NullPointerException in ProxyExchangeArgumentResolver

Describe the bug

I have a proxy gateway setup very similar to the one in the documentation. https://cloud.spring.io/spring-cloud-gateway/reference/html/#building-a-simple-gateway-by-using-spring-mvc-or-webflux

In version 2.2.1 everything works fine. In version 2.2.2 there following error occurs:

Caused by: java.lang.NullPointerException: null
	at org.springframework.cloud.gateway.mvc.config.ProxyExchangeArgumentResolver.resolveArgument(ProxyExchangeArgumentResolver.java:81)

Every version since 2.2.2 results in the same error. I believe this will happen whenever org.springframework.cloud.gateway.mvc.config.ProxyExchangeArgumentResolver::autoForwardedHeaders is null. https://github.com/spring-cloud/spring-cloud-gateway/blob/d2ea0ebd822b71c70a6e1bb4bfc7bf73423ac051/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/config/ProxyExchangeArgumentResolver.java#L48

Sample

I don’t have a sample, but it should be pretty easy to prove with a unit test and the code problem seems obvious (although I have very little experience looking at spring source code).

Code indicating autoForwardedHeaders can be null:

    public void setAutoForwardedHeaders(Set<String> autoForwardedHeaders) {
        this.autoForwardedHeaders = autoForwardedHeaders == null ? null
                : autoForwardedHeaders.stream().map(String::toLowerCase).collect(toSet());
    }

https://github.com/spring-cloud/spring-cloud-gateway/blob/d2ea0ebd822b71c70a6e1bb4bfc7bf73423ac051/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/config/ProxyExchangeArgumentResolver.java#L60

Code where there’s no null check:

        if (this.autoForwardedHeaders.size() > 0) {
            proxy.headers(extractAutoForwardedHeaders(webRequest));
        }

https://github.com/spring-cloud/spring-cloud-gateway/blob/d2ea0ebd822b71c70a6e1bb4bfc7bf73423ac051/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/config/ProxyExchangeArgumentResolver.java#L79

[manueldeveloper]

Hi @spencergibb I’ve created the following PR #2163 which solves the issue. I hope that all is ok, if not, let me know and I’d try to solve asap. Thanks!

[spencergibb]

Great. Let me know if you need some help