目录

• 前言
• 什么是OKhttp
• OkHttp是如何做网络请求的
• 1.它是如何使用的？
  
• 1.1 通过构造者模式添加 url，method，header，body 等完成一个请求的信息 Request 对象
• 1.2 同样通过构造者模式创建一个 OkHttpClicent 实例，可以按需配置
• 1.3 创建 Call 并且发起网络请求
• 2.如何通过 Call 发起请求？
  
• 2.1 Call 是什么
• 2.2 发起请求-异步请求
• 2.3 同步请求 RealCall#execute()
• 3.1 拦截器是怎么拦截的？
  
• 3.2 RetryAndFollowUpInterceptor
  
• 3.3 BridgeInterceptor
  
• 3.4 CacheInterceptor
  
• 3.5 ConnectInterceptor
  
• 3.6 CallServerInterceptor
  
• 3.如何通过拦截器处理请求和响应？
• 总结

前言

这篇文章主要讲解了okhttp的主要工作流程以及源码的解析。

什么是OKhttp

简单来说 OkHttp 就是一个客户端用来发送 HTTP 消息并对服务器的响应做出处理的应用层框架。 那么它有什么优点呢？

• 易使用、易扩展。
• 支持 HTTP/2 协议，允许对同一主机的所有请求共用同一个 socket 连接。
• 如果 HTTP/2 不可用, 使用连接池复用减少请求延迟。
• 支持 GZIP，减小了下载大小。
• 支持缓存处理，可以避免重复请求。
• 如果你的服务有多个 IP 地址，当第一次连接失败，OkHttp 会尝试备用地址。
• OkHttp 还处理了代理服务器问题和SSL握手失败问题。
  

OkHttp是如何做网络请求的

1.它是如何使用的？

1.1 通过构造者模式添加 url，method，header，body 等完成一个请求的信息 Request 对象

  val request = Request.Builder()
    .url("")
    .addHeader("","")
    .get()
    .build()

1.2 同样通过构造者模式创建一个 OkHttpClicent 实例，可以按需配置

  val okHttpClient = OkHttpClient.Builder()
    .connectTimeout(15, TimeUnit.SECONDS)
    .readTimeout(15, TimeUnit.SECONDS)
    .addInterceptor()
    .build()

1.3 创建 Call 并且发起网络请求

 val newCall = okHttpClient.newCall(request)
 //异步请求数据
 newCall.enqueue(object :Callback{
 override fun onFailure(call: Call, e: IOException) {}
 override fun onResponse(call: Call, response: Response) {}
 })
 //同步请求数据
 val response = newCall.execute()

整个使用流程很简单，主要的地方在于如何通过 Call 对象发起同/异步请求，后续的源码追踪以方法开始。

2.如何通过 Call 发起请求？

2.1 Call 是什么

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

2.2 发起请求-异步请求

//RealCall#enqueue(responseCallback: Callback) 

 override fun enqueue(responseCallback: Callback) {
 synchronized(this) {
  //检查这个call是否执行过，每个 call 只能被执行一次
  check(!executed) { "Already Executed" }
  executed = true
 }
 //此方法调用了EventListener#callStart(call: Call)，
 主要是用来监视应用程序的HTTP调用的数量，大小和各个阶段的耗时
 callStart()
 //创建AsyncCall,实际是个Runnable
 client.dispatcher.enqueue(AsyncCall(responseCallback))
 }

enqueue 最后一个方法分为两步

• 第一步将响应的回调放入 AsyncCall 对象中 ，AsyncCall 对象是 RealCall 的一个内部类实现了 Runnable 接口。
• 第二步通过 Dispatcher 类的 enqueue() 将 AsyncCall 对象传入

//Dispatcher#enqueue(call: AsyncCall)

 /** Ready async calls in the order they'll be run. */
 private val readyAsyncCalls = ArrayDeque<AsyncCall>()
 
 internal fun enqueue(call: AsyncCall) {
 synchronized(this) {
  //将call添加到即将运行的异步队列
  readyAsyncCalls.add(call)
  ...
  promoteAndExecute()
 }

