OkHttp3框架解析

OKHttp3网络框架解析

本文会先简单说下OkHttp3的工作流程,然后介绍OkHttp3的一些核心类(如连接池StreamAllocation以及各式各样的Interceptor),接着从源码角度分析一次HTTP请求在OkHttp3中所经历的过程,在不同的Interceptor(拦截器)可以看到一些OkHttp3设计的一些巧妙思想,最后对上述分析做个简单的总结。

Okhttp3是Square公司开源的强大高效的Java网络请求库,旨于替换Java的HttpUrlConnection和Apache的HttpClient的轻量级网络框架,已经被运用到很多开源库以及Android的源码中(Android Studio 在6.0之后,移除了HttpClient,并且用OKHttp代替了HttpUrlConnection)。

  • 支持Http2/SPDY;
  • 默认启用长连接,使用连接池管理,支持Cache(目前仅支持GET请求的缓存);
  • 路由节点管理,提升访问速度;
  • 透明的Gzip处理,节省网络流量。
  • 灵活的拦截器,行为类似Java EE的Filter或者函数编程中的Map。

OKhttp3也主要由以下6个概念一起运作:

  1. OkHttpClient:客户端;
  2. Dispatcher:线程池;
  3. Interceptor:拦截器;
  4. Request:请求;
  5. Response:响应;
  6. CallBack:回调。

OkHttp的简单使用方法

OkHttp 使用方法,直接抄官网的 (╯-╰)/ 。

GET 请求:

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OkHttpClient client = new OkHttpClient();
String run(String url) throws IOException {
Request request = new Request.Builder()
.url(url)
.build();
Response response = client.newCall(request).execute();
return response.body().string();
}

POST 请求:

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public static final MediaType JSON
= MediaType.parse("application/json; charset=utf-8");
OkHttpClient client = new OkHttpClient();
String post(String url, String json) throws IOException {
RequestBody body = RequestBody.create(JSON, json);
Request request = new Request.Builder()
.url(url)
.post(body)
.build();
Response response = client.newCall(request).execute();
return response.body().string();
}

深入源码

在这里,先分析下同步请求的源码,之后再回过头来看异步请求的源码。

Let’s go !

同步请求

OkHttpClient

首先创建一个 OkHttpClient 对象,那我们看看在构造器中做了什么:

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public OkHttpClient() {
this(new Builder());
}
OkHttpClient(Builder builder) {
// 调度分配器
this.dispatcher = okHttpClient.dispatcher;
// 代理
this.proxy = okHttpClient.proxy;
// 协议(例如HTTP_1_1、HTTP2、SPDY等)
this.protocols = okHttpClient.protocols;
// 连接协议例如TLS
this.connectionSpecs = okHttpClient.connectionSpecs;
// 加载我们设定的所有拦截器
this.interceptors.addAll(okHttpClient.interceptors);
// 加载网络请求的拦截器(区别在于不同的时期拦截)
this.networkInterceptors.addAll(okHttpClient.networkInterceptors);
// 加载事件监听工厂类 监听请求开始 连接开始 dns解析等等事件
this.eventListenerFactory = okHttpClient.eventListenerFactory;
// 代理选择器默认获取系统代理
this.proxySelector = okHttpClient.proxySelector;
// cookiejar外部的对cookie封装使用的类
this.cookieJar = okHttpClient.cookieJar;
// 内部的缓存机制
this.internalCache = okHttpClient.internalCache;
// 用来判断这次请求的缓存类型(例如不缓存、缓存多少时间maxAgeSeconds等)
this.cache = okHttpClient.cache;
// 用于组装生成socket 这是一个抽象工厂
this.socketFactory = okHttpClient.socketFactory;
// 用于组装ssl的socket配置工厂 内部实现继承于socketfactory
boolean isTLS = false;
for (ConnectionSpec spec : connectionSpecs) {
isTLS = isTLS || spec.isTls();
}
if (builder.sslSocketFactory != null || !isTLS) {
this.sslSocketFactory = builder.sslSocketFactory;
this.certificateChainCleaner = builder.certificateChainCleaner;
} else {
X509TrustManager trustManager = systemDefaultTrustManager();
this.sslSocketFactory = systemDefaultSslSocketFactory(trustManager);
this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
}
this.hostnameVerifier = builder.hostnameVerifier;
this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
certificateChainCleaner);
this.proxyAuthenticator = builder.proxyAuthenticator;
this.authenticator = builder.authenticator;
this.connectionPool = builder.connectionPool; // 连接复用池
this.dns = builder.dns;
this.followSslRedirects = builder.followSslRedirects;
this.followRedirects = builder.followRedirects;
this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
this.connectTimeout = builder.connectTimeout; // 连接超时时间
this.readTimeout = builder.readTimeout; // 读取超时时间
this.writeTimeout = builder.writeTimeout; // 写入超时时间
this.pingInterval = builder.pingInterval;
}

