（原因：不允许有多个 ‘Access-Control-Allow-Origin‘ CORS 头）

前后端分离就会碰到跨域问题，原本我们在后端采用 CORS 解决，现在利用网关，可以放在网关解决。

版本：
spring-cloud：Hoxton.SR3

一共两个文件

CorsConfiguration.java

/**
 * 配置跨域
 */
@Configuration
public class CorsConfiguration {

    @Bean
    public CorsResponseHeaderFilter corsResponseHeaderFilter() {
        return new CorsResponseHeaderFilter();
    }

    @Bean
    public CorsWebFilter corsWebFilter() {
        UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        CorsConfiguration corsConfiguration = new CorsConfiguration();

        corsConfiguration.addAllowedHeader("*");
        corsConfiguration.addAllowedMethod("*");
        corsConfiguration.addAllowedOrigin("*");
        corsConfiguration.setAllowCredentials(true);
        corsConfiguration.setMaxAge(600L);

        source.registerCorsConfiguration("/**", corsConfiguration);
        return new CorsWebFilter(source);
    }
}

CorsResponseHeaderFilter.java

/**
 * 跨域请求头重复处理过滤器
 */
public class CorsResponseHeaderFilter implements GlobalFilter, Ordered {
    @Override
    public int getOrder() {
        // 指定此过滤器位于NettyWriteResponseFilter之后
        // 即待处理完响应体后接着处理响应头
        return NettyWriteResponseFilter.WRITE_RESPONSE_FILTER_ORDER + 1;
    }

    @Override
    @SuppressWarnings("serial")
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return chain.filter(exchange).then(Mono.defer(() -> {
            exchange.getResponse().getHeaders().entrySet().stream()
                    .filter(kv -> (kv.getValue() != null && kv.getValue().size() > 1))
                    .filter(kv -> (kv.getKey().equals(HttpHeaders.ACCESS_CONTROL_ALLOW_ORIGIN)
                            || kv.getKey().equals(HttpHeaders.ACCESS_CONTROL_ALLOW_CREDENTIALS)))
                    .forEach(kv ->
                    {
                        kv.setValue(new ArrayList<String>() {{
                            add(kv.getValue().get(0));
                        }});
                    });

            return chain.filter(exchange);
        }));
    }
}