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游戏SDK架构设计之代码实现——网络框架
OKHttp 源码解析（一）
OKHttp 源码解析（二）拦截器

前言

上一篇解读了OKHttp 的基本框架源码，其中 OKHttp 发送请求的核心是调用 getResponseWithInterceptorChain 构建拦截器链，遍历拦截器，执行请求，执行完成时返回结果。这篇看一下 OKHttp 的拦截器链。

本文查看 OKHttp 源码的版本是 3.4.2.

OkHttp 的拦截器使用了责任链设计模式，使得每个处理者都有机会处理请求，关于责任链设计模式的介绍见文章：

拦截器源码解析

无论是同步请求还是异步请求，OkHttp 都是先调用 getResponseWithInterceptorChain 方法。添加拦截器的顺序就是执行的顺序。

private Response getResponseWithInterceptorChain() throws IOException {// Build a full stack of interceptors.List<Interceptor> interceptors = new ArrayList<>();// 添加开发者自定义的拦截器interceptors.addAll(client.interceptors());// 失败重连拦截器interceptors.add(retryAndFollowUpInterceptor);// 桥接和适配器interceptors.add(new BridgeInterceptor(client.cookieJar()));//缓存interceptors.add(new CacheInterceptor(client.internalCache()));// 链接interceptors.add(new ConnectInterceptor(client));if (!retryAndFollowUpInterceptor.isForWebSocket()) {// 网络interceptors.addAll(client.networkInterceptors());}// 请求服务interceptors.add(new CallServerInterceptor(retryAndFollowUpInterceptor.isForWebSocket()));// 创建拦截器链Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest);// 拦截器链执行结果return chain.proceed(originalRequest);}

1、开发者自定义的拦截器

OkHttp 允许开发者自定义拦截器并优先执行开发者定义的拦截器。具体做法如下

1. 定义拦截器实现 OkHttp 的拦截器接口 Interceptor

   public class RetryInterceptor implements Interceptor {public Response intercept(Chain chain) throws IOException {// ...}
   }// 官网解释：观察、修改并潜在地短路发出的请求和返回的相应响应。
   // 通常，拦截器会添加、删除或转换请求或响应的标头。
   public interface Interceptor {// 实现方法，处理请求Response intercept(Chain chain) throws IOException;// 处理者串成的链：责任链模式中的一个角色，让每个处理者都有机会处理请求interface Chain {// 获取当前的请求Request request();// 处理请求Response proceed(Request request) throws IOException;// 网络链接Connection connection();}
   }
   

2. 通过OkHttpClient 的 addInterceptor 接口添加到拦截器列表里

   public Builder addInterceptor(Interceptor interceptor) {interceptors.add(interceptor);return this;
   }
   

2、RetryAndFollowUpInterceptor 重试重定向拦截器

RetryAndFollowUpInterceptor 在调用 newCall 时，返回 RealCall 对象时创建。

protected RealCall(OkHttpClient client, Request originalRequest) {this.client = client;this.originalRequest = originalRequest;this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client);}