//Dispatcher#promoteAndExecute() 
//将[readyAsyncCalls]过渡到[runningAsyncCalls]

 private fun promoteAndExecute(): Boolean {
 ...
 for (i in 0 until executableCalls.size) {
  val asyncCall = executableCalls[i]
  //这里就是通过 ExecutorService 执行 run()
  asyncCall.executeOn(executorService)
 }
 return isRunning
 }

//RealCall.kt中的内部类

 internal inner class AsyncCall(
 private val responseCallback: Callback
 ) : Runnable {
 fun executeOn(executorService: ExecutorService) {
  ...
  //执行Runnable
  executorService.execute(this)
  ...
 	}
 	
 override fun run() {
  threadName("OkHttp ${redactedUrl()}") {
  ...
  try {
   //兜兜转转 终于调用这个关键方法了
   val response = getResponseWithInterceptorChain()
   signalledCallback = true
   //通过之前传入的接口回调数据
   responseCallback.onResponse(this@RealCall, response)
  } catch (e: IOException) {
   if (signalledCallback) {
   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 {
   //移除队列
   client.dispatcher.finished(this)
  }
  }
 }
 }

2.3 同步请求 RealCall#execute()

 override fun execute(): Response {
 //同样判断是否执行过
 synchronized(this) {
  check(!executed) { "Already Executed" }
  executed = true
 }
 timeout.enter()
 //同样监听
 callStart()
 try {
  //同样执行
  client.dispatcher.executed(this)
  return getResponseWithInterceptorChain()
 } finally {
  //同样移除
  client.dispatcher.finished(this)
 }
 }

3.如何通过拦截器处理请求和响应？

无论同异步请求都会调用到 getResponseWithInterceptorChain() ，这个方法主要使用责任链模式将整个请求分为几个拦截器调用 ，简化了各自的责任和逻辑，可以扩展其它拦截器，看懂了拦截器 OkHttp 就了解的差不多了。

 @Throws(IOException::class)
 internal fun getResponseWithInterceptorChain(): Response {
 // 构建完整的拦截器
 val interceptors = mutableListOf<Interceptor>()
 interceptors += client.interceptors     //用户自己拦截器，数据最开始和最后
 interceptors += RetryAndFollowUpInterceptor(client) //失败后的重试和重定向
 interceptors += BridgeInterceptor(client.cookieJar) //桥接用户的信息和服务器的信息
 interceptors += CacheInterceptor(client.cache)   //处理缓存相关
 interceptors += ConnectInterceptor      //负责与服务器连接
 if (!forWebSocket) {
  interceptors += client.networkInterceptors   //配置 OkHttpClient 时设置，数据未经处理
 }
 interceptors += CallServerInterceptor(forWebSocket) //负责向服务器发送请求数据、从服务器读取响应数据
 //创建拦截链
 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 {
  //拦截链的执行
  val response = chain.proceed(originalRequest)
 	 ...
 } catch (e: IOException) {
	 ...
 } finally {
	 ...
 }
 }

//1.RealInterceptorChain#proceed(request: Request)
@Throws(IOException::class)
 override fun proceed(request: Request): Response {
 ...
 // copy出新的拦截链，链中的拦截器集合index+1
 val next = copy(index = index + 1, request = request)
 val interceptor = interceptors[index]

 //调用拦截器的intercept(chain: Chain): Response 返回处理后的数据 交由下一个拦截器处理
 @Suppress("USELESS_ELVIS")
 val response = interceptor.intercept(next) ?: throw NullPointerException(
  "interceptor $interceptor returned null")
 ...
 //返回最终的响应体
 return response
 }

拦截器开始操作 Request。

3.1 拦截器是怎么拦截的？

拦截器都继承自 Interceptor 类并实现了 fun intercept(chain: Chain): Response 方法。
在 intercept 方法里传入 chain 对象 调用它的 proceed() 然后 proceed() 方法里又 copy 下一个拦截器，然后双调用了 intercept(chain: Chain) 接着叒 chain.proceed(request) 直到最后一个拦截器 return response 然后一层一层向上反馈数据。

