腾讯面试官:OkHttp 用了那么久,你知道原理吗?

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很多朋友反映大厂的面试喜欢挖底层知识,像OkHttp这些都是必问的问题。这里就给大家分享一篇非常有帮助的技术文吧。

HTTP是现代应用程序网络的方式。这就是我们交换数据和媒体的方式。有效地执行HTTP可以使您的内容加载更快并节省带宽。 原文地址:blog.csdn.net/qq_40861368…

OkHttp是一个高效的HTTP库:

  • 支持HTTP / 2,允许对同一主机的所有请求共享一个套接字
  • 通过连接池可减少请求延迟(如果HTTP / 2不可用)
  • 支持GZIP压缩减少数据流量
  • 响应缓存可以完全避免网络重复请求
  • 静默恢复处理常见的连接问题

本文就以请求使用为入口,来深入学习下OkHttp。

一、请求流程分析

1. 同步请求

Okhttp同步GET请求使用:

// 新建一个Okhttp客户端(也可以通过OkHttpClient.Builder来构造)
OkHttpClient client = new OkHttpClient();
// 构造一个请求对象
Request request = new Request.Builder().url(url).build();
// 执行同步请求,返回响应
Response response = client.newCall(request).execute();
// 从响应体中获取数据
String str = response.body().string();

先来瞧瞧构建OkhttpClient的源码:

open class OkHttpClient internal constructor(
  builder: Builder
) : Cloneable, Call.Factory, WebSocket.Factory {
	//若直接实例化OkHttpClient,则调用主构造函数以默认Builder作为参数
	constructor() : this(Builder())

	// 通过builder中的值赋值
	@get:JvmName("dispatcher") val dispatcher: Dispatcher = builder.dispatcher

  	@get:JvmName("connectionPool") val connectionPool: ConnectionPool = builder.connectionPool
	...
	
	class Builder constructor() {
	    //分发器
	    internal var dispatcher: Dispatcher = Dispatcher()
	    //连接池
	    internal var connectionPool: ConnectionPool = ConnectionPool()
	    //应用拦截器集合
	    internal val interceptors: MutableList<Interceptor> = mutableListOf()
	    //网络拦截器集合
	    internal val networkInterceptors: MutableList<Interceptor> = mutableListOf()
	    //事件监听工厂
	    internal var eventListenerFactory: EventListener.Factory = EventListener.NONE.asFactory()
	    //连接失败是否重试
	    internal var retryOnConnectionFailure = true
	    internal var authenticator: Authenticator = Authenticator.NONE
	    //是否跟随重定向
	    internal var followRedirects = true
	    internal var followSslRedirects = true
	    //cookie
	    internal var cookieJar: CookieJar = CookieJar.NO_COOKIES
	    //磁盘缓存
	    internal var cache: Cache? = null
	    //dns
	    internal var dns: Dns = Dns.SYSTEM
	    //代理设置
	    internal var proxy: Proxy? = null
	    
		...
    }
    
}

OkhttpClient可以通过构造者配置参数来构建,也可以直接实例化,直接实例化其实也是内部调用构造者,只是传入的是默认builder。

再来看看OkhttpClient的newCall方法

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

发现返回的是RealCall,接下来去RealCall中看后续的execute执行方法

override fun execute(): Response {
  //确认call没有执行过并置executed为true,否则抛出异常
  check(executed.compareAndSet(false, true)) { "Already Executed" }

  timeout.enter()
  callStart()
  try {
    //标记执行中
    client.dispatcher.executed(this)
    //通过拦截器链获取网络响应
    return getResponseWithInterceptorChain()
  } finally {
    //标记执行结束
    client.dispatcher.finished(this)
  }
}

看起来比较精简,通过拦截器链获取网络响应,然后返回响应(拦截器链路在后续拦截器分析)。

2. 异步请求

Okhttp异步GET请求使用:

OkHttpClient client = new OkHttpClient();
Request request = new Request.Builder().url(url).build();
// 执行异步请求,通过回调返回响应
client.newCall(request).enqueue(new Callback() {
      @Override
      public void onFailure(@NotNull Call call, @NotNull IOException e) {}

      @Override
      public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
      	// 从回调中通过响应体获取数据
      	String str = response.body().string();
      }
});

