@ifdef 没明白前辈的意思。我的完整代码如下
public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
ServerHttpRequest request = exchange.getRequest();
final Mono<String> bodyMono = authFeignClient.checkToken("test", authProperties.getClientId(), authProperties.getClientSecret());

return bodyMono.map(body -> {
if (StrUtil.isEmpty(body)) {
return returnData(exchange, ResponseWrapper.error(401, "认证未通过"));
}

ServerHttpRequest serverHttpRequest = request.mutate().header( "X-CONTEXT-USER", "fdasfdas").build();
return chain.filter(exchange.mutate().request(serverHttpRequest).build());
}).flatMap(it -> it);
}

private Mono<Void> returnData(ServerWebExchange exchange, ResponseWrapper<String> responseWrapper) {
ServerHttpResponse response = exchange.getResponse();
DataBuffer buffer = response.bufferFactory().wrap(JSONUtil.toJsonStr(responseWrapper).getBytes());
response.getHeaders().setContentType(MediaType.APPLICATION_JSON);
response.setStatusCode(HttpStatus.OK);
return response.writeWith(Flux.just(buffer));
}