"JSON parse error: Cannot construct instance of (although at least one Creator exists): cannot deserialize from Object value - SpringBoot

Question

I am using SpringBoot with Mongo database and I am trying to save embedded documents into database.

I have this model:

Profile.java

@Data
@Document
public class Profile {

    public final City city;
    public final String imageId;

    public Profile(City city,
                   String imageId) {
        this.city = city;
        this.imageId = imageId;
    }

    @Override
    public String toString() {
        return "Profile{" +
                ", city=" + city +
                ", imageId='" + imageId + '\'' +
                '}';
    }

    private static boolean atLeast(int numChars, String s) {
        if (s == null) {
            return false;
        }
        var str = s.strip();
        return str.length() >= numChars;
    }


    public static ProfileBuilder builder() {
        return new ProfileBuilder();
    }

    public static final class ProfileBuilder {
        public City city;
        public String imageId;

        private ProfileBuilder() {
        }


        public ProfileBuilder withCity(City city) {
            this.city = city;
            return this;
        }

        public ProfileBuilder withImageId(String imageId) {
            this.imageId = imageId;
            return this;
        }

        public Profile build(){
            return new Profile(city, imageId);
        }
    }
}

City.java

public class City {

    public final String name;

    public City(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "City{" +
                ", name='" + name + '\'' +
                '}';
    }
}

ProfileController.java

 @RequestMapping( method = RequestMethod.POST)
    public Profile addUser(@RequestBody Profile profile) {
        return profileService.addProfile(profile);
    }

and with postman I am sending this JSON

{
 "city":{
   "name":"Atena"
  },
   "imageId" : "Doe",
  }
}

But I am getting following error:

"JSON parse error: Cannot construct instance of `domain.City` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator);"

Please add the code of your REST Controller where you receive this JSON — Nikolai Shevchenko
– Nikolai Shevchenko, Commented Oct 12, 2021 at 9:34

Salah dine Maham · Accepted Answer · 2022-07-25 11:08:00Z

11

In classes with only one attribute, to deserialize an object Json need a nos argument constructor from that class.

In your class city you need a nos arg constructor add this to your class you need:

 public  City () {}

answered Jul 25, 2022 at 11:08

Salah dine Maham

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1 Comment

rabong Over a year ago

Could you please tell me why no-argument constructor is required for classes with only one property, but not for classes with more than one field? Even after reading this article(stackoverflow.com/questions/64080983/…), I still don't understand.

Nikolai Shevchenko · Accepted Answer · 2021-10-12 14:09:46Z

9

There are at least two solutions.

Add @JsonCreator to constructor and @JsonProperty to its arguments (to instruct Jackson how to substitute JSON items into constructor in proper order)

class Profile {
    ...
    @JsonCreator
    public Profile(@JsonProperty("city") City city, 
                   @JsonProperty("imageId") String imageId) {
        this.city = city;
        this.imageId = imageId;
    }
    ...
}

(+ same for City class)

Unfinal class properties and provide default no-arg constructor (along with existing all-arg constructor).

class Profile {

    public City city;
    public String imageId;

    public Profile() {
    }

    public Profile(City city, String imageId) {
        this.city = city;
        this.imageId = imageId;
    }
}

(+ same for City class)

Test

class Test {
    public static void main(String[] args) throws JsonProcessingException {
        String json = "{\"city\":{\"name\":\"Atena\"},\"imageId\":\"Doe\"}";
        Profile p = new ObjectMapper().readValue(json, Profile.class);
        System.out.println(p);
    }
}

Output:

Profile{, city=City{, name='Atena'}, imageId='Doe'}

edited Oct 12, 2021 at 14:09

answered Oct 12, 2021 at 13:46

Nikolai Shevchenko

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7 Comments

user9347049 Over a year ago

I am having same error for first example you provided and for second it has to be final. So I have to find way of making it work with final

Nikolai Shevchenko Over a year ago

It works for me. I've updated my answer with unit test. PS: Make sure that you use valid JSON, because one that you've shown in your Question is invalid

user9347049 Over a year ago

Yes, I wrote test for it as well and it works fine, but when I am sending it with postman as JSON it's not working. Hmm maybe you are right my JSON is wrong. Can you please help with working one?

Nikolai Shevchenko Over a year ago

@user9347049 in my test snippet {"city":{"name":"Atena"},"imageId":"Doe"} is a valid JSON

user9347049 Over a year ago

Thank you for your help! :) it works now

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