public boolean asyncDispatch(org.apache.coyote.Request req, org.apache.coyote.Response res,
   SocketEvent status) throws Exception {
  //省略部分代码
  Request request = (Request) req.getNote(ADAPTER_NOTES);
  Response response = (Response) res.getNote(ADAPTER_NOTES);
  boolean success = true;
  AsyncContextImpl asyncConImpl = request.getAsyncContextInternal();
  try {
   if (!request.isAsync()) {
    response.setSuspended(false);
   }

   if (status==SocketEvent.TIMEOUT) {
    if (!asyncConImpl.timeout()) {
     asyncConImpl.setErrorState(null, false);
    }
   } else if (status==SocketEvent.ERROR) {
    
   }

   if (!request.isAsyncDispatching() && request.isAsync()) {
    WriteListener writeListener = res.getWriteListener();
    ReadListener readListener = req.getReadListener();
    if (writeListener != null && status == SocketEvent.OPEN_WRITE) {
     ClassLoader oldCL = null;
     try {
      oldCL = request.getContext().bind(false, null);
      res.onWritePossible();//这里执行浏览器响应，写入数据
      if (request.isFinished() && req.sendAllDataReadEvent() &&
        readListener != null) {
       readListener.onAllDataRead();
      }
     } catch (Throwable t) {
      
     } finally {
      request.getContext().unbind(false, oldCL);
     }
    } 
    }
   }
   //这里判断异步正在进行，说明这不是一个完成方法的回调，是一个正常异步请求，继续调用容器。
   if (request.isAsyncDispatching()) {
    connector.getService().getContainer().getPipeline().getFirst().invoke(
      request, response);
    Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);
    if (t != null) {
     asyncConImpl.setErrorState(t, true);
    }
   }
   //注意，这里，如果超时或者出错，request.isAsync()会返回false，这里是为了尽快的输出错误给客户端。
   if (!request.isAsync()) {
    //这里也是输出逻辑
    request.finishRequest();
    response.finishResponse();
   }
   //销毁request和response
   if (!success || !request.isAsync()) {
    updateWrapperErrorCount(request, response);
    request.recycle();
    response.recycle();
   }
  }
  return success;
 }

上面的代码就是 ctx.complete() 执行最终的方法了（当然省略了很多细节），完成了数据的输出，最终输出到浏览器。

这里有同学可能会说，我知道异步执行完后，调用 ctx.complete() 会输出到浏览器，但是，第一次doGet请求执行完成后，Tomcat是怎么知道不用返回到客户端的呢？关键代码在 CoyoteAdapter 中的 service 方法，部分代码如下：

 postParseSuccess = postParseRequest(req, request, res, response);
   //省略部分代码
   if (postParseSuccess) {
    request.setAsyncSupported(
      connector.getService().getContainer().getPipeline().isAsyncSupported());
    connector.getService().getContainer().getPipeline().getFirst().invoke(
      request, response);
   }
   if (request.isAsync()) {
    async = true;
    } else {
    //输出数据到客户端
    request.finishRequest();
    response.finishResponse();
   if (!async) {
    updateWrapperErrorCount(request, response);
    //销毁request和response
    request.recycle();
    response.recycle();
   }