异步请求流程大致和同步请求相似,但是最后的执行方法是enqueue,并传入回调对象。

我们来看看源码:

override fun enqueue(responseCallback: Callback) {
  //确认call没有执行过并置executed为true,否则抛出异常
  check(executed.compareAndSet(false, true)) { "Already Executed" }
  //监听回调
  callStart()
  //调用Dispatcher的enqueue方法,并传入一个AsyncCall对象
  client.dispatcher.enqueue(AsyncCall(responseCallback))
}

内部调用了客户端的分发器的enqueue方法,并把AsyncCall(responseCallback)作为参数传入,AsyncCall是继承自Runnable,且是RealCall的内部类,我们先看Dispatcher.enqueue()方法

class Dispatcher constructor() {
  /** '异步准备执行'队列 */
  private val readyAsyncCalls = ArrayDeque<AsyncCall>()

  /** '异步正在执行'队列,包括取消但至今还没结束的 */
  private val runningAsyncCalls = ArrayDeque<AsyncCall>()

  /** ‘同步正在执行’队列*/
  private val runningSyncCalls = ArrayDeque<RealCall>()
  ...
  internal fun enqueue(call: AsyncCall) {
	synchronized(this) {
	  //加入准备执行队列
	  readyAsyncCalls.add(call)
	  ...
	}
	// 执行
	promoteAndExecute()
  }

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

    val executableCalls = mutableListOf<AsyncCall>()
    val isRunning: Boolean
    synchronized(this) {
      val i = readyAsyncCalls.iterator()
      while (i.hasNext()) {
        val asyncCall = i.next()
        //检查请求是否超过最大请求数
        if (runningAsyncCalls.size >= this.maxRequests) break
        //检查请求是否超过一个Host对应的最大请求数
        if (asyncCall.callsPerHost.get() >= this.maxRequestsPerHost) continue

        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]
      //调用AsyncCall的executeOn()方法
      asyncCall.executeOn(executorService)
    }

    return isRunning
  }
}

可以从上面代码看出,就是将符合条件的调用从readyAsyncCalls队列提升到runningAsyncCalls,并调用 AsyncCall的executeOn() 方法,把线程池传入。

我们来看看AsyncCall:

  inner class AsyncCall(
    private val responseCallback: Callback
  ) : Runnable {
	...
    fun executeOn(executorService: ExecutorService) {
      client.dispatcher.assertThreadDoesntHoldLock()

      var success = false
      try {
        //使用线程池执行自己的run()方法
        executorService.execute(this)
        success = true
      } catch (e: RejectedExecutionException) {
        ...
        //失败回调
        responseCallback.onFailure(this@RealCall, ioException)
      } finally {
        if (!success) {
          //标记结束
          client.dispatcher.finished(this) // This call is no longer running!
        }
      }
    }

    override fun run() {
      threadName("OkHttp ${redactedUrl()}") {
        var signalledCallback = false
        timeout.enter()
        try {
          //和同步请求一样,通过拦截器链获取网络响应
          val response = getResponseWithInterceptorChain()
          signalledCallback = true
          //回调成功
          responseCallback.onResponse(this@RealCall, response)
        } catch (e: IOException) {
          if (signalledCallback) {
            ...
          } else {
            //回调失败
            responseCallback.onFailure(this@RealCall, e)
          }
        } catch (t: Throwable) {
          cancel()
          if (!signalledCallback) {
            ...
            //回调失败
            responseCallback.onFailure(this@RealCall, canceledException)
          }
          throw t
        } finally {
          //标记结束
          client.dispatcher.finished(this)
        }
      }
    }
  }

使用线程池来执行自己,接下来就看run()方法,发现和同步请求一样,通过拦截器链获取网络响应,再调用回调对象的回调方法返回响应。

二、拦截器分析

请求大致流程知道了,我们来看看重头戏,拦截器链里面做了什么操作。

@Throws(IOException::class)
internal fun getResponseWithInterceptorChain(): Response {
  // 建立一个拦截器列表
  val interceptors = mutableListOf<Interceptor>()
  // 用户设置的所有应用拦截器
  interceptors += client.interceptors
  // 处理错误恢复和重定向的拦截器
  interceptors += RetryAndFollowUpInterceptor(client)
  // 桥接拦截器,桥接应用层和网络层代码
  interceptors += BridgeInterceptor(client.cookieJar)
  // 缓存拦截器
  interceptors += CacheInterceptor(client.cache)
  // 服务器连接拦截器
  interceptors += ConnectInterceptor
  if (!forWebSocket) {
    // 用户设置的所有网络拦截器
    interceptors += client.networkInterceptors
  }
  // 服务器请求拦截器
  interceptors += CallServerInterceptor(forWebSocket)

  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 {
    //使用责任链模式开启链式调用
    val response = chain.proceed(originalRequest)
    if (isCanceled()) {
      response.closeQuietly()
      throw IOException("Canceled")
    }
    //返回响应
    return response
  } catch (e: IOException) {
    calledNoMoreExchanges = true
    throw noMoreExchanges(e) as Throwable
  } finally {
    if (!calledNoMoreExchanges) {
      noMoreExchanges(null)
    }
  }
}

我们在看下RealInterceptorChain的proceed方法:

  @Throws(IOException::class)
  override fun proceed(request: Request): Response {
	...
    // 复制一个RealInterceptorChain,用于调用链中的下一个拦截器
    val next = copy(index = index + 1, request = request)
    val interceptor = interceptors[index]

    @Suppress("USELESS_ELVIS")
    // 调用下一个拦截器的intercept方法,获取response返回给上一个拦截器
    val response = interceptor.intercept(next) ?: throw NullPointerException(
        "interceptor $interceptor returned null")
	...
    return response
  }

接下来我们来具体看看各个拦截器的作用

1. RetryAndFollowUpInterceptor

RetryAndFollowUpInterceptor 处理错误恢复和重定向,它会判断错误是否满足条件进行重试,还有根据返回的响应判断是否需要重定向请求。

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) {
      // 初始化ExchangeFinder(后续ConnectInterceptor会用到ExchangeFinder来查找连接)
      call.enterNetworkInterceptorExchange(request, newExchangeFinder)

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

        try {
          //执行下一个拦截器,获取响应
          response = realChain.proceed(request)
          newExchangeFinder = true
        } catch (e: RouteException) {
          ...
          // 满足条件则重试
          continue
        } catch (e: IOException) {
          ...
          // 满足条件则重试
          continue
        }

        // 赋上重定向之前的响应(响应体置空)
        if (priorResponse != null) {
          response = response.newBuilder()
              .priorResponse(priorResponse.newBuilder()
                  .body(null)
                  .build())
              .build()
        }

        val exchange = call.interceptorScopedExchange
        //判断是否需重定向,若需则返回重定向请求
        val followUp = followUpRequest(response, exchange)

        //不需要重定向则直接返回response
        if (followUp == null) {
          if (exchange != null && exchange.isDuplex) {
            call.timeoutEarlyExit()
          }
          closeActiveExchange = false
          return response
        }

        val followUpBody = followUp.body
        // 若该请求只可传输一次,则返回响应
        if (followUpBody != null && followUpBody.isOneShot()) {
          closeActiveExchange = false
          return response
        }

        response.body?.closeQuietly()

        //超过重定向最大次数则抛出异常
        if (++followUpCount > MAX_FOLLOW_UPS) {
          throw ProtocolException("Too many follow-up requests: $followUpCount")
        }

        //将请求重新赋值为重定向的请求,继续循环,再次发送
        request = followUp
        priorResponse = response
      } finally {
        call.exitNetworkInterceptorExchange(closeActiveExchange)
      }
    }
  }
  ...
}

2. BridgeInterceptor

BridgeInterceptor 桥接应用层和网络层的代码,对用户的请求进行加工(如对请求头进行设置添加),也对网络响应做相应的处理(如解压服务端返回的 gzip 压缩数据)。

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

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    // 获取用户请求
    val userRequest = chain.request()
    // 真正发送的网络请求的构建者
    val requestBuilder = userRequest.newBuilder()

    // 用户请求的请求体
    val body = userRequest.body
    // 对请求头的设置
    ...
    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,获取网络响应
    val networkResponse = chain.proceed(requestBuilder.build())

