OkHttp3源码解析04-失败重连

灰太狼 2023-10-17 16:29 35阅读 0赞

回到RealCall的getResponse方法

Response getResponse(Request request, boolean forWebSocket) throws IOException {
    // Copy body metadata to the appropriate request headers.
    RequestBody body = request.body();
    if (body != null) {
      Request.Builder requestBuilder = request.newBuilder();
    
      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");
      }
    
      request = requestBuilder.build();
    }
    
    // Create the initial HTTP engine. Retries and redirects need new engine for each attempt.
    engine = new HttpEngine(client, request, false, false, forWebSocket, null, null, null);
    
    int followUpCount = 0;
    while (true) {
      if (canceled) {
        engine.releaseStreamAllocation();
        throw new IOException("Canceled");
      }
    
      boolean releaseConnection = true;
      try {
        engine.sendRequest();
        engine.readResponse();
        releaseConnection = false;
      } catch (RequestException e) {
        // The attempt to interpret the request failed. Give up.
        throw e.getCause();
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        HttpEngine retryEngine = engine.recover(e.getLastConnectException(), null);
        if (retryEngine != null) {
          releaseConnection = false;
          engine = retryEngine;
          continue; //进入下一个while遍历
        }
        // Give up; recovery is not possible.
        throw e.getLastConnectException();
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
        HttpEngine retryEngine = engine.recover(e, null);
        if (retryEngine != null) {
          releaseConnection = false;
          engine = retryEngine;
          continue; //进入下一个while遍历
        }
    
        // Give up; recovery is not possible.
        throw e;
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          StreamAllocation streamAllocation = engine.close();
          streamAllocation.release();
        }
      }
    
      //...
      if (followUp == null) {
        if (!forWebSocket) {
          engine.releaseStreamAllocation();
        }
        return response;
      }
    
      //...
      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }
    
      //...
      engine = new HttpEngine(client, request, false, false, forWebSocket, streamAllocation, null,
      response);
    }
    }

可以看到在catch里，会调用engine.recover方法

public HttpEngine recover(IOException e, Sink requestBodyOut) {
        if (!streamAllocation.recover(e, requestBodyOut)) {
          return null;
        }
    
        if (!client.retryOnConnectionFailure()) {
          return null;
        }
    
        StreamAllocation streamAllocation = close();
    
        // For failure recovery, use the same route selector with a new connection.
        return new HttpEngine(client, userRequest, bufferRequestBody, callerWritesRequestBody,
            forWebSocket, streamAllocation, (RetryableSink) requestBodyOut, priorResponse);
    }

可以看到，这里重新创建一个HttpEngine并返回

最终，只要成功，就会返回response

if (followUp == null) {
        if (!forWebSocket) {
        engine.releaseStreamAllocation();
        }
        return response;
    }

如果超过20次都失败，那么就抛出ProtocolException异常

if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

#### 总结 ####

OkHttp的失败重连机制，就是发送request或读取Response的时候，出现crash，就会重新创建一个HttpEngine重新执行，最多执行20次，如果还失败，会抛出ProtocolException异常