前言

這篇文章主要講解了okhttp的主要工作流程以及源碼的解析。

什么是OKhttp

簡單來說 OkHttp 就是一個客戶端用來發送 HTTP 消息并對服務器的響應做出處理的應用層框架。 那么它有什么優點呢？

易使用、易擴展。 支持 HTTP/2 協議，允許對同一主機的所有請求共用同一個 socket 連接。 如果 HTTP/2 不可用, 使用連接池復用減少請求延遲。 支持 GZIP，減小了下載大小。 支持緩存處理，可以避免重復請求。 如果你的服務有多個 IP 地址，當第一次連接失敗，OkHttp 會嘗試備用地址。 OkHttp 還處理了代理服務器問題和SSL握手失敗問題。 OkHttp是如何做網絡請求的1.它是如何使用的？1.1 通過構造者模式添加 url，method，header，body 等完成一個請求的信息 Request 對象

val request = Request.Builder() .url('') .addHeader('','') .get() .build()1.2 同樣通過構造者模式創建一個 OkHttpClicent 實例，可以按需配置

val okHttpClient = OkHttpClient.Builder() .connectTimeout(15, TimeUnit.SECONDS) .readTimeout(15, TimeUnit.SECONDS) .addInterceptor() .build()1.3 創建 Call 并且發起網絡請求

val newCall = okHttpClient.newCall(request) //異步請求數據 newCall.enqueue(object :Callback{ override fun onFailure(call: Call, e: IOException) {} override fun onResponse(call: Call, response: Response) {} }) //同步請求數據 val response = newCall.execute()

整個使用流程很簡單，主要的地方在于如何通過 Call 對象發起同/異步請求，后續的源碼追蹤以方法開始。

2.如何通過 Call 發起請求？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 發起請求-異步請求

//RealCall#enqueue(responseCallback: Callback) override fun enqueue(responseCallback: Callback) { synchronized(this) { //檢查這個call是否執行過，每個 call 只能被執行一次 check(!executed) { 'Already Executed' } executed = true } //此方法調用了EventListener#callStart(call: Call)， 主要是用來監視應用程序的HTTP調用的數量，大小和各個階段的耗時 callStart() //創建AsyncCall,實際是個Runnable client.dispatcher.enqueue(AsyncCall(responseCallback)) }

enqueue 最后一個方法分為兩步

第一步將響應的回調放入 AsyncCall 對象中 ，AsyncCall 對象是 RealCall 的一個內部類實現了 Runnable 接口。 第二步通過 Dispatcher 類的 enqueue() 將 AsyncCall 對象傳入

//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) { //將call添加到即將運行的異步隊列 readyAsyncCalls.add(call) ... promoteAndExecute() }//Dispatcher#promoteAndExecute() //將[readyAsyncCalls]過渡到[runningAsyncCalls] private fun promoteAndExecute(): Boolean { ... for (i in 0 until executableCalls.size) { val asyncCall = executableCalls[i] //這里就是通過 ExecutorService 執行 run() asyncCall.executeOn(executorService) } return isRunning }//RealCall.kt中的內部類 internal inner class AsyncCall( private val responseCallback: Callback ) : Runnable { fun executeOn(executorService: ExecutorService) { ... //執行Runnable executorService.execute(this) ... } override fun run() { threadName('OkHttp ${redactedUrl()}') { ... try { //兜兜轉轉 終于調用這個關鍵方法了 val response = getResponseWithInterceptorChain() signalledCallback = true //通過之前傳入的接口回調數據 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 { //移除隊列 client.dispatcher.finished(this) } } } }2.3 同步請求 RealCall#execute()

override fun execute(): Response { //同樣判斷是否執行過 synchronized(this) { check(!executed) { 'Already Executed' } executed = true } timeout.enter() //同樣監聽 callStart() try { //同樣執行 client.dispatcher.executed(this) return getResponseWithInterceptorChain() } finally { //同樣移除 client.dispatcher.finished(this) } }3.如何通過攔截器處理請求和響應？

無論同異步請求都會調用到 getResponseWithInterceptorChain() ，這個方法主要使用責任鏈模式將整個請求分為幾個攔截器調用 ，簡化了各自的責任和邏輯，可以擴展其它攔截器，看懂了攔截器 OkHttp 就了解的差不多了。