3.2 RetryAndFollowUpInterceptor

这个拦截器是用来处理重定向的后续请求和失败重试，也就是说一般第一次发起请求不需要重定向会调用下一个拦截器。

@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) {
  ...//在调用下一个拦截器前的操作
  var response: Response
  try {
  ...
  try {
   //调用下一个拦截器
   response = realChain.proceed(request)
   newExchangeFinder = true
  } catch (e: RouteException) {
  ...
   continue
  } catch (e: IOException) {
   ...
   continue
  }

  ...
  //处理上一个拦截器返回的 response
  val followUp = followUpRequest(response, exchange)
	 ...
	 //中间有一些判断是否需要重新请求 不需要则返回 response
	 //处理之后重新请求 Request
  request = followUp
  priorResponse = response
  } finally {
  call.exitNetworkInterceptorExchange(closeActiveExchange)
  }
 }
 }
 
 @Throws(IOException::class)
 private fun followUpRequest(userResponse: Response, exchange: Exchange?): Request? {
 val route = exchange?.connection?.route()
 val responseCode = userResponse.code

 val method = userResponse.request.method
 when (responseCode) {
	 //3xx 重定向
  HTTP_PERM_REDIRECT, HTTP_TEMP_REDIRECT, HTTP_MULT_CHOICE, HTTP_MOVED_PERM, HTTP_MOVED_TEMP, HTTP_SEE_OTHER -> {
  //这个方法重新 构建了 Request 用于重新请求
  return buildRedirectRequest(userResponse, method)
  }
	 ... 省略一部分code
  else -> return null
 }
 }

在 followUpRequest(userResponse: Response, exchange: Exchange?): Request? 方法中判断了 response 中的服务器响应码做出了不同的操作。

3.3 BridgeInterceptor

它负责对于 Http 的额外预处理，比如 Content-Length 的计算和添加、 gzip 的⽀持（Accept-Encoding: gzip）、 gzip 压缩数据的解包等，这个类比较简单就不贴代码了，想了解的话可以自行查看。

3.4 CacheInterceptor

这个类负责 Cache 的处理，如果本地有了可⽤的 Cache，⼀个请求可以在没有发⽣实质⽹络交互的情况下就返回缓存结果，实现如下。

 @Throws(IOException::class)
 override fun intercept(chain: Interceptor.Chain): Response {
 //在Cache(DiskLruCache)类中 通过request.url匹配response
 val cacheCandidate = cache?.get(chain.request())
 //记录当前时间点
 val now = System.currentTimeMillis()
 //缓存策略 有两种类型 
 //networkRequest 网络请求
 //cacheResponse 缓存的响应
 val strategy = CacheStrategy.Factory(now, chain.request(), cacheCandidate).compute()
 val networkRequest = strategy.networkRequest
 val cacheResponse = strategy.cacheResponse
 //计算请求次数和缓存次数
 cache?.trackResponse(strategy)
 ...
 // 如果 禁止使用网络 并且 缓存不足，返回504和空body的Response
 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()
 }

 // 如果策略中不能使用网络，就把缓存中的response封装返回
 if (networkRequest == null) {
  return cacheResponse!!.newBuilder()
   .cacheResponse(stripBody(cacheResponse))
   .build()
 }
 //调用拦截器process从网络获取数据
 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()
  }
 }
 
 //如果有缓存的Response
 if (cacheResponse != null) {
  //如果网络请求返回code为304 即说明资源未修改
  if (networkResponse?.code == HTTP_NOT_MODIFIED) {
  //直接封装封装缓存的Response返回即可
  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
  } else {
  cacheResponse.body?.closeQuietly()
  }
 }

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

 if (cache != null) {
  //判断是否具有主体 并且 是否可以缓存供后续使用
  if (response.promisesBody() && CacheStrategy.isCacheable(response, networkRequest)) {
  // 加入缓存中
  val cacheRequest = cache.put(response)
  return cacheWritingResponse(cacheRequest, response)
  }
  //如果请求方法无效 就从缓存中remove掉
  if (HttpMethod.invalidatesCache(networkRequest.method)) {
  try {
   cache.remove(networkRequest)
  } catch (_: IOException) {
   // The cache cannot be written.
  }
  }
 }
 
 return response
 }

3.5 ConnectInterceptor

此类负责建⽴连接。 包含了⽹络请求所需要的 TCP 连接（HTTP），或者 TCP 之前的 TLS 连接（HTTPS），并且会创建出对应的 HttpCodec 对象（⽤于编码解码 HTTP 请求）。

