如何使用Jackson反序列化空类型JSON字段 [英] How to deserialize null type JSON fields with Jackson

问题描述

我正在开发PATCH API.如果将字段作为JSON null值发送，则需要将其另存为null.但是我无法区分它们是作为null发送还是从不发送.

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;

我有Double，Integer，Date等字段.当它们确实以null的形式发送时，我可以将它们反序列化为Double.NAN，Integer.MIN_VALUE，以了解它们是否以null的形式发送.但是，当字段为null时，反序列化器将不起作用.当然，可以选择发送"-1"或一个不可能的值来定义null，但是我不喜欢这种方法.我将不得不同意所有类型的客户.在这种情况下最好的方法是什么?

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?

推荐答案

在这种情况下，应定义属性设置为预定义的undefined like数据的POJO类.例如，对于Integer属性，如果从业务角度来看不允许使用负数，则它可以是-1.然后，将JSON反序列化为POJO时，设置为null的属性将覆盖默认值，并且您将知道它已发送.应该有3个选项:

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. JSON中的常规值-正确反序列化为POJO中的值
JSON中的
2. null值-在POJO中反序列化为null
3. 不可用对key-value-POJO中的默认值不会被覆盖.

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.

下面的示例在版本2.9.9中使用Jackson:

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 +
                '}';
    }
}

上面的代码显示:

{"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}

当然，您应该选择默认值，以最大程度地减少客户端偶然将您的API发送为undefined的值发送给客户端的冲突可能性.

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.

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