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

    val responseBuilder = networkResponse.newBuilder()
        .request(userRequest)

    // 若因配置问题,服务端返回gzip压缩的数据,则做相应的解压缩
    if (transparentGzip &&
        "gzip".equals(networkResponse.header("Content-Encoding"), ignoreCase = true) &&
        networkResponse.promisesBody()) {
      val responseBody = networkResponse.body
      if (responseBody != null) {
        // GzipSource对象,用于解压
        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()
  }
}

3. CacheInterceptor

CacheInterceptor 承担着缓存的查找与保存的职责。根据策略判断是使用缓存还是走网络请求,对于返回的响应,满足条件则进行缓存。

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

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

    val now = System.currentTimeMillis()

    // 检查缓存策略
    val strategy = CacheStrategy.Factory(now, chain.request(), cacheCandidate).compute()
    // 若还需发送网络请求,则networkRequest不为空
    val networkRequest = strategy.networkRequest
    // 若存在可用缓存,则cacheResponse不为空
    val cacheResponse = strategy.cacheResponse

    ...

    // 如果我们被禁止使用网络,并且无可用缓存,则返回失败
    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)
          }
    }
    
    // 如果不需要网络请求,缓存可用,则返回缓存
    if (networkRequest == null) {
      return cacheResponse!!.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build().also {
            listener.cacheHit(call, it)
          }
    }

    ...

    var networkResponse: Response? = null
    try {
      // 若无缓存可用,则执行下一个拦截器,获取响应
      networkResponse = chain.proceed(networkRequest)
    } finally {
      if (networkResponse == null && cacheCandidate != null) {
        cacheCandidate.body?.closeQuietly()
      }
    }
    
    // 如果我们还有缓存响应,且网络响应code为304,则更新缓存响应,并返回
    if (cacheResponse != null) {
      if (networkResponse?.code == HTTP_NOT_MODIFIED) {
      	// 合并响应头、更新为网络请求时间和网络响应时间等
        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()

        cache!!.trackConditionalCacheHit()
        // 更新缓存
        cache.update(cacheResponse, response)
        return response.also {
          listener.cacheHit(call, it)
        }
      } else {
        cacheResponse.body?.closeQuietly()
      }
    }

    // 包装网络响应
    val response = networkResponse!!.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build()

    // 若用户配置了缓存
    if (cache != null) {
      // 判断是否满足缓存条件
      if (response.promisesBody() && CacheStrategy.isCacheable(response, networkRequest)) {
        // 将网络响应写入缓存,并返回
        val cacheRequest = cache.put(response)
        return cacheWritingResponse(cacheRequest, response).also {
          if (cacheResponse != null) {
            listener.cacheMiss(call)
          }
        }
      }

      // 根据请求方法判断是否为无效请求,是则从缓存移除相对应响应
      if (HttpMethod.invalidatesCache(networkRequest.method)) {
        try {
          cache.remove(networkRequest)
        } catch (_: IOException) {
          // cache无法被写
        }
      }
    }

    return response
  }
}

4. ConnectInterceptor

ConnectInterceptor 主要是给网络请求提供一个连接,并交给下一个拦截器处理,这里还没有发送请求到服务器获取响应。在获取连接对象的时候,使用了连接池ConnectionPool来复用连接。

object ConnectInterceptor : Interceptor {
  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    // 查找新连接或池里的连接以承载即将到来的请求和响应
    val exchange = realChain.call.initExchange(chain)
    val connectedChain = realChain.copy(exchange = exchange)
    return connectedChain.proceed(realChain.request)
  }
}

ConnectInterceptor 看似代码很少,其实代码都在深处,看下initExchange方法

internal fun initExchange(chain: RealInterceptorChain): Exchange {
  ...
  // codec(ExchangeCodec) 是一个连接所用的编码解码器,用于编码HTTP请求和解码HTTP响应
  val codec = exchangeFinder.find(client, chain)
  // result(Exchange)是封装这个编码解码器的一个工具类,用于管理ExchangeCodec,处理实际的 I/O
  val result = Exchange(this, eventListener, exchangeFinder, codec)
  ...
  return result
}

