Differentiate a null value from an absent one in JsonDeserializer

Hi,

I have a class representing a field update:

public class OptionalUpdate<T> {

    private final Optional<T> value;

    private Present(Optional<T> value) {
        this.value = value;
    }

    public boolean isPresent() {
        return value != null;
    }

    public Optional<T> getValue() {
        return value;
    }

    public static <T> OptionalUpdate<T> unset() {
        return new OptionalUpdate<>(Optional.empty());
    }

    public static <T> OptionalUpdate<T> set(T value) {
        return new OptionalUpdate<>(Optional.of(value));
    }

    public static <T> OptionalUpdate<T> empty() {
        return new OptionalUpdate<>(null);
    }

}

and an entity update:

public class UserUpdate {

    private final OptionalUpdate<String> nickname;

    @JsonCreator
    public UserUpdate(OptionalUpdate<String> nickname) {
        this.nickname = nickname;
    }

    public OptionalUpdate<String> getNickname() {
        return nickname;
    }

}

I would like to handle three types of updates:

1. to set a nickname to “lolcontrol”: { "nickname": "lolcontrol" }
2. to unset a nickname: { "nickname": null }
3. not to update a nickname: { }

How relevant is it to add a new method to JsonDeserializer that can be overridden in the custom deserializer?

/**
 * Method called to determine value to be used for absent values.
 * Usually this is same as {@link #getNullValue} (which in turn
 * is usually simply Java null), but it can be overridden
 * for types.
 *<p>
 * Default implementation simple calls {@link #getNullValue} and
 * returns value.
 */
public T getAbsentValue() {
    return getNullValue();
}

[garretwilson]

This is indeed an interesting problem. @cowtowncoder maybe you can point me to where the discussion should take place.

This becomes a more important issue when patching a resource using JSON Merge Patch. If I want to update a FooBar resource on the server, I can send via HTTP PATCH:

{
  "foo": 123
}

This would update FooBar.foo on the server to 123. But what would it do with the existing FooBar.bar value, which might already be set to 789? If my Java DTO object assumes that bar here is null because it is absent, it would set FooBar.bar to null on the server. But in reality I want to recognize that bar wasn’t specified at all, so that FooBar.bar should remain unchanged.

Where is the best place to discuss this issue further? Thanks.

[garretwilson]

Ah, maybe I can use the behavior indicated in https://github.com/FasterXML/jackson-datatype-jdk8/issues/2 to set Optional<> fields defaulting to null, that are let as null if no value is specified in the JSON. I’ll try that.