How to distinguish between null and not provided values for partial updates in Spring Rest Controller

I'm trying to distinguish between null values and not provided values when partially updating an entity with PUT request method in Spring Rest Controller.

Consider the following entity, as an example:

@Entityprivate 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):

@Repositorypublic 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 firstName= "John" (leave lastName 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.