How to deserialize null type JSON fields with Jackson

In cases like this you should define POJO class with properties set to predefined undefined like data. For example, for Integer property if negative numbers are not allowed from business point of view, it could be -1. Then, when JSON is deserialised to POJO properties set to null override default values and you will know it was sent. There should be 3 options:

1. Regular value in JSON - properly deserialised to a value in POJO
2. null value in JSON - deserialised to null in POJO
3. Not available pair key-value - default value in POJO is not overridden.

Below example uses Jackson in version 2.9.9:

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;

import java.time.LocalDateTime;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new JavaTimeModule());

        String[] jsons = {
                "{\"number\":1,\"date\":\"2019-01-01T22:23:11\",\"max_amount\":4.9}",
                "{\"number\":null,\"date\":null,\"max_amount\":null}",
                "{}",
                "{\"number\":1}",
                "{\"date\":\"2019-01-01T22:23:11\"}",
                "{\"max_amount\":4.9}",
                "{\"number\":1,\"date\":null,\"max_amount\":null}"
        };
        for (String json : jsons) {
            System.out.println(json + " => " + mapper.readValue(json, Pojo.class));
        }
    }
}

class Pojo {

    private static final LocalDateTime NULL_DATE = LocalDateTime.of(1900, 1, 1, 12, 13);

    @JsonProperty("max_amount")
    private Double maxAmount = Double.MIN_VALUE;
    private Integer number = Integer.MIN_VALUE;

    @JsonFormat(shape = JsonFormat.Shape.STRING)
    private LocalDateTime date = NULL_DATE;

    public Double getMaxAmount() {
        return maxAmount;
    }

    public void setMaxAmount(Double maxAmount) {
        this.maxAmount = maxAmount;
    }

    public Integer getNumber() {
        return number;
    }

    public void setNumber(Integer number) {
        this.number = number;
    }

    public LocalDateTime getDate() {
        return date;
    }

    public void setDate(LocalDateTime date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "{" +
                "number=" + number +
                ", date=" + date +
                ", maxAmount=" + maxAmount +
                '}';
    }
}

Above code prints:

{"number":1,"date":"2019-01-01T22:23:11","max_amount":4.9} => {number=1, date=2019-01-01T22:23:11, maxAmount=4.9}
{"number":null,"date":null,"max_amount":null} => {number=null, date=null, maxAmount=null}
{} => {number=-2147483648, date=1900-01-01T12:13, maxAmount=4.9E-324}
{"number":1} => {number=1, date=1900-01-01T12:13, maxAmount=4.9E-324}
{"date":"2019-01-01T22:23:11"} => {number=-2147483648, date=2019-01-01T22:23:11, maxAmount=4.9E-324}
{"max_amount":4.9} => {number=-2147483648, date=1900-01-01T12:13, maxAmount=4.9}
{"number":1,"date":null,"max_amount":null} => {number=1, date=null, maxAmount=null}

Of course, you should pick default values in the way it will minimise possibility of collision that client by accident send value treated as undefined by your API.