ExchangeCodec持有连接,可通过其编码请求到服务端和获取服务端的响应并解码,我们依方法进入到最深处,看看是连接是如何获取的(代码已做简化处理)。

private fun findConnection(): RealConnection {

  // 1、复用当前连接
  val callConnection = call.connection 
  if (callConnection != null) {
      //检查这个连接是否可用和可复用
      if (callConnection.noNewExchanges || !sameHostAndPort(callConnection.route().address.url)) {
        toClose = call.releaseConnectionNoEvents()
      }
    return callConnection
  }

 //2、从连接池中获取可用连接
  if (connectionPool.callAcquirePooledConnection(address, call, null, false)) {
    val result = call.connection!!
    eventListener.connectionAcquired(call, result)
    return result
  }

  //3、从连接池中获取可用连接,通过一组路由routes(涉及知识点Http2多路复用)
  if (connectionPool.callAcquirePooledConnection(address, call, routes, false)) {
      val result = call.connection!!
      return result
    }
  route = localRouteSelection.next()


  // 4、创建新连接,进行tcp连接
  val newConnection = RealConnection(connectionPool, route)
  newConnection.connect

  // 5、再获取一次连接,在新建连接过程中可能有其他竞争连接被创建了,如可用防止浪费
  if (connectionPool.callAcquirePooledConnection(address, call, routes, true)) {
  	val result = call.connection!! 
  	// 关闭刚刚创建的新连接
  	newConnection.socket().closeQuietly()
    return result
  }

  //6、还是要使用创建的新连接,放入连接池,并返回
  connectionPool.put(newConnection)
  return newConnection
}

5. CallServerInterceptor

CallServerInterceptor 是真正向服务器发起请求并获取响应的,它是拦责任链的最后一个拦截器,拿到响应后返回给上一个拦截器。代码已做简化(省略了很多条件判断和处理)。

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

  @Throws(IOException::class)
  override fun intercept(chain: Interceptor.Chain): Response {
    val realChain = chain as RealInterceptorChain
    // ConnectInterceptor获取到的,持有编码解码器
    val exchange = realChain.exchange!!
    val request = realChain.request
    val requestBody = request.body
    val sentRequestMillis = System.currentTimeMillis()
    
    var responseBuilder: Response.Builder? = null
    try {
      // 写入请求头
      exchange.writeRequestHeaders(request)

	  if (HttpMethod.permitsRequestBody(request.method) && requestBody != null) {
	  	if (...) {
	  		// 写入请求体
	  		val bufferedRequestBody = exchange.createRequestBody(request, false).buffer()
            requestBody.writeTo(bufferedRequestBody)
            bufferedRequestBody.close()
	  	} else {
	  		...
	  	}
	  } else {
	    // 无请求体
        exchange.noRequestBody()
      }
    } catch (e: IOException) {...}

    try {
      if (responseBuilder == null) {
        // 读取响应头
        responseBuilder = exchange.readResponseHeaders(expectContinue = false)!!
        if (invokeStartEvent) {
          exchange.responseHeadersStart()
          invokeStartEvent = false
        }
      }
      // 构建响应
      var response = responseBuilder
          .request(request)
          .handshake(exchange.connection.handshake())
          .sentRequestAtMillis(sentRequestMillis)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build()
      var code = response.code
      // 读取响应体
      response = if (forWebSocket && code == 101) {
        response.newBuilder()
            .body(EMPTY_RESPONSE)
            .build()
      } else {
        response.newBuilder()
            .body(exchange.openResponseBody(response))
            .build()
      }
      ...
      return response
    } catch (e: IOException) {
      ...
    }
  }
}

总结

以上就是对OkHttp的源码解析,可以看出它是一个结构清晰的优质源码库,各个模块通过设计模式解耦。总结下流程:首先通过OkHttpClient对象调用newCall方法得到RealCall实例,再通过调用RealCall的execute方法或enqueue方法,这两个方法最终都会调用到getResponseWithInterceptorChain方法,运用责任链模式,开始一层层传入各个拦截器,每个拦截器都有着自己都职责,最终在CallServerInterceptor发出请求并获取响应,然后层层返回响应。

参考文献

OkHttp源码 v4.9.1 《Android应用开发进阶》 Andriod 网络框架 OkHttp 源码解析 B站大厂面试真题解析视频合集