The following WebFilter redirects incoming requests for / to a static HTML file, index.html.

ToIndexPageRedirection.java

package com.hascode.tutorial;

import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import reactor.core.publisher.Mono;

@Component
public class ToIndexPageRedirection implements WebFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        if (exchange.getRequest().getURI().getPath().equals("/")) {
            return chain.filter(
                exchange.mutate().request(
                    exchange.getRequest().mutate().path("/index.html").build()
                ).build()
            );
        }

        return chain.filter(exchange);
    }
}

Ressources: