How can I simply redirect a url if a specific query parameter is missing?

```
@RestController
public class PersonController {
    //only in case the "sort" query parameter is missing
    @GetMapping("/persons")
    public String unsorted() {
         return "redirect:/persons?sort=name";
    }

    //only in case the "sort" query parameter exists
    @GetMapping("/persons")
    public String sorted() {
         //...
    }
}

```