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

Question

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:

@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 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.

Rosenthal

Answer 1

Actually,if ignore the validation,you can solve your problem like this.

   public class BusDto {
       private Map<String, Object> changedAttrs = new HashMap<>();

       /* getter and setter */
   }

• First, write a super class for your dto,like BusDto.
• Second, change your dto to extend the super class, and change the dto's set method,to put the attribute name and value to the changedAttrs(beacause the spring would invoke the set when the attribute has value no matter null or not null).
• Third,traversal the map.

Turning

Answer 2

Another option is to use java.util.Optional.

import com.fasterxml.jackson.annotation.JsonInclude;
import java.util.Optional;

@JsonInclude(JsonInclude.Include.NON_NULL)
private class PersonDTO {
    private Optional<String> firstName;
    private Optional<String> lastName;
    /* getters and setters ... */
}

If firstName is not set, the value is null, and would be ignored by the @JsonInclude annotation. Otherwise, if implicitly set in the request object, firstName would not be null, but firstName.get() would be. I found this browsing the solution @laffuste linked to a little lower down in a different comment (garretwilson's initial comment saying it didn't work turns out to work).

You can also map the DTO to the Entity with Jackson's ObjectMapper, and it will ignore properties that were not passed in the request object:

import com.fasterxml.jackson.databind.ObjectMapper;

class PersonController {
    // ...
    @Autowired
    ObjectMapper objectMapper

    @Transactional
    @RequestMapping(path = "/{personId}", method = RequestMethod.PUT)
    public ResponseEntity<?> update(
            @PathVariable String personId,
            @RequestBody PersonDTO dto
    ) {
        Person p = people.findOne(personId);
        objectMapper.updateValue(p, dto);
        personRepository.save(p);
        // return ...
    }
}

Validating a DTO using java.util.Optional is a little different as well. It's documented here, but took me a while to find:

// ...
import javax.validation.constraints.NotNull;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.Pattern;
// ...
private class PersonDTO {
    private Optional<@NotNull String> firstName;
    private Optional<@NotBlank @Pattern(regexp = "...") String> lastName;
    /* getters and setters ... */
}

In this case, firstName may not be set at all, but if set, may not be set to null if PersonDTO is validated.

//...
import javax.validation.Valid;
//...
public ResponseEntity<?> update(
        @PathVariable String personId,
        @RequestBody @Valid PersonDTO dto
) {
    // ...
}

Also might be worth mentioning the use of Optional seems to be highly debated, and as of writing Lombok's maintainer(s) won't support it (see this question for example). This means using lombok.Data/lombok.Setter on a class with Optional fields with constraints doesn't work (it attempts to create setters with the constraints intact), so using @Setter/@Data causes an exception to be thrown as both the setter and the member variable have constraints set. It also seems better form to write the Setter without an Optional parameter, for example:

//...
import lombok.Getter;
//...
@Getter
private class PersonDTO {
    private Optional<@NotNull String> firstName;
    private Optional<@NotBlank @Pattern(regexp = "...") String> lastName;

    public void setFirstName(String firstName) {
        this.firstName = Optional.ofNullable(firstName);
    }
    // etc...
}

Pham

is it possible to cascade @Valid ? (cause it doesn't seem to work) I mean If I head Controller -> @Valid PersonDTO --inside--> @Valid private Optional<List<AnimalDTO>> animals; -> {@NotNull private String animalName;} – Jecoa

My only concern about this apprach is that Option API clearly prohibits its use. "A variable whose type is Optional should never itself be null; it should always point to an Optional instance." docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/… – Estreat

Answer 3

There is a better option, that does not involve changing your DTO's or to customize your setters.

It involves letting Jackson merge data with an existing data object, as follows:

MyData existingData = ...
ObjectReader readerForUpdating = objectMapper.readerForUpdating(existingData);

MyData mergedData = readerForUpdating.readValue(newData);    

Any fields not present in newData will not overwrite data in existingData, but if a field is present it will be overwritten, even if it contains null.

