Json 如何区分 Spring Rest Controller 中部分更新的 null 值和未提供的值

在使用Spring睡觉控制器中的PUT请求方法部分更新实体时，我try 区分空值和未提供的值.

以以下实体为例:

@Entity
private class Person {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    /* let's assume the following attributes may be null */
    private String firstName;
    private String lastName;

    /* getters and setters ... */
}

My Person repository (Spring Data):

@Repository
public interface PersonRepository extends CrudRepository<Person, Long> {
}

The DTO I use:

private class PersonDTO {
    private String firstName;
    private String lastName;

    /* getters and setters ... */
}

My Spring RestController:

@RestController
@RequestMapping("/api/people")
public class PersonController {

    @Autowired
    private PersonRepository people;

    @Transactional
    @RequestMapping(path = "/{personId}", method = RequestMethod.PUT)
    public ResponseEntity<?> update(
            @PathVariable String personId,
            @RequestBody PersonDTO dto) {

        // get the entity by ID
        Person p = people.findOne(personId); // we assume it exists

        // update ONLY entity attributes that have been defined
        if(/* dto.getFirstName is defined */)
            p.setFirstName = dto.getFirstName;

        if(/* dto.getLastName is defined */)
            p.setLastName = dto.getLastName;

        return ResponseEntity.ok(p);
    }
}

Request with missing property

{"firstName": "John"}

Expected behaviour: update 100 (leave 101 unchanged).

Request with null property

{"firstName": "John", "lastName": null}

Expected behaviour: update firstName="John" and set lastName=null.

I cannot distinguish between these two cases, sincelastName in the DTO is always set to null by Jackson.

Note: I know that REST best practices (RFC 6902) recommend using PATCH instead of PUT for partial updates, but in my particular scenario I need to use PUT.