Mandatory but nullable fields with Jackson

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I am implementing a detector - deserializer for JSON files using Jackson.

Those JSON files can contain null values. However, for the detection phase, I'd like jackson to differentiate whether the field is present but null, or not present at all.

Example: I have this Vehicle Java class:

    class Vehicle {

        // mandatory fields
        private String id;
        private String brand;
        private String model;

        // optional fields
        private String color;
        private double length;
        private double width;

    }

I want these JSONs to be successfully parsed

    {
      "id": 123456,
      "brand": "Ford",
      "model": null, // field is present (null, but present)

      "color": "white",
      "length": 410,
      "width": 230,
    }

    {
      "id": 123456,
      "brand": "Ford",
      "model": null, // field is present (null, but present)

      "color": "white", // missing optional fields, but they are optional
    }

    {
      "id": 123456,
      "brand": "Ford",
      "model": "FordModel",

      "color": "white",
      "length": 410,
      "width": 230,
      "height": 210 // additional field not in Java class
    }

However, I want the following JSON not to be parsed at all (jackson throwing an exception is fine):

    {
      "id": 123456,
      "brand": "Ford", // missing the model field here

      "color": "white",
      "length": 410,
      "width": 230,
    }

I found a way to ignore null values (either by using Optional or with Jackson annotations), but the problem is that I can't find a way to differentiate between "the vehicle field being present but null" and "the vehicle field not being present at all".

What's the best way to achieve that?

jackson json java
1 Answers

Jackson can fail on missing json creator properties.

You can achieve in multiple ways based on the level of customization you want to achieve.

ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_MISSING_CREATOR_PROPERTIES, true);

Then add a @JsonCreator constructor that contains all the properties that should fail if the property is missing.

    public class Vehicle {
        private Long id;
        private String brand;
        private String model;

        private String color;
        private double length;
        private double width;

        @JsonCreator
        public Vehicle(@JsonProperty(value = "id", required = true) Long id,
                       @JsonProperty(value = "brand", required = true) String brand,
                       @JsonProperty(value = "model", required = true) String model) {
            this.id = id;
            this.brand = brand;
            this.model = model;
        }

    // getters and setters
    }
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