OKHttp原理讲解之RetryAndFollowUpInterceptor

一、前言:

1.1 本篇主要讲解内容

1.RetryAndFollowUpInterceptor中主要成员介绍

2.拦截器中重试机制

3.拦截器中执行流程

1.2 OKHttp项目地址:

https://github.com/square/okhttp

1.3 OKHttp系列讲解简介:

OKHttp项目本身还是很大的,而且涉及到的知识点很多,所以一篇文章是很难概括全的,所以会以一个系列文章的方式来详细讲解OKHttp。

系列准备写以下几篇。

OKHttp原理讲解之基本概念_失落夏天的博客-CSDN博客

OKHttp原理讲解之责任链模式及扩展_失落夏天的博客-CSDN博客

OKHttp原理讲解之重试拦截器(预计03.22发布)

OKHttp原理讲解之路由拦截器

OKHttp原理讲解之缓存拦截器

OKHttp原理讲解之连接池拦截器

OKHttp原理讲解之请求拦截器

PS:预计每周更新1-2篇的进度进行

二、主要成员介绍

Route:路由。当我们请求一个服务的时候,中间要经过DNS域名解析返回多个IP供我们去尝试连接。并且中间有可能会经过多层的DNS,

StreamAllocation:StreamAllocation在其注释中写道,主要是协调Connections,Streams和Call之间的关系。

StreamAllocation中包含几个主要的对象,如下:

ConnectionPool:连接池,具体会在ConnectInterceptor那一章细讲。一个请求结束后,并不会立马关闭连接,还可以利用这个socket继续进行数据的传输。

Address:请求地址的封装对象。

Call:请求体

EventListener:监听对象,方便监听状态

RouteSelector:路由选择器。我们请求一个服务的时候,中间要经过DNS域名解析返回多个

三、RetryAndFollowUpInterceptor中重试机制

3.1重试机制

这里的重试机制我感觉设计的是特别的巧妙的。首先外层是一个while(true)循环,然后发送请求,如果返回值有问题或者抛出异常,则消耗一次重试次数,如果没有超过重试次数上限,则在进行一遍循环发送请求。

如果当次请求成功,并且如果进行了通讯(没有使用缓存),则通过 streamAllocation.release();关闭和释放此次的链接和资源。

如果失败,也要关闭响应中的数据流。

//代码3.1
while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
        ...
        //代码3.2
        continue;
      } catch (IOException e) {
        ...
        continue;
      } finally {
        ...
      }
      ...
      Request followUp = followUpRequest(response, streamAllocation.route());

      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

      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());
      }

      if (!sameConnection(response, followUp.url())) {
        streamAllocation.release();
        streamAllocation = new StreamAllocation(client.connectionPool(),
            createAddress(followUp.url()), call, eventListener, callStackTrace);
        this.streamAllocation = streamAllocation;
      } else if (streamAllocation.codec() != null) {
        throw new IllegalStateException("Closing the body of " + response
            + " didn't close its backing stream. Bad interceptor?");
      }

      request = followUp;
      priorResponse = response;
    }

3.2 如何判断是否请求成功?

即使请求成功,也不一定真的发送了请求,因为还存在命中缓存的可能。所以这需要综合的判断,在拦截器中,判断方法在followUpRequest方法中。

这里的设计是很巧妙的,如果followUpRequest方法返回的request不为空,则代表请求失败,下一次请求使用如果该方法返回值followUp不为空,则代表此次结果是OK的。否则下一次请求就是这一次的返回值followUp。

另外我们详细看代码,发现这个判断方法里面竟然没有我们所常知的200,404,这是为何呢?

我们可以看上面的代码段3.2,这里进行了异常捕获,其实原理就在这里。如果是404错误,那么会在后续的迭代器中直接抛出异常,而在3.2处进行了捕获。所以就不会执行到后面的followUpRequest方法当中。

那如果是200成功呢?如果reponseCode=200的话,则会走到代码3.3的逻辑,直接返回null。从而代表是响应是符合要求。

private Request followUpRequest(Response userResponse, Route route) throws IOException {
    if (userResponse == null) throw new IllegalStateException();
    int responseCode = userResponse.code();

    final String method = userResponse.request().method();
    switch (responseCode) {
      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);

      case HTTP_UNAUTHORIZED:
        return client.authenticator().authenticate(route, userResponse);

      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"
        if (!method.equals("GET") && !method.equals("HEAD")) {
          return null;
        }
        // fall-through
      case HTTP_MULT_CHOICE:
      case HTTP_MOVED_PERM:
      case HTTP_MOVED_TEMP:
      case HTTP_SEE_OTHER:
        // Does the client allow redirects?
        if (!client.followRedirects()) return null;

        String location = userResponse.header("Location");
        if (location == null) return null;
        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.
        boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
        if (!sameScheme && !client.followSslRedirects()) return null;

        // Most redirects don't include a request body.
        Request.Builder requestBuilder = userResponse.request().newBuilder();
        if (HttpMethod.permitsRequestBody(method)) {
          final boolean maintainBody = HttpMethod.redirectsWithBody(method);
          if (HttpMethod.redirectsToGet(method)) {
            requestBuilder.method("GET", null);
          } else {
            RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
            requestBuilder.method(method, requestBody);
          }
          if (!maintainBody) {
            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.
        if (!sameConnection(userResponse, url)) {
          requestBuilder.removeHeader("Authorization");
        }

        return requestBuilder.url(url).build();

      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.)
        if (!client.retryOnConnectionFailure()) {
          // The application layer has directed us not to retry the request.
          return null;
        }

        if (userResponse.request().body() instanceof UnrepeatableRequestBody) {
          return null;
        }

        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        if (retryAfter(userResponse, 0) > 0) {
          return null;
        }

        return userResponse.request();

      case HTTP_UNAVAILABLE:
        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_UNAVAILABLE) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        if (retryAfter(userResponse, Integer.MAX_VALUE) == 0) {
          // specifically received an instruction to retry without delay
          return userResponse.request();
        }

        return null;

      default:
        //代码3.3
        return null;
    }
  }

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