Mono<Flux> ->如何返回ResponseEntity.NOT_FOUND？

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

@RestController
public class ExampleController {

    @GetMapping("/example/{id}")
    public Mono<ResponseEntity<String>> getExample(@PathVariable String id) {
        // 模拟一个空的Flux
        Flux<String> emptyFlux = Flux.empty();

        return emptyFlux
                .flatMap(data -> Mono.just(ResponseEntity.ok(data))) // 如果存在数据，则返回正常的响应
                .switchIfEmpty(Mono.just(ResponseEntity.notFound().build())); // 如果不存在数据，则返回NOT_FOUND
    }
}