@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)
 }

看似短短四行实际工作还是比较多的。

/** Finds a new or pooled connection to carry a forthcoming request and response. */
 internal fun initExchange(chain: RealInterceptorChain): Exchange {
	...
	//codec是对 HTTP 协议操作的抽象，有两个实现：Http1Codec和Http2Codec，对应 HTTP/1.1 和 HTTP/2。
 val codec = exchangeFinder.find(client, chain)
 val result = Exchange(this, eventListener, exchangeFinder, codec)
 ...
 return result
 }
 
 #ExchangeFinder.find
 fun find(client: OkHttpClient,chain: RealInterceptorChain):ExchangeCodec {
 try {
  //寻找一个可用的连接
  val resultConnection = findHealthyConnection(
   connectTimeout = chain.connectTimeoutMillis,
   readTimeout = chain.readTimeoutMillis,
   writeTimeout = chain.writeTimeoutMillis,
   pingIntervalMillis = client.pingIntervalMillis,
   connectionRetryEnabled = client.retryOnConnectionFailure,
   doExtensiveHealthChecks = chain.request.method != "GET"
  )
  return resultConnection.newCodec(client, chain)
 } catch (e: RouteException) {
  trackFailure(e.lastConnectException)
  throw e
 } catch (e: IOException) {
  trackFailure(e)
  throw RouteException(e)
 }
 }
 
 @Throws(IOException::class)
 private fun findHealthyConnection(
 connectTimeout: Int,
 readTimeout: Int,
 writeTimeout: Int,
 pingIntervalMillis: Int,
 connectionRetryEnabled: Boolean,
 doExtensiveHealthChecks: Boolean
 ): RealConnection {
 while (true) {
 	//寻找连接
  val candidate = findConnection(
   connectTimeout = connectTimeout,
   readTimeout = readTimeout,
   writeTimeout = writeTimeout,
   pingIntervalMillis = pingIntervalMillis,
   connectionRetryEnabled = connectionRetryEnabled
  )

  //确认找到的连接可用并返回
  if (candidate.isHealthy(doExtensiveHealthChecks)) {
  return candidate
  }
	 ...
  throw IOException("exhausted all routes")
 }
 }
 
 @Throws(IOException::class)
 private fun findConnection(
 connectTimeout: Int,
 readTimeout: Int,
 writeTimeout: Int,
 pingIntervalMillis: Int,
 connectionRetryEnabled: Boolean
 ): RealConnection {
 if (call.isCanceled()) throw IOException("Canceled")

 // 1. 尝试重用这个call的连接 比如重定向需要再次请求 那么这里就会重用之前的连接
 val callConnection = call.connection
 if (callConnection != null) {
  var toClose: Socket? = null
  synchronized(callConnection) {
  if (callConnection.noNewExchanges || !sameHostAndPort(callConnection.route().address.url)) {
   toClose = call.releaseConnectionNoEvents()
  }
  }
	 //返回这个连接
  if (call.connection != null) {
  check(toClose == null)
  return callConnection
  }

  // The call's connection was released.
  toClose?.closeQuietly()
  eventListener.connectionReleased(call, callConnection)
 }
	...
 // 2. 尝试从连接池中找一个连接 找到就返回连接
 if (connectionPool.callAcquirePooledConnection(address, call, null, false)) {
  val result = call.connection!!
  eventListener.connectionAcquired(call, result)
  return result
 }

