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; //進入下一個while遍歷
    }
    // 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; //進入下一個while遍歷
    }

    // 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裡，會呼叫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，這裡重新建立一個HttpEngine並返回

最終，只要成功，就會返回response

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

如果超過20次都失敗，那麼就丟擲ProtocolException異常

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

總結

OkHttp的失敗重連機制，就是傳送request或讀取Response的時候，出現crash，就會重新建立一個HttpEngine重新執行，最多執行20次，如果還失敗，會丟擲ProtocolException異常