定向的后续请求工作。

@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) {
      //这里创建一个exchangeFinder，ConnectInterceptor会用到
      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) {
          // The attempt to connect via a route failed. The request will not have been sent.
          //当请求还未完全发送时连接失败，可以尝试检查是否能重新连接
          if (!recover(e.lastConnectException, call, request, requestSendStarted = false)) {
            throw e.firstConnectException.withSuppressed(recoveredFailures)
          } else {
            recoveredFailures += e.firstConnectException
          }
          newExchangeFinder = false
          continue
        } catch (e: IOException) {
          // An attempt to communicate with a server failed. The request may have been sent.
          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.
        //尝试使用上一个已经构建好的response，body为空
        if (priorResponse != null) {
          response = response.newBuilder()
              .priorResponse(priorResponse.newBuilder()
                  .body(null)
                  .build())
              .build()
        }

        val exchange = call.interceptorScopedExchange
        //根据responseCode来判断，是否构建一个新的response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
        }

        val followUpBody = followUp.body
        if (followUpBody != null && followUpBody.isOneShot()) {
          closeActiveExchange = false
          return response
        }

        response.body?.closeQuietly()

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

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

  /**
   * Report and attempt to recover from a failure to communicate with a server. Returns true if
   * `e` is recoverable, or false if the failure is permanent. Requests with a body can only
   * be recovered if the body is buffered or if the failure occurred before the request has been
   * sent.
   */
  //判断是否需要重连
  private fun recover(
    e: IOException,
    call: RealCall,
    userRequest: Request,
    requestSendStarted: Boolean
  ): Boolean {
    // The application layer has forbidden retries.
    //客户端禁止重试
    if (!client.retryOnConnectionFailure) return false
    
    //不能再次发送该请求体
    // We can't send the request body again.
    if (requestSendStarted && requestIsOneShot(e, userRequest)) return false
    
    //发生的异常时致命的，无法恢复
    // This exception is fatal.
    if (!isRecoverable(e, requestSendStarted)) return false

    //没有跟多的路由来尝试重连
    // No more routes to attempt.
    if (!call.retryAfterFailure()) return false

    // For failure recovery, use the same route selector with a new connection.
    return true
  }

BridgeInterceptor：这是客户端与服务器之间的沟通桥梁，负责将用户构建的请求转换为服务器需要的请求，以及将网络请求返回回来的响应转换为用户可用的相应

@Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val userRequest = chain.request()
    val requestBuilder = userRequest.newBuilder()

    val body = userRequest.body
    if (body != null) {
      val contentType = body.contentType()
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString())
      }

      val contentLength = body.contentLength()
      if (contentLength != -1L) {
        requestBuilder.header("Content-Length", contentLength.toString())
        requestBuilder.removeHeader("Transfer-Encoding")
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked")
        requestBuilder.removeHeader("Content-Length")
      }
    }

    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", userRequest.url.toHostHeader())
    }

    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.
    var transparentGzip = false
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true
      requestBuilder.header("Accept-Encoding", "gzip")
    }

    val cookies = cookieJar.loadForRequest(userRequest.url)
    if (cookies.isNotEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies))
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", userAgent)
    }
    //执行下一个拦截器
    val networkResponse = chain.proceed(requestBuilder.build())

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

    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()
        responseBuilder.headers(strippedHeaders)
        val contentType = networkResponse.header("Content-Type")
        responseBuilder.body(RealResponseBody(contentType, -1L, gzipSource.buffer()))
      }
    }

    return responseBuilder.build()
  }

CacheInterceptor：这里是缓存相关的处理，会根据用户在OkhttpClient里定义的缓存配置，然后结合请求新建一个缓存策略，由他来判断使用网络还是缓存来构建response

@Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val call = chain.call()
    val cacheCandidate = cache?.get(chain.request())

    val now = System.currentTimeMillis()
    //创建一个缓存策略
    val strategy = CacheStrategy.Factory(now, chain.request(), cacheCandidate).compute()
    //使用网络
    val networkRequest = strategy.networkRequest
    //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
    //关闭不适用的缓存数据
    if (cacheCandidate != null && cacheResponse == null) {
      // The cache candidate wasn't applicable. Close it.
      cacheCandidate.body?.closeQuietly()
    }
    //如果网络被禁止，同时缓存也是空的，构建一个code为504的response返回
    // If we're forbidden from using the network and the cache is insufficient, fail.
    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)
          }
    }

    //没有网络，由缓存数据，则构建缓存内容并返回
    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse!!.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build().also {
            listener.cacheHit(call, it)
          }
    }

    if (cacheResponse != null) {
      listener.cacheConditionalHit(call, cacheResponse)
    } else if (cache != null) {
      listener.cacheMiss(call)
    }

    var networkResponse: Response? = 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) {
        cacheCandidate.body?.closeQuietly()
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      //如果由缓存，且返回的code为304时，使用缓存构建数据并返回
      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 {
        cacheResponse.body?.closeQuietly()
      }
    }

    val response = networkResponse!!.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build()

    if (cache != null) {
      if (response.promisesBody() && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        val cacheRequest = cache.put(response)
        return cacheWritingResponse(cacheRequest, response).also {
          if (cacheResponse != null) {
            // This will log a conditional cache miss only.
            listener.cacheMiss(call)
          }
        }
      }
      //根据请求方法来判定缓存是否有效，只对Get请求进行缓存
      if (HttpMethod.invalidatesCache(networkRequest.method)) {
        try {
          cache.remove(networkRequest)
        } catch (_: IOException) {
          // The cache cannot be written.
        }
      }
    }

    return response
  }

ConnectInterceptor：这里主要是负责建立连接，会建立tcp或者tls连接

@Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    val exchange = realChain.call.initExchange(chain)
    val connectedChain = realChain.copy(exchange = exchange)
    return connectedChain.proceed(realChain.request)
}

client.networkInterceptors：这也是开发者自己设置的，但所处位置不同，用处也不相同

CallServerInterceptor：这里是进行网络请求和响应的地方。将请求头与请求体发送给服务器，以及解析服务器返回的response。

以下是整体调用流程

知识点：

建造者模式：不论是OkHttpClient、Request还是Response，都是用了建造者模式，因为这几个类中都有很多参数，需要供用户选择需要的参数来构建其想要的实例。

工厂方法模式：帮助生成复杂对象，比如OkHttpClient.newCall(request Request)来创建call对象

责任链模式：这里将拦截器构建成拦截器责任链，然后按顺序从上往下执行，得到response后，从下往上传。

线程安全

使用原子类，重入锁等实现线程安全

数据结构

采用队列实现，更符合整体网络请求的要求，比如先到先得。