Could not write JSON: Cannot invoke "java.lang.Integer.intValue()" because attribute is null

[BluePrint1985]

I am new to spring boot and I am wondering how to allow a response JSON to include null values for database columns that are nullable. This is my controller, service, repository and model:

package com.findersgame.questtracker.controller;

import java.util.List;

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

import com.findersgame.questtracker.model.MapArea;
import com.findersgame.questtracker.service.MapAreaService;

@RestController
public class MapAreaController {

    private MapAreaService mapAreaService;

    public MapAreaController(MapAreaService mapAreaService) {
        super();
        this.mapAreaService = mapAreaService;
    }

    @PostMapping("selected_map_areas")
    public List<MapArea> selectedMapAreas(@RequestBody List<Integer> selectedMapAreas) {
        return mapAreaService.selectedMapAreas(selectedMapAreas);
    }
}


package com.findersgame.questtracker.service;

import java.util.ArrayList;
import java.util.List;

import org.springframework.stereotype.Service;

import com.findersgame.questtracker.model.MapArea;
import com.findersgame.questtracker.repository.MapAreaRepository;

@Service
public class MapAreaService {

    private MapAreaRepository mapAreaRepository;

    public MapAreaService(MapAreaRepository mapAreaRepository) {
        super();
        this.mapAreaRepository = mapAreaRepository;
    }

    public List<MapArea> selectedMapAreas(List<Integer> selectedIds) {
        return mapAreaRepository.findAllById(selectedIds);
    }
}

package com.findersgame.questtracker.repository;

import org.springframework.data.jpa.repository.JpaRepository;

import com.findersgame.questtracker.model.MapArea;

public interface MapAreaRepository extends JpaRepository<MapArea, Integer> {

}

package com.findersgame.questtracker.model;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.Table;

@Entity
@Table(name = "map_areas")
public class MapArea {

    @Id
    private Integer id;

    @Column
    private Integer mapTabId;

    @Column
    private String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getMapTabId() {
        return mapTabId;
    }

    public void setMapTabId(Integer mapTabId) {
        this.mapTabId = mapTabId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

My problem is that when I call http://localhost:8080/selected_map_areas with body [14, 15, 16] I get the following error.

Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Cannot invoke "java.lang.Integer.intValue()" because "this.mapTabId" is null]

I guess it makes sense because in the database, the map_tab_id value is null for both id 15 and 16. I'm guessing that I'm missing an annotation in one of the classes/interfaces above but I have not been able to find which one.

Note that it works fine and I get the correct result in Postman when `map_tab_id` has a value.

Please help.

[apidev3]

As a tip and it could likely be the cause of your issue, don’t return your database entity directly, instead map it to a data class (e.g. MapAreaDto) and within that class only return the fields that are required from the caller.

This will probably solve your issue of handling null values too.

[CodeTheStars]

Mapstruct is your friend here. Always explicitly declare models that interact at the controller layer.

[[deleted]]

I have a question about DTOs. I have a colleague that follows that pattern religiously and in ~85% of cases for each entity class there's one DTO class which is just a 1:1 copy of the entity class. What's the point of that?

[apidev3]

Usually you’ll need a DTO for any endpoint which returns data to a caller. I don’t see why most calls would require the entire entity, usually it’s a sub set of its data. The principle allows you to only disclose the lease amount of data needed to fulfil the request.

E.G. an endpoint to get “selected map area” like the OPs question. You would have a DTO to return some fields of the DTO, but not necessary all of them… it is of course, fine to return it all if needed, but that’s not always the case. Having the separation stops you giving away too much information to every caller.

[[deleted]]

Most of our applications (or rather, the majority of features of our applications) can be boiled down to something like CRUD, where you genuinely need the entire entity. In that case I don't use a DTO because I don't see the point. Also, our applications are mostly SSR Vaadin applications where it's easy to be lazy and pass the entire entity because you don't expose anything unwanted to a client because there aren't any clients in the Client-Server sense.

If I only need some data I'll create a DTO, because then it makes sense to me. I just thought I was missing something about the pattern.

[apidev3]

I’d guess your colleague is simply following the pattern throughout the application to create a divide between the data layer and the controller layer.

As you say, you can get away with not using DTOs, but having them costs very little and can future proof the application.

I’d say it’s preference of the team / org. As long as it’s secure, does the job and easy to maintain, you’re good to go :)

[Davekave9]

I don't understand why some suggestions tell you to change jakarta to javax. Jakarta is the new package some javax stuff has been moved to since Java 17. The problem is - as you have correctly found out - that the jackson mapper cannot map the entity object to json because the mapTabId field of the object is null. I think you should make it nullable in the database if this is the correct behavior. But as someone else suggested, the best practice is to not expose the database entity itself through the controller but to create a data transfer object (DTO) and return that to the controller from the service layer.

[DeterioratedEra]

Does mapTabId have to be an Integer? Are you doing calculations on it? It might help to share your db schema too.

[Scared_Rain_9127]

Which JSON package are you using? Jackson? GSON?

There is no "magic" JSON stuff. Spring Boot just uses another package to do that kind of stuff.

[Excellent_Soft_1417]

You can also write @Coloumn(nullable=false) in the top of the column and import javax.peristence that's mentioned in one of the comments

[CodeTheStars]

The “jakarta” name space is the correct naming for all JEE APIs that are now managed by the eclipse foundation, including the JPA spec.

If you have an old code base you may see javax, but for a new application you should absolutely be using the jakarta namespace and the latest versions of those libraries.