/*** This interceptor recovers from failures and follows redirects as necessary. It may throw an* {@link IOException} if the call was canceled.
主要管理请求重试次数和重定向的*/
public final class RetryAndFollowUpInterceptor implements Interceptor {/*** How many redirects and auth challenges should we attempt? Chrome follows 21 redirects; Firefox,* curl, and wget follow 20; Safari follows 16; and HTTP/1.0 recommends 5.*/// 定义最大重试和重定向次数private static final int MAX_FOLLOW_UPS = 20;@Override public Response intercept(Chain chain) throws IOException {Request request = chain.request();// 这是一个路由、链接池和流的调用逻辑管理类，负责调度协调streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()));int followUpCount = 0;Response priorResponse = null;// 无限循环while (true) {// 如果取消掉，释放链接池if (canceled) {streamAllocation.release();throw new IOException("Canceled");}Response response = null;boolean releaseConnection = true;try {// 执行拦截器链，获取结果response = ((RealInterceptorChain) chain).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.// 通过 recover 方法判断是否需要进行重试，否则中断此次循环，开始下一次循环// 如果是一些永久故障，则不尝试。1.设置了不允许重试，2.不可再次发送请求异常， 3. 致命的异常 4.没有更多的路由可以尝试if (!recover(e.getLastConnectException(), true, request)) throw e.getLastConnectException();releaseConnection = false;continue;} catch (IOException e) {// An attempt to communicate with a server failed. The request may have been sent.// 处理同上if (!recover(e, false, request)) throw e;releaseConnection = false;continue;} finally {// We're throwing an unchecked exception. Release any resources.// 不是以上异常情况，不进行重试if (releaseConnection) {streamAllocation.streamFailed(null);streamAllocation.release();}}// Attach the prior response if it exists. Such responses never have a body.// 关联前一个响应： 责任链模式的特点之一，之前的请求结果统一返回if (priorResponse != null) {response = response.newBuilder().priorResponse(priorResponse.newBuilder().body(null).build()).build();}// 处理重定向Request followUp = followUpRequest(response);// 不需要重定向，此次请求结束，释放资源if (followUp == null) {if (!forWebSocket) {streamAllocation.release();}return response;}closeQuietly(response.body());
// 重定向次数限制，超过一定次数抛出异常if (++followUpCount > MAX_FOLLOW_UPS) {streamAllocation.release();throw new ProtocolException("Too many follow-up requests: " + followUpCount);}if (followUp.body() instanceof UnrepeatableRequestBody) {throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());}if (!sameConnection(response, followUp.url())) {streamAllocation.release();streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(followUp.url()));} else if (streamAllocation.stream() != null) {throw new IllegalStateException("Closing the body of " + response+ " didn't close its backing stream. Bad interceptor?");}request = followUp;priorResponse = response;}}}

followUpRequest 根据结果响应码处理是否需要重定向。

当服务端数据迁移网址以后，客户端使用原地址请求数据时，服务端会将迁移后的新地址通过 header 返回给客户端，同时携带 3 开头的错误码通知客户端目标地址已修改，需要重定向地址。

客户端收到后根据响应码和新地址重新发出请求，获取目标数据。

/*** Figures out the HTTP request to make in response to receiving {@code userResponse}. This will* either add authentication headers, follow redirects or handle a client request timeout. If a* follow-up is either unnecessary or not applicable, this returns null.*/private Request followUpRequest(Response userResponse) throws IOException {if (userResponse == null) throw new IllegalStateException();Connection connection = streamAllocation.connection();Route route = connection != null? connection.route(): null;int responseCode = userResponse.code();final String method = userResponse.request().method();switch (responseCode) {//407 客户端使用HTTP代理服务器，需要代理身份认证case HTTP_PROXY_AUTH:Proxy selectedProxy = route != null? route.proxy(): client.proxy();if (selectedProxy.type() != Proxy.Type.HTTP) {throw new ProtocolException("Received HTTP_PROXY_AUTH (407) code while not using proxy");}return client.proxyAuthenticator().authenticate(route, userResponse);// 401 身份验证，服务器需要验证者身份，case HTTP_UNAUTHORIZED:return client.authenticator().authenticate(route, userResponse);// 3xx ：重定向case HTTP_PERM_REDIRECT: case HTTP_TEMP_REDIRECT:// "If the 307 or 308 status code is received in response to a request other than GET// or HEAD, the user agent MUST NOT automatically redirect the request"
// 看以上注释：307、308 ，除 get 、head 外，不会重定向if (!method.equals("GET") && !method.equals("HEAD")) {return null;}// fall-throughcase HTTP_MULT_CHOICE: //300case HTTP_MOVED_PERM: //301case HTTP_MOVED_TEMP: // 302case HTTP_SEE_OTHER: //303// Does the client allow redirects?   客户端是否允许重定向if (!client.followRedirects()) return null;String location = userResponse.header("Location");if (location == null) return null;// 将旧地址替换为新地址HttpUrl url = userResponse.request().url().resolve(location);// Don't follow redirects to unsupported protocols.if (url == null) return null;// If configured, don't follow redirects between SSL and non-SSL.// http 和 https 之间的切换，不允许重定向boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());if (!sameScheme && !client.followSslRedirects()) return null;// Redirects don't include a request body.Request.Builder requestBuilder = userResponse.request().newBuilder();if (HttpMethod.permitsRequestBody(method)) {// 除 PROPFIND 之外的所有请求都应重定向到 GET 请求。if (HttpMethod.redirectsToGet(method)) {requestBuilder.method("GET", null);} else {requestBuilder.method(method, null);}// 将请求体中有关body的信息删除requestBuilder.removeHeader("Transfer-Encoding");requestBuilder.removeHeader("Content-Length");requestBuilder.removeHeader("Content-Type");}// When redirecting across hosts, drop all authentication headers. This// is potentially annoying to the application layer since they have no// way to retain them.// 当跨主机重定向时,删除请求头Authorization身份验证信息if (!sameConnection(userResponse, url)) {requestBuilder.removeHeader("Authorization");}return requestBuilder.url(url).build();
// 4xx 不是重定向case HTTP_CLIENT_TIMEOUT:// 408's are rare in practice, but some servers like HAProxy use this response code. The// spec says that we may repeat the request without modifications. Modern browsers also// repeat the request (even non-idempotent ones.)// 408 很少见。但一些服务器（如 HAProxy）使用此响应代码。 规范说我们可以不加修改地重复请求。 现代浏览器也会重复请求（甚至是非幂等的）。if (userResponse.request().body() instanceof UnrepeatableRequestBody) {return null;}return userResponse.request();default:return null;}}