@Throws(IOException::class) internal fun getResponseWithInterceptorChain(): Response { // 構建完整的攔截器 val interceptors = mutableListOf<Interceptor>() interceptors += client.interceptors //用戶自己攔截器，數據最開始和最后 interceptors += RetryAndFollowUpInterceptor(client) //失敗后的重試和重定向 interceptors += BridgeInterceptor(client.cookieJar) //橋接用戶的信息和服務器的信息 interceptors += CacheInterceptor(client.cache) //處理緩存相關 interceptors += ConnectInterceptor //負責與服務器連接 if (!forWebSocket) { interceptors += client.networkInterceptors //配置 OkHttpClient 時設置，數據未經處理 } interceptors += CallServerInterceptor(forWebSocket) //負責向服務器發送請求數據、從服務器讀取響應數據 //創建攔截鏈 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 { //攔截鏈的執行 val response = chain.proceed(originalRequest) ... } catch (e: IOException) { ... } finally { ... } }//1.RealInterceptorChain#proceed(request: Request)@Throws(IOException::class) override fun proceed(request: Request): Response { ... // copy出新的攔截鏈，鏈中的攔截器集合index+1 val next = copy(index = index + 1, request = request) val interceptor = interceptors[index] //調用攔截器的intercept(chain: Chain): Response 返回處理后的數據 交由下一個攔截器處理 @Suppress('USELESS_ELVIS') val response = interceptor.intercept(next) ?: throw NullPointerException( 'interceptor $interceptor returned null') ... //返回最終的響應體 return response }

攔截器開始操作 Request。

3.1 攔截器是怎么攔截的？

攔截器都繼承自 Interceptor 類并實現了 fun intercept(chain: Chain): Response 方法。在 intercept 方法里傳入 chain 對象 調用它的 proceed() 然后 proceed() 方法里又 copy 下一個攔截器，然后雙調用了 intercept(chain: Chain) 接著? chain.proceed(request) 直到最后一個攔截器 return response 然后一層一層向上反饋數據。

3.2 RetryAndFollowUpInterceptor

這個攔截器是用來處理重定向的后續請求和失敗重試，也就是說一般第一次發起請求不需要重定向會調用下一個攔截器。

@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) { ...//在調用下一個攔截器前的操作 var response: Response try { ... try { //調用下一個攔截器 response = realChain.proceed(request) newExchangeFinder = true } catch (e: RouteException) { ... continue } catch (e: IOException) { ... continue } ... //處理上一個攔截器返回的 response val followUp = followUpRequest(response, exchange) ... //中間有一些判斷是否需要重新請求 不需要則返回 response //處理之后重新請求 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 -> { //這個方法重新 構建了 Request 用于重新請求 return buildRedirectRequest(userResponse, method) } ... 省略一部分code else -> return null } }

在 followUpRequest(userResponse: Response, exchange: Exchange?): Request? 方法中判斷了 response 中的服務器響應碼做出了不同的操作。

3.3 BridgeInterceptor

它負責對于 Http 的額外預處理，比如 Content-Length 的計算和添加、 gzip 的⽀持（Accept-Encoding: gzip）、 gzip 壓縮數據的解包等，這個類比較簡單就不貼代碼了，想了解的話可以自行查看。

3.4 CacheInterceptor

這個類負責 Cache 的處理，如果本地有了可⽤的 Cache，⼀個請求可以在沒有發⽣實質⽹絡交互的情況下就返回緩存結果，實現如下。

@Throws(IOException::class) override fun intercept(chain: Interceptor.Chain): Response { //在Cache(DiskLruCache)類中 通過request.url匹配response val cacheCandidate = cache?.get(chain.request()) //記錄當前時間點 val now = System.currentTimeMillis() //緩存策略 有兩種類型 //networkRequest 網絡請求 //cacheResponse 緩存的響應 val strategy = CacheStrategy.Factory(now, chain.request(), cacheCandidate).compute() val networkRequest = strategy.networkRequest val cacheResponse = strategy.cacheResponse //計算請求次數和緩存次數 cache?.trackResponse(strategy) ... // 如果 禁止使用網絡 并且 緩存不足，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網絡，就把緩存中的response封裝返回 if (networkRequest == null) { return cacheResponse!!.newBuilder() .cacheResponse(stripBody(cacheResponse)) .build() } //調用攔截器process從網絡獲取數據 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() } } //如果有緩存的Response if (cacheResponse != null) { //如果網絡請求返回code為304 即說明資源未修改 if (networkResponse?.code == HTTP_NOT_MODIFIED) { //直接封裝封裝緩存的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) { //判斷是否具有主體 并且 是否可以緩存供后續使用 if (response.promisesBody() && CacheStrategy.isCacheable(response, networkRequest)) { // 加入緩存中 val cacheRequest = cache.put(response) return cacheWritingResponse(cacheRequest, response) } //如果請求方法無效 就從緩存中remove掉 if (HttpMethod.invalidatesCache(networkRequest.method)) { try { cache.remove(networkRequest) } catch (_: IOException) { // The cache cannot be written. } } } return response }3.5 ConnectInterceptor

