How to distinguish between JSON null value and default java null in java pojo

Question

I am understanding below JSON

{ "id": "1", "value": "some value" }

{ "id": "2", "value": null }

{ "id": "3" }

To hold the above JSON data I have java class :

    class myClass
    {
       private String id;
       private String value;
       public String getId() {
        return id;
       }
       public void setId(String id) {
        this.id = id;
       }
       public String getValue() {
        return value;
       }
       public void setValue(String value) {
        this.value = value;
      }
}

In above example : id 2 and id 3 both have value as NULL

In java it is very difficult to identify the distinguish between passed null value in JSON and other field which are not passed hence java pojo has its default value as NULL.

For me value of id 2 & id 3 should not be same.

Is it possible to distinguish between provided JSON NULL & and default NULL in java?

You need to specify which json library you're using to unmarshall the json. It's likely not possible after unmarshalling. The question is: why do you want this? If you really need this, it's most likely a better idea to change your approach. — Erwin Bolwidt, Aug 24 '18 at 05:42
Why do you need to distinguish between those situations? Personally, I usually configure my JSON serialization to just not write null values. Null is null. It should not matter whether that is 'default' or explicit set to null. — Mark Rotteveel, Aug 24 '18 at 07:29

kapex · Accepted Answer · 2021-03-18T08:56:18.917

It's better not to distinguish null and missing values at all for the very reason you have to ask this question: This distinction is not easy and may not be possible with all JSON libraries, especially when automatically converting to POJO.

It depends on the JSON library if it works, but you can try to add an additional flag to your POJOs, that is set when the setter is called. For example like this:

class MyClass {

    private String value;
    private boolean isSetValue;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
        isSetValue = true;
    }

    public boolean getIsSetValue() {
        return isSetValue;
    }
}

If you plan to serialize this POJO, you need to exclude getIsSetValue from serialization and you likely need to modify the JSON serializer to use getIsSetValue to decide whether to write a null value or no value at all.

By adding @JsonIgnore on isSetValue i am able to ignore serialization/de-serialization. — sudheer mishra, Aug 27 '18 at 09:57

jokster · Answer 2 · 2018-08-24T07:25:54.047

0

The only possible solution I can imagine: Track calls to the setter and ensure that the jSON library you use sets explicit null values.

Would give the following results:

id 1: setter called with "some value"
id 2: setter called with null
id 3: setter NOT called

Alternative after Torbens comment: If the library doesn't use getters/setters, you might be able to use a special value for "unset" and mask that in the getter to return null instead.

edited Aug 24 '18 at 07:25

answered Aug 24 '18 at 05:59

jokster

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5
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2

Doesn't work if the JSON library sets the field directly using reflection (like some of them do). – Torben Aug 24 '18 at 06:54
@Torben you're right of course. Perhaps there are config options to make them use getters/setters. – jokster Aug 24 '18 at 07:24

score 0 · Answer 3 · answered Aug 24 '18 at 07:53

Use this code:

JSONObject jsonObject = new JSONObject(response);
Sting s =  jsonObject.optString("value");

for this { "id": "2", "value": null } it will return null as JAVA Sting

jsonObject.optString("value"); // s="null"

and for this { "id": "3" } it will return null value

jsonObject.optString("value"); // s==null

How to distinguish between JSON null value and default java null in java pojo

3 Answers3