重试重定向拦截器总结：

1. 开始一个无限循环，调用之后的拦截器获取响应结果response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
2. 请求或链接过程中抛出异常，在异常内通过 recover 方法判断是否需要重试。否则返回priorResponse （链接之前的 response）。
3. 根据响应码判断是否需要重定向 followUpRequest
4. 不需要重定向结束请求，返回响应，释放资源，需要重定向执行有限次数重定向，超过一定次数抛出异常。

3、BridgeInterceptor桥连接拦截器

桥梁拦截器是应用程序和HTTP网络的桥梁。根据用户请求构建网络请求，发送到服务器。将服务器的响应转换为用户能够识别的响应。

public final class BridgeInterceptor implements Interceptor {private final CookieJar cookieJar;public BridgeInterceptor(CookieJar cookieJar) {this.cookieJar = cookieJar;}@Override public Response intercept(Chain chain) throws IOException {Request userRequest = chain.request();Request.Builder requestBuilder = userRequest.newBuilder();RequestBody body = userRequest.body();if (body != null) { // 请求体不为空时，将请求体设置到请求头中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 {
// 请求长度不确定时，使用分块传输 chunked，减少资源消耗，提高效率requestBuilder.header("Transfer-Encoding", "chunked");requestBuilder.removeHeader("Content-Length");}}
// 向header中添加 url 域名if (userRequest.header("Host") == null) {requestBuilder.header("Host", hostHeader(userRequest.url(), false));}
// 默认设置长链接。长链接就是在 tcp 发送完消息后不关闭连接，而是持续连接，
// 因为TCP 连接需要三次握手四次挥手，为节省资源减少消耗，在后续传输中重用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.boolean transparentGzip = false;if (userRequest.header("Accept-Encoding") == null) {transparentGzip = true;requestBuilder.header("Accept-Encoding", "gzip");}// 获取 客户端的cookie信息List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());if (!cookies.isEmpty()) {requestBuilder.header("Cookie", cookieHeader(cookies));}
// 请求的用户信息if (userRequest.header("User-Agent") == null) {requestBuilder.header("User-Agent", Version.userAgent());}Response networkResponse = chain.proceed(requestBuilder.build());
// 根据返回的信息是否保存到cookie中
// CookieJar.NO_COOKIES 或者没有要保存的信息时，不保存
// 否则调用 saveFromResponse ，由客户端保存HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());Response.Builder responseBuilder = networkResponse.newBuilder().request(userRequest);
// 如果有响应体且是gzip类型，将响应体解压为 gzip 对象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桥连接拦截器总结：

