How to deserialize null type JSON fields with Jackson

问题

I'm developing a PATCH API. If the fields are sent as JSON null value I need to save them as null. However I can't distinguish if they're sent as null or never sent.

{
      "max_amount": null
}

Double maxAmount;

I have Double, Integer, Date etc. fields. I can deserialize them to Double.NAN, Integer.MIN_VALUE when they're really sent as null to understand if they're sent as null. But Deserializers don't work when the field is null. Of course it's an option to send "-1" or an impossible value to define null but I didn't like this approach. I will have to agree with the clients for all types. What's the best approach in this case?

回答1:

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.

来源：https://stackoverflow.com/questions/57911836/how-to-deserialize-null-type-json-fields-with-jackson