How to correctly configure JsonMapper to deserialize `Optional.empty()` instead of `null` in case of missing property

[wendigo]

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Describe the bug

Optional handling differs between Jackson 2.x and 3.0 despite the information that Jdk8 module was pulled in to core.

Version Information

3.0

Reproduction

I have a record class with an Optional field. When I use JsonMapper from Jackson 3.0, I'm unable to configure it in a way that will inject Optional.empty if either value is null or absent in the JSON.

Works in Jackson 2.x:

public class TestOptional
{
    public static void main(String[] args) throws JsonProcessingException {
        JsonMapper.Builder builder = JsonMapper.builder();
        builder.addModule(new Jdk8Module());
        builder.defaultPropertyInclusion(JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS));

        Person person = new Person("Alice", Optional.empty());
        ObjectMapper mapper = builder.build();

        System.out.println(mapper.writeValueAsString(person));
        System.out.println(mapper.readValue(mapper.writeValueAsString(person), Person.class));
        System.out.println(mapper.readValue("{\"name\": \"Alice\"}}", Person.class));
        System.out.println(mapper.readValue("{\"name\": \"Alice\", \"nickname\": null}}", Person.class));
    }

    record Person(String name, Optional<String> nickname)
    {
        public Person {
            requireNonNull(name, "name is null");
            requireNonNull(nickname, "nickname is null");
        }
    }
}

Doesn't work in Jackson 3:

public class TestOptional
{
    public static void main(String[] args)
    {
        JsonMapper.Builder builder = JsonMapper.builder();
        builder.changeDefaultPropertyInclusion(_ -> JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS));

        Person person = new Person("Alice", Optional.empty());
        ObjectMapper mapper = builder.build();

        System.out.println(mapper.writeValueAsString(person));
        System.out.println(mapper.readValue(mapper.writeValueAsString(person), Person.class));
        System.out.println(mapper.readValue("{\"name\": \"Alice\"}}", Person.class));
        System.out.println(mapper.readValue("{\"name\": \"Alice\", \"nickname\": null}}", Person.class));
    }

    record Person(String name, Optional<String> nickname)
    {
        public Person {
            requireNonNull(name, "name is null");
            requireNonNull(nickname, "nickname is null");
        }
    }
}

Result:

{"name":"Alice"}
Exception in thread "main" tools.jackson.databind.exc.ValueInstantiationException: Cannot construct instance of `io.airlift.json.TestOptional$Person`, problem: nickname is null
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); byte offset: #UNKNOWN]
	at tools.jackson.databind.exc.ValueInstantiationException.from(ValueInstantiationException.java:44)
	at tools.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:2076)
	at tools.jackson.databind.deser.std.StdValueInstantiator.wrapAsDatabindException(StdValueInstantiator.java:581)
	at tools.jackson.databind.deser.std.StdValueInstantiator.rewrapCtorProblem(StdValueInstantiator.java:602)
	at tools.jackson.databind.deser.std.StdValueInstantiator.createFromObjectWith(StdValueInstantiator.java:289)
	at tools.jackson.databind.deser.ValueInstantiator.createFromObjectWith(ValueInstantiator.java:270)
	at tools.jackson.databind.deser.bean.PropertyBasedCreator.build(PropertyBasedCreator.java:252)
	at tools.jackson.databind.deser.bean.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:697)
	at tools.jackson.databind.deser.bean.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1417)
	at tools.jackson.databind.deser.bean.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:480)
	at tools.jackson.databind.deser.bean.BeanDeserializer.deserialize(BeanDeserializer.java:200)
	at tools.jackson.databind.deser.DeserializationContextExt.readRootValue(DeserializationContextExt.java:265)
	at tools.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2610)
	at tools.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1522)
	at io.airlift.json.TestOptional.main(TestOptional.java:22)
Caused by: java.lang.NullPointerException: nickname is null
	at java.base/java.util.Objects.requireNonNull(Objects.java:246)
	at io.airlift.json.TestOptional$Person.<init>(TestOptional.java:31)
	at java.base/java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:735)
	at tools.jackson.databind.introspect.AnnotatedConstructor.call(AnnotatedConstructor.java:113)
	at tools.jackson.databind.deser.std.StdValueInstantiator.createFromObjectWith(StdValueInstantiator.java:287)
	... 10 more

Process finished with exit code 1

Expected behavior

Optionals should work :)

Additional context

No response

[pjfanning]

@wendigo I created a unit test but it seems to pass - #5336 - could you review it?