1. 添加请求头，将用户请求转换为能够进行网络访问的请求。
2. 执行拦截器链的下一个拦截器方法。
3. 获取响应，将响应转成应用可识别的response。

4、CacheInterceptor缓存拦截器

public final class CacheInterceptor implements Interceptor {public CacheInterceptor(InternalCache cache) {this.cache = cache;}@Override public Response intercept(Chain chain) throws IOException {
// 如果客户端实现缓存对象，则根据url获取本地缓存对象Response cacheCandidate = cache != null? cache.get(chain.request()): null;long now = System.currentTimeMillis();// 获取缓存策略对象，见下方介绍CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();Request networkRequest = strategy.networkRequest;Response cacheResponse = strategy.cacheResponse;if (cache != null) {cache.trackResponse(strategy);}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.
// 不需要访问网络也不需要缓存，返回 504if (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(EMPTY_BODY).sentRequestAtMillis(-1L).receivedResponseAtMillis(System.currentTimeMillis()).build();}// If we don't need the network, we're done.
// 网络请求是空的，返回响应if (networkRequest == null) {return cacheResponse.newBuilder().cacheResponse(stripBody(cacheResponse)).build();}Response networkResponse = null;try {
// 执行请求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());}}// If we have a cache response too, then we're doing a conditional get.
// 本地缓存不为空，可以继续使用本地资源if (cacheResponse != null) {if (validate(cacheResponse, networkResponse)) {Response response = cacheResponse.newBuilder().headers(combine(cacheResponse.headers(), networkResponse.headers())).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 {closeQuietly(cacheResponse.body());}}Response response = networkResponse.newBuilder().cacheResponse(stripBody(cacheResponse)).networkResponse(stripBody(networkResponse)).build();if (HttpHeaders.hasBody(response)) {CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);response = cacheWritingResponse(cacheRequest, response);}return response;}}

1. 判断本地是否有缓存；
2. 根据

缓存策略对象

public Factory(long nowMillis, Request request, Response cacheResponse) {this.nowMillis = nowMillis;this.request = request;this.cacheResponse = cacheResponse;if (cacheResponse != null) {
// 发送请问的时间this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
// 获取响应的时间 
// 记录时间是为了计算当前缓存是否有效this.receivedResponseMillis = cacheResponse.receivedResponseAtMillis();Headers headers = cacheResponse.headers();for (int i = 0, size = headers.size(); i < size; i++) {String fieldName = headers.name(i);String value = headers.value(i);// 请求发送时间if ("Date".equalsIgnoreCase(fieldName)) {servedDate = HttpDate.parse(value);servedDateString = value;
// 缓存过期时间} else if ("Expires".equalsIgnoreCase(fieldName)) {expires = HttpDate.parse(value);
// 目标资源最后修改时间} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {lastModified = HttpDate.parse(value);lastModifiedString = value;
// 目标资源的标识，如果目标资源被修改过之后，标识会变，客户端需要用标识对比是否一致，不一致说明资源已被修改，需要重新请求} else if ("ETag".equalsIgnoreCase(fieldName)) {etag = value;
// 缓存的年龄} else if ("Age".equalsIgnoreCase(fieldName)) {ageSeconds = HttpHeaders.parseSeconds(value, -1);}}}
}// 获取缓存策略对象
public CacheStrategy get() {CacheStrategy candidate = getCandidate();// 如果需要网络请求，且客户端只取缓存信息，条件冲突返回一个if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {// We're forbidden from using the network and the cache is insufficient.return new CacheStrategy(null, null);}return candidate;}

