Webflux应用经过Zuul网关后空字符串参数变成了null的问题

一 背景

1 spring webflux应用 Controller方法参数加了@RequestParam即参数必须有，否则会报错
2 应用接口经过了zuul网关做转发
controller方法如下：

@GetMapping(value = "/api/coupons/activitySimples")
public Mono<List<CouponActivitySimple>> getCouponActivitySimples(
    @RequestParam Long productCode,
    @RequestParam String profitLevel) {
    ....
}

zuul网关如下：

zuul:
  routes:
    coupon-service-v2:
      path: /coupon/**
      url: http://【coupon-service-host】/api
      sensitiveHeaders:
      customSensitiveHeaders: true
      addHostHeader: true

curl -XGET http://【zuul-gateway-host】/coupon/coupons/activitySimples?productCode=1&profitLevel=

二 现象

报如下异常：

org.springframework.web.server.ServerWebInputException: 400 BAD_REQUEST "Required String parameter 'profitLevel' is not present"
at org.springframework.web.reactive.result.method.annotation.RequestParamMethodArgumentResolver.handleMissingValue(RequestParamMethodArgumentResolver.java:114) ~[spring-webflux-5.2.1.RELEASE.jar!/:5.2.1.RELEASE]
at org.springframework.web.reactive.result.method.annotation.AbstractNamedValueArgumentResolver.lambda$getDefaultValue$1(AbstractNamedValueArgumentResolver.java:215) ~[spring-webflux-5.2.1.RELEASE.jar!/:5.2.1.RELEASE]
at reactor.core.publisher.MonoSupplier.subscribe(MonoSupplier.java:56) [reactor-core-3.3.0.RELEASE.jar!/:3.3.0.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:4087) ~[reactor-core-3.3.0.RELEASE.jar!/:3.3.0.RELEASE]
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onComplete(FluxSwitchIfEmpty.java:75...

三 原因

通过断点方式，发现通过了zuul网关之后，请求的uri变成了如下，即profitLevel后面的等于号不见了，然后从request中取出来的值是null，并非空字符串

/coupon/coupons/activitySimples?productCode=1&profitLevel

四 解决方案

1 不使用zuul网关，用spring-cloud-gateway2.x网关，不会有如上问题
2 允许profitLevel字段为空 @RequestParam(require = false) String profitLevel