此類負責建⽴連接。 包含了⽹絡請求所需要的 TCP 連接（HTTP），或者 TCP 之前的 TLS 連接（HTTPS），并且會創建出對應的 HttpCodec 對象（⽤于編碼解碼 HTTP 請求）。

@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) }

看似短短四行實際工作還是比較多的。

/** Finds a new or pooled connection to carry a forthcoming request and response. */ internal fun initExchange(chain: RealInterceptorChain): Exchange {...//codec是對 HTTP 協議操作的抽象，有兩個實現：Http1Codec和Http2Codec，對應 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 { //尋找一個可用的連接 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) { //尋找連接 val candidate = findConnection( connectTimeout = connectTimeout, readTimeout = readTimeout, writeTimeout = writeTimeout, pingIntervalMillis = pingIntervalMillis, connectionRetryEnabled = connectionRetryEnabled ) //確認找到的連接可用并返回 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. 嘗試重用這個call的連接 比如重定向需要再次請求 那么這里就會重用之前的連接 val callConnection = call.connection if (callConnection != null) { var toClose: Socket? = null synchronized(callConnection) { if (callConnection.noNewExchanges || !sameHostAndPort(callConnection.route().address.url)) { toClose = call.releaseConnectionNoEvents() } } //返回這個連接 if (call.connection != null) { check(toClose == null) return callConnection } // The call’s connection was released. toClose?.closeQuietly() eventListener.connectionReleased(call, callConnection) }... // 2. 嘗試從連接池中找一個連接 找到就返回連接 if (connectionPool.callAcquirePooledConnection(address, call, null, false)) { val result = call.connection!! eventListener.connectionAcquired(call, result) return result } // 3. 如果連接池中沒有 計算出下一次要嘗試的路由 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.到這里還沒有找到可用的連接 但是找到了 route 即路由 進行socket/tls連接 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.查找是否有多路復用(http2)的連接,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) { //放入連接池中 connectionPool.put(newConnection) call.acquireConnectionNoEvents(newConnection) } eventListener.connectionAcquired(call, newConnection) return newConnection }

接下來看看是如何建立連接的

fun connect( connectTimeout: Int, readTimeout: Int, writeTimeout: Int, pingIntervalMillis: Int, connectionRetryEnabled: Boolean, call: Call, eventListener: EventListener ) { ... while (true) { try { if (route.requiresTunnel()) { //創建tunnel，用于通過http代理訪問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 { //不創建tunnel就創建socket連接 獲取到數據流 connectSocket(connectTimeout, readTimeout, call, eventListener) } //建立協議連接tsl establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener) eventListener.connectEnd(call, route.socketAddress, route.proxy, protocol) break } catch (e: IOException) { ... } }... }

建立tsl連接

@Throws(IOException::class) private fun establishProtocol( connectionSpecSelector: ConnectionSpecSelector, pingIntervalMillis: Int, call: Call, eventListener: EventListener ) { //ssl為空 即http請求 明文請求 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 }//否則為https請求 需要連接sslSocket 驗證證書是否可被服務器接受 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，創建好了連接，返回到最開始的 find() 方法返回 ExchangeCodec 對象，再包裝為 Exchange 對象用來下一個攔截器操作。

3.6 CallServerInterceptor

這個類負責實質的請求與響應的 I/O 操作，即往 Socket ⾥寫⼊請求數據，和從 Socket ⾥讀取響應數據。

總結

用一張 @piasy 的圖來做總結，圖很干練結構也很清晰。

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