根据不同的条件返回缓存策略对象（重点）

private CacheStrategy getCandidate() {// No cached response.
//没有缓存，请求网络if (cacheResponse == null) {return new CacheStrategy(request, null);}// Drop the cached response if it's missing a required handshake.
// 缺少需要的握手，删除缓存的响应if (request.isHttps() && cacheResponse.handshake() == null) {return new CacheStrategy(request, null);}// If this response shouldn't have been stored, it should never be used// as a response source. This check should be redundant as long as the// persistence store is well-behaved and the rules are constant./*凡是响应码为200, 203, 204, 300, 301, 404, 405, 410, 414, 501, 308,若请求头或响应头中不包含noStore(不允许存储缓存),返回true凡是响应码为303,307,响应头中包含Expires(值为缓存过期时间),或Cache-Control中带有max-age,public,private其中一个,同样不包含noStore,返回true否则,一律返回false*/if (!isCacheable(cacheResponse, request)) {return new CacheStrategy(request, null);}/*  Cache-Control:noCache,请求头信息,noCache并不是表面看起来的不缓存数据,数据也会进行缓存,只是每次在使用本地缓存前需要先进行一次网络请求验证缓存If-Modified-Since:请求头中携带的存储客户端缓存最后修改的时间,服务器将实际文件修改时间与请求头中时间进行对比,若相同返回304码表示目标资源未更改,客户端可以使用本地缓存,若不同返回200,同时将目标资源返回给客户端If-None-Match:与If-Modified-Since类似作用,value为缓存资源的ETag值(资源唯一标识),到服务端时同样会进行比较,相同返回304,不同返回目标资源*/
// 请求包含noCache请求头或If-Modified-Since或者有If-None-Match,访问网络CacheControl requestCaching = request.cacheControl();if (requestCaching.noCache() ||hasConditions(request)) {return new CacheStrategy(request, null);}// 缓存产生到现在的时间long ageMillis = cacheResponseAge();
// 响应缓存最小可用时间long freshMillis = computeFreshnessLifetime();if (requestCaching.maxAgeSeconds() != -1) {freshMillis = Math.min(freshMillis,SECONDS.toMillis(requestCaching.maxAgeSeconds()));}
//客户端设置的缓存剩余有效可用时间long minFreshMillis = 0;if (requestCaching.minFreshSeconds() != -1) {minFreshMillis =SECONDS.toMillis(requestCaching.minFreshSeconds());}
// 修改过期后还可以使用的时长，未设置时表示过期多久都可以使用long maxStaleMillis = 0;CacheControl responseCaching = cacheResponse.cacheControl();if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {maxStaleMillis =SECONDS.toMillis(requestCaching.maxStaleSeconds());}
// 如果不需要访问网络，且缓存年龄+客户端认为的最小缓存有效时间<缓存实际有效时长+缓存后仍可使用时长 ————> 可以使用缓存if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {Response.Builder builder = cacheResponse.newBuilder();
// 缓存年龄+客户端认为的最小缓存有效时间 超过响应缓存的最小有效时间if (ageMillis + minFreshMillis >= freshMillis) {builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");}
// 缓存年龄超过1天且没有设置过时间，抛出警告long oneDayMillis = 24 * 60 * 60 * 1000L;if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");}return new CacheStrategy(null, builder.build());}// Find a condition to add to the request. If the condition is satisfied, the response body// will not be transmitted.
// 需要请求网络，请求网络，验证头信息String conditionName;String conditionValue;if (etag != null) {conditionName = "If-None-Match";conditionValue = etag;} else if (lastModified != null) {conditionName = "If-Modified-Since";conditionValue = lastModifiedString;} else if (servedDate != null) {conditionName = "If-Modified-Since";conditionValue = servedDateString;} else {return new CacheStrategy(request, null); // No condition! Make a regular request.}Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);Request conditionalRequest = request.newBuilder().headers(conditionalRequestHeaders.build()).build();return new CacheStrategy(conditionalRequest, cacheResponse);
}

