FasterXML / FasterXML/jackson-modules-java8
`Optional<JsonNode>` deserialization from "absent" value does not work in the expected way
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Mô tả
Example:
```java
public record MyRecord(
Optional myField
) {
}
```
When deserialized from: `{}`
Expected:
`myField.isPresent() == false`
Actual:
`myField.isPresent() == true`
This is because `myField` gets set to an `Optional` of a `NullNode`
After spending some time looking into the source code of both the `jackson-databind` and the `jackson-datatype-jdk8` libraries, the problem seems to lie in the `OptionalDeserializer` (or higher).
During deserialization, when a property is missing, the `PropertyValueBuffer::_findMissing` method is called and in it, this piece of code is called:
https://github.com/FasterXML/jackson-databind/blob/0fe97e0d69b7d5362907b094d5b979bc2216dc4a/src/main/java/com/fasterxml/jackson/databind/deser/impl/PropertyValueBuffer.java#L203
```java
// Third: NullValueProvider? (22-Sep-2019, [databind#2458])
// 08-Aug-2021, tatu: consider [databind#3214]; not null but "absent" value...
Object absentValue = prop.getNullValueProvider().getAbsentValue(_context);
if (absentValue != null) {
return absentValue;
}
```
The `OptionalDeserializer` is not overriding its inherited `getAbsentValue` method to return `Optional.ofNullable(_valueDeserializer.getAbsentValue(ctxt));` (or similar).
Due to the lack of the overriding, the inherited `getAbsentValue` method actually calls `getNullValue` instead as can be seen here:
https://github.com/FasterXML/jackson-databind/blob/0fe97e0d69b7d5362907b094d5b979bc2216dc4a/src/main/java/com/fasterxml/jackson/databind/JsonDeserializer.java#L349
```java
@Override
public Object getAbsentValue(DeserializationContext ctxt) throws JsonMappingException {
return getNullValue(ctxt);
}
```
In the case of a `JsonNode`, the `JsonNodeDeserializer` is used. This deserializer overrides the `getNullValue` method to return a `NullNode`.
https://github.com/FasterXML/jackson-databind/blob/0fe97e0d69b7d5362907b094d5b979bc2216dc4a/src/main/java/com/fasterxml/jackson/databind/deser/std/JsonNodeDeserializer.java#L73
```java
@Override
public JsonNode getNullValue(DeserializationContext ctxt) {
return ctxt.getNodeFactory().nullNode();
}
```
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