在构造器中利用建造者模式来构建 OkHttpClient 的对象。当然,如果你想自定义 OkHttpClient 配置的话,就要 new 一个 OkHttpClient.Builder 来配置自己的参数了。相信大家都干过这种事情了(∩_∩)。

OkHttpClient 的构造器中主要是扎堆扎堆的配置,没别的。

所以OKHttpClient这个类作用就是把配置全部生成好相当于一个机关枪,工厂已经把所有需要的配件都生成好了。就差扣扳机钥匙来开枪了。那么子弹就是我们的Request这个类。但是看这个类之前我们先去看看这里面为什么能做连续发射(Dispatcher)。

之后再调用 newCall(Request request)

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@Override
public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}

在方法里面其实是创建了一个 RealCall 的对象,那么我们就进入 RealCall 中去看看吧。

RealCall

RealCall 的构造器中只是给一些变量赋值或初始化而已,没什么:

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static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
// Safely publish the Call instance to the EventListener.
RealCall call = new RealCall(client, originalRequest, forWebSocket);
call.eventListener = client.eventListenerFactory().create(call);
return call;
}
private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
}

然后再把目光转向 RealCall 中的 execute() 方法:

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@Override
public Response execute() throws IOException {
synchronized (this) {
// 如果该 call 已经被执行过了,就设置 executed 为 true
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
try {
// 加入 runningSyncCalls 队列中
client.dispatcher().executed(this);
// 得到响应 result
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} finally {
// 从 runningSyncCalls 队列中移除
client.dispatcher().finished(this);
}
}

execute() 方法为执行该 RealCall,在方法里面一开始检查了该 call 时候被执行。

然后又加入了 DispatcherrunningSyncCalls 中。runningSyncCalls 队列只是用来记录正在同步请求中的 call ,在 call 完成请求后又会从 runningSyncCalls 中移除。

可见,在同步请求中 Dispatcher 参与的部分很少。但是在异步请求中, Dispatcher 可谓是大展身手。

最重要的方法,那就是 getResponseWithInterceptorChain() 。我们可以看到这方法是直接返回 Response 对象的,所以,在这个方法中一定做了很多很多的事情。

那就继续深入吧:

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Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors()); // 加入用户自定义的拦截器
interceptors.add(retryAndFollowUpInterceptor); // 重试和重定向拦截器
interceptors.add(new BridgeInterceptor(client.cookieJar())); // 加入转化请求响应的拦截器
interceptors.add(new CacheInterceptor(client.internalCache())); // 加入缓存拦截器
interceptors.add(new ConnectInterceptor(client)); // 加入连接拦截器
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors()); // 加入用户自定义的网络拦截器
}
interceptors.add(new CallServerInterceptor(forWebSocket)); // 加入发出请求和读取响应的拦截器
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.readTimeoutMillis());
// 利用 chain 来链式调用拦截器,最后的返回结果就是 Response 对象
return chain.proceed(originalRequest);
}

getResponseWithInterceptorChain() 方法中有一堆的拦截器!!!

我们都知道,拦截器是 OkHttp 的精髓。

  1. client.interceptors() ,首先加入 interceptors 的是用户自定义的拦截器,比如修改请求头的拦截器等;
  2. RetryAndFollowUpInterceptor 是用来重试和重定向的拦截器,在下面我们会讲到;
  3. BridgeInterceptor 是用来将用户友好的请求转化为向服务器的请求,之后又把服务器的响应转化为对用户友好的响应;
  4. CacheInterceptor 是缓存拦截器,若存在缓存并且可用就直接返回该缓存,否则会向服务器请求;
  5. ConnectInterceptor 用来建立连接的拦截器;
  6. client.networkInterceptors() 加入用户自定义的 networkInterceptors ;
  7. CallServerInterceptor 是真正向服务器发出请求且得到响应的拦截器;

最后在聚合了这些拦截器后,利用 RealInterceptorChain 来链式调用这些拦截器,利用的就是责任链模式。

RealInterceptorChain

RealInterceptorChain 可以说是真正把这些拦截器串起来的一个角色。一个个拦截器就像一颗颗珠子,而 RealInterceptorChain 就是把这些珠子串连起来的那根绳子。

