UserResource.java

import io.swagger.annotations.ApiModelProperty;
import javax.validation.constraints.NotNull;

@RestController
@RequestMapping("/users")
class UserResource {

  @GetMapping()
  List<User> allUsers() { /* ... */ }

  static class User {
    @NotNull
    public String email;
    public String firstName;
    @ApiModelProperty(notes = "User's surname")
    public String lastName;
    @NotNull
    public EnumSet<Role> roles;
  }
  enum Role {
    USER, ADMIN
  }
}