[JooHyukKim]

Some findings...

1. Seems like Jdk8OptionalDeserializer is returning absent value as null.
2. In our case is actually "absent".
3. Our shared configuration indeed should only include NON_ABSENT values?
   builder.defaultPropertyInclusion(JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS));

I am trying hard to understand,

1. Should NON_ABSENT be ALWAYS? -- To adhere to documentation
2. Or should the behavior be preserved?

[wendigo]

@JooHyukKim I've added some test cases to #5336 which works under 2.x but fails on 3.0. The existing behavior doesn't make sense at all to me (this is common for a POJO that has an Optional field to be deserialized from a JSON that doesn't contain that optional field at all - because what's the point in sending "field": null

[JooHyukKim]

Okay so it turns out this check in 2.x is removed in 3.x

[JooHyukKim]

because what's the point in sending "field": null

Yes, I agree with you, what's the point sending null value?
But contradictoriliy, your configuration says only "ONLY include sending null value" value (as far as I understand).

        builder.defaultPropertyInclusion(JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS));

This is why I am re-reading through my findings in above comment

[wendigo]

        /**
         * Value that indicates that properties are included unless their value
         * is:
         *<ul>
         *  <li>null</li>
         *  <li>"absent" value of a referential type (like Java 8 `Optional`, or
         *     {@link java.util.concurrent.atomic.AtomicReference}); that is, something
         *     that would not deference to a non-null value.
         * </ul>
         * This option is mostly used to work with "Optional"s (Java 8, Guava).
         *
         * @since 2.6
         */
        NON_ABSENT,

[wendigo]

Which I read as "emit a field if the Optional has a value"

[JooHyukKim]

Yes, but it doesn't here "{\"name\": \"Alice\"}}?
Some examples in the JavaDoc would've been useful here actually.

[JooHyukKim]

Seems like this something has to do with #3601. Heading there.

[wendigo]

    record RecordWithOptionalsOnly(Optional<String> optional)
    {
        public RecordWithOptionalsOnly
        {
            requireNonNull(optional, "optional is null");
        }
    }

    record RecordWithOptionalField(RecordWithOptionalsOnly record)
    {
        public RecordWithOptionalField
        {
            requireNonNull(record, "record is null");
        }
    }

    @Test
    void deserializeNestedOptional() throws Exception
    {
        JsonMapper mapper = JsonMapper.builder()
                .changeDefaultPropertyInclusion(_ -> JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS))
                .build();

        assertEquals(mapper.writeValueAsString(new RecordWithOptionalField(new RecordWithOptionalsOnly(Optional.empty()))), a2q("{'record':{}}"));
        mapper.readValue(mapper.writeValueAsString(new RecordWithOptionalField(new RecordWithOptionalsOnly(Optional.empty()))), RecordWithOptionalField.class);
        mapper.readValue(a2q("{'record':{}}"), RecordWithOptionalField.class);
    }

That doesn't even round trip. It correctly serializes this POJO to {"record": {}} but it fails to deserialize it back:

tools.jackson.databind.exc.ValueInstantiationException: Cannot construct instance of `io.airlift.log.RecordWithOptionalTest$RecordWithOptionalsOnly`, problem: optional is null
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); byte offset: #UNKNOWN] (through reference chain: io.airlift.log.RecordWithOptionalTest$RecordWithOptionalField["record"])

[JooHyukKim]

I understand that your previous behavior is failing, but we should be working towards what right and designed behavior.
If I understand correctly, I see two complications here....

First, configuration issue.
I am not 100% sure if your configuration is correct?

JsonMapper mapper = JsonMapper.builder()
                .changeDefaultPropertyInclusion(_ -> JsonInclude.Value.construct(JsonInclude.Include.NON_ABSENT, JsonInclude.Include.ALWAYS))
                .build();

.. says include only NON_ABSENT values.

Second, regardless of confiuration, even using ALWAYS, OptionalDeserializer would return null because it is hard-coded, like below.

[wendigo]

Yes, it is correct. We've used that in Jackson 2.x and we don't want Optional.empty() fields to be serialized (Optional.empty() was treated as absent)

[JooHyukKim]

So JsonInclude is used correctly, then./

Re-reading through #3601, seems like we COULD change behavior if needed.
But need to solve the complexity of handling explicit-null vs absent value.