进入 RealInterceptorChain ,主要是 proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, RealConnection connection) 这个方法:

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public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
// If we already have a stream, confirm that the incoming request will use it.
if (this.httpCodec != null && !this.connection.supportsUrl(request.url())) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must retain the same host and port");
}
// If we already have a stream, confirm that this is the only call to chain.proceed().
if (this.httpCodec != null && calls > 1) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must call proceed() exactly once");
}
// 得到下一次对应的 RealInterceptorChain
RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
connection, index + 1, request, call, eventListener, readTimeout);
// 当前次数的 interceptor
Interceptor interceptor = interceptors.get(index);
// 进行拦截处理,并且在 interceptor 链式调用 next 的 proceed 方法
Response response = interceptor.intercept(next);
// 确认下一次的 interceptor 调用过 chain.proceed()
if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
throw new IllegalStateException("network interceptor " + interceptor
+ " must call proceed() exactly once");
}
// Confirm that the intercepted response isn't null.
if (response == null) {
throw new NullPointerException("interceptor " + interceptor + " returned null");
}
if (response.body() == null) {
throw new IllegalStateException(
"interceptor " + interceptor + " returned a response with no body");
}
return response;
}

下面就要进入分析拦截器的步骤了,至于用户自定义的拦截器在这就略过了。还有,拦截器只分析主要的 intercept(Chain chain) 代码。

RetryAndFollowUpInterceptor

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@Override
public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Call call = realChain.call();
EventListener eventListener = realChain.eventListener();
streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
call, eventListener, callStackTrace);
int followUpCount = 0;
Response priorResponse = null;
// 如果取消,就释放资源
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response = null;
boolean releaseConnection = true;
try {
// 调用下一个拦截器
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
// 路由连接失败,请求将不会被发送
if (!recover(e.getLastConnectException(), false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
// 服务器连接失败,请求可能已被发送
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, requestSendStarted, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
// 抛出未检查的异常,释放资源
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
// 如果不需要重定向,那么 followUp 为空,会根据响应码判断
Request followUp = followUpRequest(response);
// 释放资源,返回 response
if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}
// 关闭 response 的 body
closeQuietly(response.body());
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
// response 和 followUp 比较是否为同一个连接
// 若为重定向就销毁旧连接,创建新连接
if (!sameConnection(response, followUp.url())) {
streamAllocation.release();
streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(followUp.url()), call, eventListener, callStackTrace);
} else if (streamAllocation.codec() != null) {
throw new IllegalStateException("Closing the body of " + response
+ " didn't close its backing stream. Bad interceptor?");
}
// 将重定向操作得到的新请求设置给 request
request = followUp;
priorResponse = response;
}
}

总体来说,RetryAndFollowUpInterceptor 是用来失败重试以及重定向的拦截器。

BridgeInterceptor

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@Override
public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
// 将用户友好的 request 构造为发送给服务器的 request
RequestBody body = userRequest.body();
// 若有请求体,则构造
if (body != null) {
MediaType contentType = body.contentType();
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());
}
long contentLength = body.contentLength();
if (contentLength != -1) {
requestBuilder.header("Content-Length", Long.toString(contentLength));
requestBuilder.removeHeader("Transfer-Encoding");
} else {
requestBuilder.header("Transfer-Encoding", "chunked");
requestBuilder.removeHeader("Content-Length");
}
}
if (userRequest.header("Host") == null) {
requestBuilder.header("Host", hostHeader(userRequest.url(), false));
}
// Keep Alive
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.
// 使用 gzip 压缩
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}
// 设置 cookie
List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}
// UA
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
// 构造完后,将 request 交给下一个拦截器去处理。最后又得到服务端响应 networkResponse
Response networkResponse = chain.proceed(requestBuilder.build());
// 保存 networkResponse 的 cookie
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
// 将 networkResponse 构造为对用户友好的 response
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
// 如果 networkResponse 使用 gzip 并且有响应体的话,给用户友好的 response 设置响应体
if (transparentGzip
&& "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
&& HttpHeaders.hasBody(networkResponse)) {
GzipSource responseBody = new GzipSource(networkResponse.body().source());
Headers strippedHeaders = networkResponse.headers().newBuilder()
.removeAll("Content-Encoding")
.removeAll("Content-Length")
.build();
responseBuilder.headers(strippedHeaders);
responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}
return responseBuilder.build();
}

