Spring MVC PATCH method: partial updates

This could be very late, but for the sake of newbies and people who encounter the same problem, let me share you my own solution.

In my past projects, to make it simple, I just use the native java Map. It will capture all the new values including the null values that the client explicitly set to null. At this point, it will be easy to determine which java properties needs to be set as null, unlike when you use the same POJO as your domain model, you won't be able to distinguish which fields are set by the client to null and which are just not included in the update but by default will be null.

In addition, you have to require the http request to send the ID of the record you want to update, and do not include it on the patch data structure. What I did, is set the ID in the URL as path variable, and the patch data as a PATCH body.Then with the ID, you would get first the record via a domain model,then with the HashMap, you can just use a mapper service or utility to patch the changes to the concerned domain model.

Update

You can create a abstract superclass for your services with this kind of generic code, you must use Java Generics. This is just a segment of possible implementation, I hope you get the idea.Also it is better to use mapper framework such as Orika or Dozer.

public abstract class AbstractService<Entity extends BaseEntity, DTO extends BaseDto> {
    @Autowired
    private MapperService mapper;

    @Autowired
    private BaseRepo<Entity> repo;

    private Class<DTO> dtoClass;

    private Class<Entity> entityCLass;

    public AbstractService(){
       entityCLass = (Class<Entity>) SomeReflectionTool.getGenericParameter()[0];
       dtoClass = (Class<DTO>) SomeReflectionTool.getGenericParameter()[1];
    }

    public DTO patch(Long id, Map<String, Object> patchValues) {
        Entity entity = repo.get(id);
        DTO dto = mapper.map(entity, dtoClass);
        mapper.map(patchValues, dto);
        Entity updatedEntity = toEntity(dto);
        save(updatedEntity);
        return dto;
    }
}

The correct way to do this is the way proposed in JSON PATCH RFC 6902

A request example would be:

PATCH http://example.com/api/entity/1 HTTP/1.1
Content-Type: application/json-patch+json 

[
  { "op": "replace", "path": "aBoolean", "value": true }
]

After digging around a bit I found an acceptable solution using the same approach currently used by a Spring MVC DomainObjectReader see also: JsonPatchHandler

import org.springframework.data.rest.webmvc.mapping.Associations

@RepositoryRestController
public class BookCustomRepository {
    private final DomainObjectReader domainObjectReader;
    private final ObjectMapper mapper;

    private final BookRepository repository;


    @Autowired
    public BookCustomRepository(BookRepository bookRepository, 
                                ObjectMapper mapper,
                                PersistentEntities persistentEntities,
                                Associations associationLinks) {
        this.repository = bookRepository;
        this.mapper = mapper;
        this.domainObjectReader = new DomainObjectReader(persistentEntities, associationLinks);
    }


    @PatchMapping(value = "/book/{id}", consumes = {MediaType.APPLICATION_JSON_UTF8_VALUE, MediaType.APPLICATION_JSON_VALUE})
    public ResponseEntity<?> patch(@PathVariable String id, ServletServerHttpRequest request) throws IOException {

        Book entityToPatch = repository.findById(id).orElseThrow(ResourceNotFoundException::new);
        Book patched = domainObjectReader.read(request.getBody(), entityToPatch, mapper);
        repository.save(patched);

        return ResponseEntity.noContent().build();
    }

}

The whole point of PATCH is that you are not sending the entire entity representation, so I don't understand your comments about the empty string. You would have to handle some sort of simple JSON such as:

{ aBoolean: true }

and apply that to the specified resource. The idea is that what has been received is a diff of the desired resource state and the current resource state.