code: public CompletionHandler createAsyncHandler() {
          return Utility.createAsyncHandler((v, a) -> {
              finish(v);
          }, (t, a) -> {
              context.getLogger().log(Level.WARNING, "Servlet occur, force to close channel. request = " + request + ", result is CompletionHandler", (Throwable) t);
              finish(500, null);
          });
      }
docs: 创建CompletionHandler实例
  
  @return CompletionHandler