JavaOkHttp框架源码超详细解析

王子 · 2022-11-8 17:22:32

一、自己的理解的OkHttp

我理解的http的本质就是基于socket连接，把要传输的数据按照http协议的格式去封装后，传输在网络中，以此来实现的网络通信。
而OkHttp协议就是帮助我们，把我们把要传输的数据请求，按照http协议的格式，传输在Socket上，当然还有很多优化管理这些请求和连接的方法，例如：对于这些请求的管理：最多同时进行64个请求，同域名的最多同时进行5个请求。还有Socket连接池的管理。

二、OkHttp的使用方法

1.创建一个client，构建一个request

OkHttpClient client = new OkHttpClient();
Request request = new Request.Builder()
.url("https://www.baidu.com/")
.build();

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2.同步请求

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

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3.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
}
});

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三、基本对象介绍

1.OkHttpClient

一个请求的配置类，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 {
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···

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2.request

同样是请求参数的配置类，也同样采用了建造者模式，但相比于OkHttpClient，Request就十分简单了，只有四个参数，分别是请求URL、请求方法、请求头、请求体。

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···

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3.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 {
/** 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
}
}

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4.RealCall

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

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RealCall是Call接口的具体实现类，是应用端与网络层的连接桥，展示应用端原始的请求与连接数据，以及网络层返回的response及其它数据流。通过使用方法也可知，创建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。（后面具体展开分析）

5.AsyncCall

异步请求调用，是RealCall的一个内部类，就是一个Runnable，被dispatcher调度器中的线程池所执行。

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)
}
}
}
}

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6.Dispatcher

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

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,这个线程池的核心线程数是0，等待队列的长度也是0，意味着
//线程池会直接创建新的线程去处理请求
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···
}

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四、流程分析

1.同步请求

client.newCall(request).execute();

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

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)
}
}
Dispatcher.kt
@Synchronized internal fun executed(call: RealCall) {
runningSyncCalls.add(call)
}

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

2.异步请求

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))
}

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然后调用调度器的enqueue方法

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
}

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调度器的enqueue方法就是将AsyncCall加入到readyAsyncCalls队列中，然后调用promoteAndExecute方法来执行请求，promoteAndExecute方法做的其实就是遍历readyAsyncCalls队列，然后将符合条件的请求用线程池执行，也就是会执行AsyncCall.run()方法。
AsyncCall 方法的具体代码看上面的这边就不在此展示了，简单来说就是调用getResponseWithInterceptorChain方法拿到response，然后通过Callback.onResponse方法传递出去。反之，如果请求失败，捕获了异常，就通过Callback.onFailure将异常信息传递出去。最终，请求结束，调用调度器finish方法。

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()
}
}

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请求结束，异步请求，把当前同域名的计数减一，然后后面和同步一样，都是把请求从正在执行的队列中移除，然后继续执行剩余请求。

3.获取Response

接着就是看看getResponseWithInterceptorChain方法是如何拿到response的。

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)
}
}
}

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简单概括一下：这里采用了责任链设计模式，通过拦截器构建了以RealInterceptorChain责任链，然后执行proceed方法来得到response。
那么，这又涉及拦截器是什么？拦截器责任链又是什么？

五、Interceptor

只声明了一个拦截器方法，在子类中具体实现，还包含一个Chain接口，核心方法是proceed(request)处理请求来获取response。

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
}
}

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六、RealInterceptorChain

拦截器链就是实现Interceptor.Chain接口，重点就是复写的proceed方法。

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
}
}

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链式调用，最终会向下执行拦截器列表中的每个拦截器，然后向上返回Response。

七、拦截器

各类拦截器的总结，按顺序：

接下来我们按顺序，从上往下，对这些拦截器进行一一解读。

1.client.interceptors

这是用户自己定义的拦截器，称为应用拦截器，会保存在OkHttpClient的interceptors: List<Interceptor>列表中。他是拦截器责任链中的第一个拦截器，也就是说会第一个执行拦截方法，我们可以通过它来添加自定义Header信息，如：

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();

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2.RetryAndFollowUpInterceptor

第二个拦截器，从它的名字也可知道，它负责请求失败的重试工作与重定向的后续请求工作，同时它会对连接做一些初始化工作。

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···

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3.BridgeInterceptor

从它的名字可以看出，他的定位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，比如：添加Content-Type，添加Cookie，添加User-Agent等等。再将服务器返回的response做一些处理转换为客户端需要的response。比如：移除响应头中的Content-Encoding、Content-Length等等。

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···

复制代码

4.CacheInterceptor

用户可以通过OkHttpClient.cache来配置缓存，缓存拦截器通过CacheStrategy来判断是使用网络还是缓存来构建response。

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···

复制代码

5.ConnectInterceptor

负责实现与服务器真正建立起连接，

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)
}
}

复制代码

一扫下来，代码十分简单，拦截方法里就只有三步。

那这个exchange对象又是什么呢？

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
}

复制代码

具体看看ExchangeFinder.find()这一步，

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)
}
}

复制代码

继续往下看findHealthyConnection方法

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...
}
}

复制代码

所以核心方法就是findConnection，我们继续深入看看该方法：

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
}

复制代码

在代码中可以看出，一共做了5次尝试去得到连接：

这一步就是为了建立连接。

6.client.networkInterceptors

该拦截器称为网络拦截器，与client.interceptors一样也是由用户自己定义的，同样是以列表的形式存在OkHttpClient中。
那这两个拦截器有什么不同呢？
其实他两的不同都是由于他们所处的位置不同所导致的，应用拦截器处于第一个位置，所以无论如何它都会被执行，而且只会执行一次。而网络拦截器处于倒数第二的位置，它不一定会被执行，而且可能会被执行多次，比如：在RetryAndFollowUpInterceptor失败或者CacheInterceptor直接返回缓存的情况下，我们的网络拦截器是不会被执行的。

7.CallServerInterceptor

到了这里，客户端与服务器已经建立好了连接，接着就是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了。

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···

复制代码

简单概括一下：写入发送请求头，然后根据条件是否写入发送请求体，请求结束。解析服务器返回的请求头，然后构建一个新的response，并返回。这里CallServerInterceptor是拦截器责任链中最后一个拦截器了，所以他不会再调用chain.proceed()方法往下执行，而是将这个构建的response往上传递给责任链中的每个拦截器。
总结一下流程：

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