public class User {
    private String name;

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

@RestController
public class DemoController {

    @GetMapping("/hello")
    public Mono<String> hello(@ModelAttribute(binding = false) User user) {
        return Mono.just(user.getName());
    }
}