缓存拦截器作用：是请求网络获取新数据还是使用缓存的数据，缓存拦截器的核心在于返回一个什么样的缓存策略。

5、ConnectInterceptor 链接拦截器

@Override public Response intercept(Chain chain) throws IOException {RealInterceptorChain realChain = (RealInterceptorChain) chain;Request request = realChain.request();
// 获取一个 StreamAllocationStreamAllocation streamAllocation = realChain.streamAllocation();// We need the network to satisfy this request. Possibly for validating a conditional GET.boolean doExtensiveHealthChecks = !request.method().equals("GET");
// 创建获取一个健康的链接 findHealthyConnectionHttpStream httpStream = streamAllocation.newStream(client, doExtensiveHealthChecks);RealConnection connection = streamAllocation.connection();return realChain.proceed(request, streamAllocation, httpStream, connection);
}private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,int writeTimeout, boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)throws IOException {
// 无限循环获取 connection 链接while (true) {RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,connectionRetryEnabled);// If this is a brand new connection, we can skip the extensive health checks.
// 如果是一个没有过链接记录的链接，无需检查，直接返回synchronized (connectionPool) {if (candidate.successCount == 0) {return candidate;}}// Do a (potentially slow) check to confirm that the pooled connection is still good. If it// isn't, take it out of the pool and start again.
// 检查查找到的链接是否健康，比如是否已经closed、shutdown、outputshutdown，链接是否超时等问题if (!candidate.isHealthy(doExtensiveHealthChecks)) {noNewStreams();continue;}return candidate;}}

**StreamAllocation：**是链接、流、路由之间的桥梁。官方解释如下

连接：到远程服务器的物理套接字连接。 这些建立起来可能很慢，因此有必要能够取消当前正在连接的连接。
流：在连接上分层的逻辑 HTTP 请求/响应对。 每个连接都有自己的分配限制，它定义了该连接可以承载多少个并发流。 HTTP/1.x 连接一次可以携带 1 个流，SPDY 和 HTTP/2 通常携带多个。
调用：流的逻辑序列，通常是初始请求及其后续请求。 我们更愿意将单个呼叫的所有流保持在同一连接上，以获得更好的行为和位置。

此类的实例代表调用，在一个或多个连接上使用一个或多个流。 此类具有用于释放上述每个资源的 API：
noNewStreams() 防止连接在未来被用于新的流。 在 Connection: close 标头之后或连接可能不一致时使用它。
streamFinished() 从此分配中释放活动流。 请注意，在给定时间只有一个流可能处于活动状态，因此有必要在使用 newStream() 创建后续流之前调用 streamFinished()。
release() 删除呼叫对连接的保持。 请注意，如果仍然存在流，这不会立即释放连接。 当调用完成但其响应主体尚未完全消耗时，就会发生这种情况。

支持异步取消。

ConnectionPool ：管理 HTTP 和 SPDY 连接的重用以减少网络延迟。 共享同一地址的 HTTP 请求可能共享一个连接。 此类实现了哪些连接保持打开以供将来使用的策略。

public ConnectionPool() {
// 一个连接池最多有个 5 个空闲链接，每个链接超出 5 分钟无动作被移除this(5, 5, TimeUnit.MINUTES);
}
// 循环遍历获取连接
RealConnection get(Address address, StreamAllocation streamAllocation) {assert (Thread.holdsLock(this));for (RealConnection connection : connections) {
// 1.达到最大不可复用限制
// 2.保证和上次请求的是一个地址
// 3.链接不能添加新的流if (connection.allocations.size() < connection.allocationLimit&& address.equals(connection.route().address)&& !connection.noNewStreams) {streamAllocation.acquire(connection);return connection;}}return null;}// 放入链接
void put(RealConnection connection) {assert (Thread.holdsLock(this));
// 先判断是否有需要清理的连接 if (!cleanupRunning) {cleanupRunning = true;
// 执行清理回收算法。
// 内部逻辑：找到最不活跃的连接，当空闲的连接超过5个后删除这个不活跃的连接。
// 内部是死循环，当所有连接都是活跃状态时，暂停执行回收机制，直到池中无连接。executor.execute(cleanupRunnable);}
// 将连接添加到连接池connections.add(connection);}

连接拦截器：

1. 根据地址、证书、DNS 、最大限制流、超空闲时间等条件确定一个连接是否能复用。
2. 创建一个新的连接。