okhttp3实例源码浅析(3)-重试、重定向

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okhttp封装了具有重试、重定向功能的拦截器RetryAndFollowUpInterceptor,它是okhttp自带的第一层拦截器。

下面分析它的intercept方法内做了哪些操作:

@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();

	//创建StreamAllocation对象
    streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
        call, eventListener, callStackTrace);

    int followUpCount = 0;
    Response priorResponse = null;
    //开启循环,请求完成或遇到异常则退出
    while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
	    //调用下一层拦截器
        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.
        //路由连接异常,判断能否重试,无法重试抛出异常,否则continue
        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.
        //服务器通信异常,判断能否重试
        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.
        // 正常情况此时releaseConnection=false。如遇异常且无法重试,释放资源
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      //保存重定向前的响应,第一次循环priorResponse=null
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

	  //判断是否需要重定向。如果需要,会对request做一些修改,返回新的request;否则返回null
      Request followUp = followUpRequest(response);

      if (followUp == null) {
        //无重定向,释放资源,直接返回reponse给上层
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

	  //释放此response的body中开启的字节流
      closeQuietly(response.body());

	  //判断次数是否超过上限20次
      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

	  //request的body通常情况下不为UnrepeatableRequestBody
      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }

      if (!sameConnection(response, followUp.url())) {
	    //比较重定向前后接口地址scheme、host、port是否一致,一致的话可重用,否则释放资源重新创建
        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?");
      }
	  
      request = followUp;
      priorResponse = response;
    }
}
1.StreamAllocation对象

StreamAllocation主要用来管理Connections/Streams的资源分配和释放,这里主要用它来释放资源。在底层的拦截器中会更深入的使用StreamAllocation,后面再具体分析。

2.recover方法

在try-catch中执行chain.proceed方法,如果捕获到RouteException和IOException异常,就会调用recover方法判断该请求能否重试。

private boolean recover(IOException e, boolean requestSendStarted, Request userRequest) {
    streamAllocation.streamFailed(e);

    // The application layer has forbidden retries.
    //retryOnConnectionFailure默认返回true(在构建OkHttpClient时,可通过retryOnConnectionFailure方法设置)
    if (!client.retryOnConnectionFailure()) return false;

    // We can't send the request body again.
    //当捕获到的异常是RouteException和ConnectionShutdownException(继承自IOException)时requestSendStarted为false,其他情况为true;request的body通常情况下不为UnrepeatableRequestBody
    if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;

    // This exception is fatal.
    //判断具体的异常类型能否进行重试
    if (!isRecoverable(e, requestSendStarted)) return false;

    // No more routes to attempt.
    //是否还有可用路由线路
    if (!streamAllocation.hasMoreRoutes()) return false;

    // For failure recovery, use the same route selector with a new connection.
    return true;
}
private boolean isRecoverable(IOException e, boolean requestSendStarted) {
    // If there was a protocol problem, don't recover.
    if (e instanceof ProtocolException) {
      return false;
    }

    // If there was an interruption don't recover, but if there was a timeout connecting to a route
    // we should try the next route (if there is one).
    if (e instanceof InterruptedIOException) {
      return e instanceof SocketTimeoutException && !requestSendStarted;
    }

    // Look for known client-side or negotiation errors that are unlikely to be fixed by trying
    // again with a different route.
    if (e instanceof SSLHandshakeException) {
      // If the problem was a CertificateException from the X509TrustManager,
      // do not retry.
      if (e.getCause() instanceof CertificateException) {
        return false;
      }
    }
    if (e instanceof SSLPeerUnverifiedException) {
      // e.g. a certificate pinning error.
      return false;
    }

    // An example of one we might want to retry with a different route is a problem connecting to a
    // proxy and would manifest as a standard IOException. Unless it is one we know we should not
    // retry, we return true and try a new route.
    return true;
}
  • ProtocolException 协议异常不可重试
  • 捕获到RouteException,传入RouteException的getLastConnectException,若lastException为SocketTimeoutException可重试,否则不可重试。即出现连接超时,可尝试下一个路由重试
  • SSLHandshakeException ssl握手异常,若是由来自X509TrustManager的CertificateException引起则不可重试
  • SSLPeerUnverifiedException ssl证书校验异常不可重试
3.followUpRequest方法
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) {
      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?
        // 判断是否允许重定向,默认允许
        if (!client.followRedirects()) return null;

		// 从header中取出Location值,它就是重定向地址
        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.
        // 判断请求方式,将除了PROPFIND的重定向请求转为GET请求。请求方式若为PROPFIND,则保持原来的method和requestBody,否则method设为GET且丢弃requestBody。
        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) {
            //请求方式不为PROPFIND,移除以下三个属性
            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.
        // 若重定向地址和原来的地址scheme、host、port一致,则移除身份验证请求头
        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;
        }

		// 如果经历过重定向,且上一轮状态码是同样的HTTP_CLIENT_TIMEOUT,则放弃
        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        return userResponse.request();

      default:
        return null;
    }
}

followUpRequest方法内部会判断服务端返回的状态码来决定是否重定向,若返回新的request表示需要重定向,返回null表示不进行重定向。

三种情况下会重新发起请求:

  1. HTTP_PROXY_AUTH(407)、HTTP_UNAUTHORIZED(401),返回包含证书信息的request重新请求进行认证。
  2. HTTP_MULT_CHOICE(300)、HTTP_MOVED_PERM(301)、HTTP_MOVED_TEMP(302)、HTTP_SEE_OTHER(303),修改request的method、url等参数,返回进行重定向。
  3. HTTP_CLIENT_TIMEOUT(408),请求超时,直接返回request进行重试。

总结

RetryAndFollowUpInterceptor的intercept中先是创建了StreamAllocation对象,然后开启while(true)无限循环。接着在这个循环中先调用下层拦截器去网络请求,若请求期间发生异常,判断能否重试,能就continue进行下一轮循环,否则抛异常退出循环结束方法。如果下层拦截器请求完成返回response,通过response的状态码判断是否需要重定向,若需要重定向,修改request后进行下一轮循环,否则返回response结束方法。