Spring Cloud Gateway filter for SPAs nested routes would be redirected to index.html and resolve them properly.

Filters.kt

@Component
class SPAGatewayFilter : GatewayFilter {
  val logger: Logger = LoggerFactory.getLogger(SPAGatewayFilter::class.java)
  companion object { const val INDEX_PATH = "/index.html" }

  override fun filter(exchange: ServerWebExchange, chain: GatewayFilterChain): Mono<Void> {
    val req = exchange.request
    val ex = if(req.headers.accept.contains(MediaType.TEXT_HTML)) {
      exchange.mutate().request {
        logger.debug("Replace path: ${req.path} by $INDEX_PATH")
        it.path(INDEX_PATH)
      }.build()
    } else exchange
    return chain.filter(ex)
  }
}

@Component
class SPAGatewayFilterFactory(
        private val spaGatewayFilter: SPAGatewayFilter
) : AbstractGatewayFilterFactory<SPAGatewayFilterFactory.Config>(Config::class.java) {
  class Config { }
  override fun apply(config: Config) = spaGatewayFilter
}