BridgeInterceptor 这一步,先把用户友好的请求进行重新构造,变成了向服务器发送的请求。

之后调用 chain.proceed(requestBuilder.build()) 进行下一个拦截器的处理。

等到后面的拦截器都处理完毕,得到响应。再把 networkResponse 转化成对用户友好的 response

CacheInterceptor

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@Override
public Response intercept(Chain chain) throws IOException {
// 得到 request 对应缓存中的 response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
// 获取当前时间,会和之前缓存的时间进行比较
long now = System.currentTimeMillis();
// 得到缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
// 追踪缓存,其实就是计数
if (cache != null) {
cache.trackResponse(strategy);
}
// 缓存不适用,关闭
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
// If we're forbidden from using the network and the cache is insufficient, fail.
// 禁止网络并且没有缓存的话,返回失败
if (networkRequest == null && cacheResponse == null) {
return new Response.Builder()
.request(chain.request())
.protocol(Protocol.HTTP_1_1)
.code(504)
.message("Unsatisfiable Request (only-if-cached)")
.body(Util.EMPTY_RESPONSE)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
// If we don't need the network, we're done.
// 不用网络请求,返回缓存
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
// 交给下一个拦截器,返回 networkResponse
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) {
closeQuietly(cacheCandidate.body());
}
}
// 如果我们同时有缓存和 networkResponse ,根据情况使用
if (cacheResponse != null) {
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
Response 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();
// 更新原来的缓存至最新
// 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 {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
// 保存之前未缓存的缓存
if (cache != null) {
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
CacheRequest cacheRequest = cache.put(response);
return cacheWritingResponse(cacheRequest, response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}

CacheInterceptor 做的事情就是根据请求拿到缓存,若没有缓存或者缓存失效,就进入网络请求阶段,否则会返回缓存。

ConnectInterceptor

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@Override
public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Request request = realChain.request();
StreamAllocation streamAllocation = realChain.streamAllocation();
// We need the network to satisfy this request. Possibly for validating a conditional GET.
boolean doExtensiveHealthChecks = !request.method().equals("GET");
// 创建 httpCodec (抽象类),分别对应着 http1.1 和 http 2
HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

这里调用了 streamAllocation.newStream 创建了一个 HttpCodec 的对象。

HttpCodec 是一个抽象类,其实现类分别是 Http1CodecHttp2Codec 。相对应的就是 HTTP/1.1 和 HTTP/2.0 。

我们来看下 streamAllocation.newStream 的代码:

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public HttpCodec newStream(OkHttpClient client, boolean doExtensiveHealthChecks) {
int connectTimeout = client.connectTimeoutMillis();
int readTimeout = client.readTimeoutMillis();
int writeTimeout = client.writeTimeoutMillis();
boolean connectionRetryEnabled = client.retryOnConnectionFailure();
try {
// 在连接池中找到一个可用的连接,然后创建出 HttpCodec 对象
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, connectionRetryEnabled, doExtensiveHealthChecks);
HttpCodec resultCodec = resultConnection.newCodec(client, this);
synchronized (connectionPool) {
codec = resultCodec;
return resultCodec;
}
} catch (IOException e) {
throw new RouteException(e);
}
}

newStream(OkHttpClient client, boolean doExtensiveHealthChecks) 中先在连接池中找到可用的连接 resultConnection ,再结合 sinksource 创建出 HttpCodec 的对象。

CallServerInterceptor

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@Override
public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
HttpCodec httpCodec = realChain.httpStream();
StreamAllocation streamAllocation = realChain.streamAllocation();
RealConnection connection = (RealConnection) realChain.connection();
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
// 整理请求头并写入
httpCodec.writeRequestHeaders(request);
Response.Builder responseBuilder = null;
// 检查是否为有 body 的请求方法
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
// If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
// Continue" response before transmitting the request body. If we don't get that, return what
// we did get (such as a 4xx response) without ever transmitting the request body.
// 如果有 Expect: 100-continue 在请求头中,那么要等服务器的响应
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
responseBuilder = httpCodec.readResponseHeaders(true);
}
if (responseBuilder == null) {
// Write the request body if the "Expect: 100-continue" expectation was met.
// 写入请求体
Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
} else if (!connection.isMultiplexed()) {
// If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection from
// being reused. Otherwise we're still obligated to transmit the request body to leave the
// connection in a consistent state.
streamAllocation.noNewStreams();
}
}
httpCodec.finishRequest();
// 得到响应头
if (responseBuilder == null) {
responseBuilder = httpCodec.readResponseHeaders(false);
}
// 构造 response
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
int code = response.code();
// 如果为 web socket 且状态码是 101 ,那么 body 为空
if (forWebSocket && code == 101) {
// Connection is upgrading, but we need to ensure interceptors see a non-null response body.
response = response.newBuilder()
.body(Util.EMPTY_RESPONSE)
.build();
} else {
// 读取 body
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
// 如果请求头中有 close 那么断开连接
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}
// 抛出协议异常
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}

CallServerInterceptor 中可见,关于请求和响应部分都是通过 HttpCodec 来实现的。而在 HttpCodec 内部又是通过 sinksource 来实现的。所以说到底还是 IO 流在起作用。

小结

到这里,我们也完全明白了 OkHttp 中的分层思想,每一个 interceptor 只处理自己的事,而剩余的就交给其他的 interceptor 。这种思想可以简化一些繁琐复杂的流程,从而达到逻辑清晰、互不干扰的效果。

异步请求

与同步请求直接调用 execute() 不同的是,异步请求是调用了 enqueue(Callback responseCallback) 这个方法。那么我们对异步请求探究的入口就是 enqueue(Callback responseCallback) 了。

RealCall

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@Override
public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
// 加入到 dispatcher 中,这里包装成了 AsyncCall
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}

主要的方法就是调用了 Dispatcherenqueue(AsyncCall call) 方法。这里需要注意的是,传入的是 AsyncCall 对象,而不是同步中的 RealCall

那么我们就跟进到 Dispatcher 的源码中吧,至于 AsyncCall 我们会在下面详细讲到。

Dispatcher

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synchronized void enqueue(AsyncCall call) {
// 如果当前正在运行的异步 call 数 < 64 && 队列中请求同一个 host 的异步 call 数 < 5
// maxRequests = 64,maxRequestsPerHost = 5
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
// 加入正在运行异步队列
runningAsyncCalls.add(call);
// 加入到线程池中
executorService().execute(call);
} else {
// 加入预备异步队列
readyAsyncCalls.add(call);
}
}
// 创建线程池
public synchronized ExecutorService executorService() {
if (executorService == null) {
executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
}
return executorService;
}

enqueue(AsyncCall call) 中可以知道,OkHttp 在运行中的异步请求数最多为 63 ,而同一个 host 的异步请求数最多为 4 。否则会加入到 readyAsyncCalls 中。

在加入到 runningAsyncCalls 后,就会进入线程池中被执行。到了这里,我们就要到 AsyncCall 中一探究竟了。

AsyncCall

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final class AsyncCall extends NamedRunnable {
private final Callback responseCallback;
AsyncCall(Callback responseCallback) {
super("OkHttp %s", redactedUrl());
this.responseCallback = responseCallback;
}
String host() {
return originalRequest.url().host();
}
Request request() {
return originalRequest;
}
RealCall get() {
return RealCall.this;
}
@Override
protected void execute() {
boolean signalledCallback = false;
try {
// 调用一连串的拦截器,得到响应
Response response = getResponseWithInterceptorChain();
if (retryAndFollowUpInterceptor.isCanceled()) {
// 回调失败
signalledCallback = true;
responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
} else {
// 回调结果
signalledCallback = true;
responseCallback.onResponse(RealCall.this, response);
}
} catch (IOException e) {
if (signalledCallback) {
// Do not signal the callback twice!
Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
} else {
// 回调失败
responseCallback.onFailure(RealCall.this, e);
}
} finally {
// 在 runningAsyncCalls 中移除,并作推进其他 call 的工作
client.dispatcher().finished(this);
}
}
}

AsyncCallexecute() 方法中,也是调用了 getResponseWithInterceptorChain() 方法来得到 Response 对象。从这里开始,就和同步请求的流程是一样的,就没必要讲了。

在得到 Response 后,进行结果的回调。

最后,调用了 Dispatcherfinished 方法:

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void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
int runningCallsCount;
Runnable idleCallback;
synchronized (this) {
// 移除该 call
if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
// 将 readyAsyncCalls 中的 call 移动到 runningAsyncCalls 中,并加入到线程池中
if (promoteCalls) promoteCalls();
runningCallsCount = runningCallsCount();
idleCallback = this.idleCallback;
}
if (runningCallsCount == 0 && idleCallback != null) {
idleCallback.run();
}
}

finished(Deque<T> calls, T call, boolean promoteCalls) 中对该 call 移除。

若在 readyAsyncCalls 中其他的 call ,就移动到 runningAsyncCalls 中并加入线程池中。

这样,完整的流程就循环起来了。

基本上 OkHttp 的请求响应的流程就讲完了,篇幅有点长长长啊。不过还有很多点没有涉及到的

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