Demo code:

    ObjectMapper objectMapper = new ObjectMapper();
    MyDTO dto = new MyDTO();

    dto.setText("text");
    dto.setAddress("address");
    dto.setCity("city");

    String json = "{\"text\": \"patched text\", \"city\": null}";

    ObjectReader readerForUpdating = objectMapper.readerForUpdating(dto);

    MyDTO merged = readerForUpdating.readValue(json);

Results in {"text": "patched text", "address": "address", "city": null}

Note that text and city were patched (city is now null) and that address was left alone.

In a Spring Rest Controller you will need to get the original JSON data instead of having Spring deserialize it in order to do this. So change your endpoint like this:

@Autowired ObjectMapper objectMapper;

@RequestMapping(path = "/{personId}", method = RequestMethod.PATCH)
public ResponseEntity<?> update(
        @PathVariable String personId,
        @RequestBody JsonNode jsonNode) {

   RequestDTO existingData = getExistingDataFromSomewhere();

   ObjectReader readerForUpdating = objectMapper.readerForUpdating(existingData);
   
   RequestDTO mergedData = readerForUpdating.readValue(jsonNode);

   ...
}

Macaque

why did you use a plain string as json? I think you break the example here. You should answer with an entity decoded already. – Reinstate

@Reinstate I really don't understand what you are asking here -- to demo how it works, I used a string, what is the issue? See the last example with the Spring controller, there is no json string there. – Macaque

Decent answer. The only issue here is, you have to manually validate the mergedData DTO and then you might get unwanted errors if validation fails because of wrong existing data. – Afloat

@Michał Jabłoński In any case, I think you would need to revalidate the merged entity after applying a partial update, as there may be cross-field validations of modified AND unmodified fields. – Norenenorfleet

Answer 4

Use boolean flags as jackson's author recommends.

class PersonDTO {
    private String firstName;
    private boolean isFirstNameDirty;

    public void setFirstName(String firstName){
        this.firstName = firstName;
        this.isFirstNameDirty = true;
    }

    public String getFirstName() {
        return firstName;
    }

    public boolean hasFirstName() {
        return isFirstNameDirty;
    }
}

Paniagua

This solution works but I would argue this is a failing of Jackson and results in a huge amount of code bloat... good reason to not use it. Looks like GSON is a good alternative: github.com/google/gson/blob/master/… – Frumpish

@Frumpish and how does GSON solve this then? – Macaque

Answer 5

I have tried to solve the same problem. I found it quite easy to use JsonNode as the DTOs. This way you only get what is submitted.

You will need to write a MergeService yourself that does the actual work, similar to the BeanWrapper. I haven't found an existing framework that can do exactly what is needed. (If you use only Json requests you might be able to use Jacksons readForUpdate method.)

We actually use another node type as we need the same functionality from "standard form submits" and other service calls. Additionally the modifications should be applied within a transaction inside something called EntityService.

This MergeService will unfortunately become quite complex, as you will need to handle properties, lists, sets and maps yourself :)

The most problematic piece for me was to distinguish between changes within an element of a list/set and modifications or replacements of lists/sets.

And also validation will not be easy as you need to validate some properties against another model (the JPA entities in my case)

EDIT - Some mapping code (pseudo-code):

class SomeController { 
   @RequestMapping(value = { "/{id}" }, method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE)
    @ResponseBody
    public void save(
            @PathVariable("id") final Integer id,
            @RequestBody final JsonNode modifications) {
        modifierService.applyModifications(someEntityLoadedById, modifications);
    }
}

class ModifierService {

    public void applyModifications(Object updateObj, JsonNode node)
            throws Exception {