 // 3. 如果连接池中没有 计算出下一次要尝试的路由
 val routes: List<Route>?
 val route: Route
 if (nextRouteToTry != null) {
  // Use a route from a preceding coalesced connection.
  routes = null
  route = nextRouteToTry!!
  nextRouteToTry = null
 } else if (routeSelection != null && routeSelection!!.hasNext()) {
  // Use a route from an existing route selection.
  routes = null
  route = routeSelection!!.next()
 } else {
  // Compute a new route selection. This is a blocking operation!
  var localRouteSelector = routeSelector
  if (localRouteSelector == null) {
  localRouteSelector = RouteSelector(address, call.client.routeDatabase, call, eventListener)
  this.routeSelector = localRouteSelector
  }
  val localRouteSelection = localRouteSelector.next()
  routeSelection = localRouteSelection
  routes = localRouteSelection.routes

  if (call.isCanceled()) throw IOException("Canceled")

  // Now that we have a set of IP addresses, make another attempt at getting a connection from
  // the pool. We have a better chance of matching thanks to connection coalescing.
  if (connectionPool.callAcquirePooledConnection(address, call, routes, false)) {
  val result = call.connection!!
  eventListener.connectionAcquired(call, result)
  return result
  }

  route = localRouteSelection.next()
 }

 // Connect. Tell the call about the connecting call so async cancels work.
 // 4.到这里还没有找到可用的连接 但是找到了 route 即路由 进行socket/tls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())

 // If we raced another call connecting to this host, coalesce the connections. This makes for 3
 // different lookups in the connection pool!
 // 4.查找是否有多路复用(http2)的连接,有就返回
 if (connectionPool.callAcquirePooledConnection(address, call, routes, true)) {
  val result = call.connection!!
  nextRouteToTry = route
  newConnection.socket().closeQuietly()
  eventListener.connectionAcquired(call, result)
  return result
 }

 synchronized(newConnection) {
  //放入连接池中
  connectionPool.put(newConnection)
  call.acquireConnectionNoEvents(newConnection)
 }

 eventListener.connectionAcquired(call, newConnection)
 return newConnection
 }
 

接下来看看是如何建立连接的

fun connect(
 connectTimeout: Int,
 readTimeout: Int,
 writeTimeout: Int,
 pingIntervalMillis: Int,
 connectionRetryEnabled: Boolean,
 call: Call,
 eventListener: EventListener
 ) {
 ...
 while (true) {
  try {
  if (route.requiresTunnel()) {
   //创建tunnel，用于通过http代理访问https
   //其中包含connectSocket、createTunnel
   connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener)
   if (rawSocket == null) {
   // We were unable to connect the tunnel but properly closed down our resources.
   break
   }
  } else {
   //不创建tunnel就创建socket连接 获取到数据流
   connectSocket(connectTimeout, readTimeout, call, eventListener)
  }
  //建立协议连接tsl
  establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener)
  eventListener.connectEnd(call, route.socketAddress, route.proxy, protocol)
  break
  } catch (e: IOException) {
  ...
  }
 }
	...
 }

建立tsl连接

@Throws(IOException::class)
 private fun establishProtocol(
 connectionSpecSelector: ConnectionSpecSelector,
 pingIntervalMillis: Int,
 call: Call,
 eventListener: EventListener
 ) {
 	//ssl为空 即http请求 明文请求
 if (route.address.sslSocketFactory == null) {
  if (Protocol.H2_PRIOR_KNOWLEDGE in route.address.protocols) {
  socket = rawSocket
  protocol = Protocol.H2_PRIOR_KNOWLEDGE
  startHttp2(pingIntervalMillis)
  return
  }

  socket = rawSocket
  protocol = Protocol.HTTP_1_1
  return
 }
	//否则为https请求 需要连接sslSocket 验证证书是否可被服务器接受 保存tsl返回的信息
 eventListener.secureConnectStart(call)
 connectTls(connectionSpecSelector)
 eventListener.secureConnectEnd(call, handshake)

 if (protocol === Protocol.HTTP_2) {
  startHttp2(pingIntervalMillis)
 }
 }

至此，创建好了连接，返回到最开始的 find() 方法返回 ExchangeCodec 对象，再包装为 Exchange 对象用来下一个拦截器操作。

3.6 CallServerInterceptor

这个类负责实质的请求与响应的 I/O 操作，即往 Socket ⾥写⼊请求数据，和从 Socket ⾥读取响应数据。

总结

用一张 @piasy 的图来做总结，图很干练结构也很清晰。

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