Minimal application.java file with static and dynamic header manipulation for Spring Cloud Gateway.

Application.java

@SpringBootApplication
public class Application {

	@Bean
	public RouteLocator staticRoutes(RouteLocatorBuilder builder) {
		return builder
				.routes()
				.route(defaultRoute())
				.build();
	}

	private Function<PredicateSpec, Route.Builder> defaultRoute() {
		return r -> r
                .path("/**")
                .filters(defaultFilters())
                .uri("lb://application:8080");
	}

	private Function<GatewayFilterSpec, UriSpec> defaultFilters() {
		return f -> f
                .preserveHostHeader()
				        .filter(this::addCustomHeader)
                .addResponseHeader("x-static-router-header", random() + "");
	}

	private Mono<Void> addCustomHeader(ServerWebExchange exchange, GatewayFilterChain chain) {
		exchange.getResponse().getHeaders().add("x-dynamic-router-header", random() + "");
        return chain.filter(exchange);
	}

	private double random(){
		return Math.random();
	}

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}
}