Keep null fields of a POJO as keys in map java

Question

I have a POJO and using object mapper to convert it into a map. I want the map to contain all fields of the POJO in it's entryset even if their value is null. So that entry might be null in the map I am creating.

Map<String, Object> oldDetails = objectMapper.convertValue(oldFields, Map.class);

I've tried using below snippet but that didn't help me.

objectMapper.setDefaultPropertyInclusion(JsonInclude.Value.construct(JsonInclude.Include.ALWAYS, JsonInclude.Include.ALWAYS));

Any other idea that I can use here?

My POJO:

@JsonInclude(NON_NULL)
@JsonIgnoreProperties(ignoreUnknown = true)
public class BaseFields {

    @JsonProperty("number") protected String number;
    @JsonProperty("name") protected String name;
    @JsonProperty("dob") protected String dob;

}

There are other classes inheriting from BaseFields such as.

@JsonInclude(NON_NULL)
@JsonIgnoreProperties(ignoreUnknown = true)
public class CustomerBaseFields extends BaseFields {
    @JsonProperty("email") protected String email;         
}

Now there is method that converts an instance of ChangedBaseFields into a map.

ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, true);
        objectMapper.enable(SerializationFeature.INDENT_OUTPUT);
        objectMapper.setDefaultPropertyInclusion(JsonInclude.Value.construct(JsonInclude.Include.ALWAYS, JsonInclude.Include.ALWAYS));
        Map<String, Object> oldDetails = objectMapper.convertValue(baseFields, Map.class);

But say if a field like email is null in object, I'm not getting a key email in my map.

Answer 1

Here's simple code demonstrating that your code in fact works without any additional configuration with default ObjectMapper.

package java17test;

import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.Map;

public class Test {
    public static void main(String[] args) {
        var objectMapper = new ObjectMapper();
        
        var oldFields = new TestClass();
        oldFields.setFirstName("first name");

        Map<?, ?> oldDetails = objectMapper.convertValue(oldFields, Map.class);
        oldDetails.forEach((key, value) -> System.out.println(key + "=" + value));
    }

    static class TestClass {
        private String firstName;
        private String lastName;

        public String getFirstName() {
            return firstName;
        }

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

        public String getLastName() {
            return lastName;
        }

        public void setLastName(String lastName) {
            this.lastName = lastName;
        }
    }
}

Output:

firstName=first name
lastName=null