        BeanWrapperImpl bw = new BeanWrapperImpl(updateObj);
        Iterator<String> fieldNames = node.fieldNames();

        while (fieldNames.hasNext()) {
            String fieldName = fieldNames.next();
            Object valueToBeUpdated = node.get(fieldName);
            Class<?> propertyType = bw.getPropertyType(fieldName);
            if (propertyType == null) {
               if (!ignoreUnkown) {
                    throw new IllegalArgumentException("Unkown field " + fieldName + " on type " + bw.getWrappedClass());
                }
            } else if (Map.class.isAssignableFrom(propertyType)) {
                    handleMap(bw, fieldName, valueToBeUpdated, ModificationType.MODIFY, createdObjects);
            } else if (Collection.class.isAssignableFrom(propertyType)) {
                    handleCollection(bw, fieldName, valueToBeUpdated, ModificationType.MODIFY, createdObjects);
            } else {
                    handleObject(bw, fieldName, valueToBeUpdated, propertyType, createdObjects);
            }
        }
    }
}

Homeroom

Your answer seems interesting, but I haven't got it very well. Could you please enhance it with a pertinent example? – Rosenthal

Which part, jsonnode requestmapping, the mergeservice or the validation? – Homeroom

It is not clear how to use JsonNode within my update method in the Controller class. Should my DTOs inherit from JsonNode? – Rosenthal

Added some code to show you the idea. Hope it helps :) – Homeroom

Thanks Martin, your solution is clever, but it puts me into troubles due to the lack of automatic JSR-303 validation on DTOs. From this point of view, @demon 's proposal, even if less elaborate, is bets suited. However, I'll consider your solution for other occasions :) – Rosenthal

Answer 6

Maybe too late for an answer, but you could:

• By default, don't unset 'null' values. Provide an explicit list via query params what fields you want to unset. In such a way you can still send JSON that corresponds to your entity and have flexibility to unset fields when you need.

• Depending on your use case, some endpoints may explicitly treat all null values as unset operations. A little bit dangerous for patching, but in some circumstances might be an option.

Inlaid

Answer 7

Another solution would be to imperatively deserialize the request body. By doing it, you will be able to collect user provided fields and selectively validate them.

So your DTO might look like this:

public class CatDto {
    @NotBlank
    private String name;

    @Min(0)
    @Max(100)
    private int laziness;

    @Max(3)
    private int purringVolume;
}

And your controller can be something like this:

@RestController
@RequestMapping("/api/cats")
@io.swagger.v3.oas.annotations.parameters.RequestBody(
        content = @Content(schema = @Schema(implementation = CatDto.class)))
// ^^ this passes your CatDto model to swagger (you must use springdoc to get it to work!)
public class CatController {
    @Autowired
    SmartValidator validator; // we'll use this to validate our request

    @PatchMapping(path = "/{id}", consumes = "application/json")
    public ResponseEntity<String> updateCat(
            @PathVariable String id,
            @RequestBody Map<String, Object> body
            // ^^ no Valid annotation, no declarative DTO binding here!
    ) throws MethodArgumentNotValidException {
        CatDto catDto = new CatDto();
        WebDataBinder binder = new WebDataBinder(catDto);
        BindingResult bindingResult = binder.getBindingResult();
        List<String> patchFields = new ArrayList<>();

        binder.bind(new MutablePropertyValues(body));
        // ^^ imperatively bind to DTO
        body.forEach((k, v) -> {
            patchFields.add(k);
            // ^^ collect user provided fields if you need
            validator.validateValue(CatDto.class, k, v, bindingResult);
            // ^^ imperatively validate user input
        });
        if (bindingResult.hasErrors()) {
            throw new MethodArgumentNotValidException(null, bindingResult);
            // ^^ this can be handled by your regular exception handler
        }
        // Here you can do normal stuff with your catDto.
        // Map it to cat model, send to cat service, whatever.
        return ResponseEntity.ok("cat updated");
    }
}

No need for Optional's, no extra dependencies, your normal validation just works, your swagger looks good. The only problem is, you don't get proper merge patch on nested objects, but in many use cases that's not even required.

Marque

Answer 8

Probably to late but following code works for me to distinguish between null and not provided values

if(dto.getIban() == null){
  log.info("Iban value is not provided");
}else if(dto.getIban().orElse(null) == null){
  log.info("Iban is provided and has null value");
}else{
  log.info("Iban value is : " + dto.getIban().get());
}

Catechin

You are using optionals here – Polley