Add @NotNull annotation on a field (you may need to change type to Integer), and add @Valid annotation on the method parameter of the controller.
Mono<String> question(@Valid @RequestBody Foo foo) {
...
}
public class Foo {
@NotNull
private Integer important;
private String something;
//constructors, getter, seters, toString
}
You can find more information here: https://lmonkiewicz.medium.com/the-power-of-spring-rest-api-validation-77be83edef