Constructor not executed when deserializing JSON

Question

I have the following JSON data

{
    "objectName": "CustomObject__c",
    "fields": [
        {
            "name": "Field1__c",
            "extName": "ABC",
            "master": "System1"
        },
        {
            "name": "Field2__c",
            "extName": "DEF",
            "master": "System2"
        }
    ]
}

which I wanted to deserialize and added some more information in the target Apex object via its constructor:

public class JsonTest {

    public static void test() {
        String jsonData = '....';
        Schema s = (Schema)JSON.deserialize(jsonData, Schema.class);
        System.debug(s); // custom = null
    }

    public class Schema {
        String objectName;
        List<FieldItem> fields;
        String custom;
            
        public Schema() {
            custom = 'test';
        }
    }

    public class FieldItem {
        String name;
        String extName;
        String master;
    }
}

When I run my code JsonTest.test() via anonymous apex, the custom property is still null (although it has been set via the class constructor). Why is it so? Thanks.

Answer 1

It will remain null as you are attempting to deserialise JSON into the inner object that is either an empty object or an object without a value for the custom key.

If you call new Schema() (suggest a name change away from platform Schema) you will see custom=test. If you construct valid JSON to support the inner Schema object properties you will see the correct value also i.e String jsonData = '{"custom":"testing"}';

See JSON.deserialize won't call default constructor for more.