Cannot deserialize Scala case class when JsonProperty annotation is used with var constructor parameter

[aimboden]

Hi all, we encountered the following regression while migrating our Scala 2.13.x projects to Jackson 3.

When deserializing JSON to Scala case classes with mutable var fields, Jackson now throws an IllegalStateException if a var is declared as a constructor parameter, but works correctly when declared in the class body.

So this is supported, for both serialization and de-serialization

    case class VarTestBody() {
        @JsonProperty("t") var test: Int = _
    }

But this fails

    case class VarTestConstructor(
        @JsonProperty("t") var test: Int
    )

with

java.lang.IllegalStateException: Conflicting/ambiguous property name definitions (implicit name 'test'): found multiple explicit names: [test, t], but also implicit accessor

[pjfanning]

Jackson 3 is a major release. It has behaviour changes so this is not necessarily a regression - that is, some changes are deliberate. I, for one, have not switched to Jackson 3 yet because it is in its early days and if you want stability, you stick with Jackson 2 - which is still maintained.
It is relatively unlikely that the change that leads to this change in behaviour is in jackson-module-scala - it may turn out to be in jackson-databind. Don't cross report for now but we may end up having to move this issue.
We probably don't have enough test cases for edge cases like this. Case classes are usually immutable. Your POC class would not be regarded as standard or expected Scala usage.
I will have a look but it could take a few days.

[pjfanning]

Jackson 3 has a mode where you can configure it to behave more like Jackson 2.

https://www.javadoc.io/doc/tools.jackson.core/jackson-databind/latest/tools.jackson.databind/tools/jackson/databind/json/JsonMapper.html#builderWithJackson2Defaults()

Something like:

val mapper = JsonMapper.builderWithJackson2Defaults().build()

[pjfanning]

I did some testing and it does seem like it's only when you have the @JsonProperty annotation that this happens.
It also appears that if you use JsonMapper.builderWithJackson2Defaults().build() to create the ObjectMapper that everything is ok.

@cowtowncoder the deserialization test exception is

[info] - should deserialize AnnotatedVarTestConstructor *** FAILED ***
[info]   java.lang.IllegalStateException: Conflicting/ambiguous property name definitions (implicit name 'test'): found multiple explicit names: [test, t], but also implicit accessor: [method tools.jackson.module.scala.deser.CaseClassDeserializerTest$AnnotatedVarTestConstructor#test()][visible=true,ignore=false,explicitName=false]
[info]   at tools.jackson.databind.introspect.POJOPropertyBuilder._explode(POJOPropertyBuilder.java:1317)
[info]   at tools.jackson.databind.introspect.POJOPropertyBuilder.explode(POJOPropertyBuilder.java:1296)
[info]   at tools.jackson.databind.introspect.POJOPropertiesCollector._renameProperties(POJOPropertiesCollector.java:1539)
[info]   at tools.jackson.databind.introspect.POJOPropertiesCollector.collectAll(POJOPropertiesCollector.java:410)
[info]   at tools.jackson.databind.introspect.POJOPropertiesCollector.getPotentialCreators(POJOPropertiesCollector.java:202)
[info]   at tools.jackson.databind.introspect.BasicBeanDescription.getPotentialCreators(BasicBeanDescription.java:336)
[info]   at tools.jackson.databind.deser.BasicDeserializerFactory._constructDefaultValueInstantiator(BasicDeserializerFactory.java:203)
[info]   at tools.jackson.databind.deser.BasicDeserializerFactory.findValueInstantiator(BasicDeserializerFactory.java:171)

Any idea what configuration in Jackson 3 makes this happen? When we use Jackson 2 config defaults in Jackson 3, it does not happen. This test is on 3.x branch.

[pjfanning]

So I was wrong about JsonMapper.builderWithJackson2Defaults().build() - it works but it only works because the ScalaModule is not registered.
Everything works on in jackson 2 but with jackson 3 and with the ScalaModule registered, something is throwing off the JsonProperty annotation support. The jackson-module-scala annotation introspector was not changed much in v3 but maybe jackson-databind expects v3 annotation introspectors to work differently.

[cowtowncoder]

I can't point to anything obvious straight away; although embedding of jackson-module-parameter-names could trigger the issue. If so, disabling MapperFeature.DETECT_PARAMETER_NAMES might work around it on short term & help figure out root cause.

[pjfanning]

Disabling MapperFeature.DETECT_PARAMETER_NAMES doesn't help at the moment. So far, the problem use case fails in Jackson 3 if ScalaModule is enabled regardless of feature settings.
If the JsonProperty annotation is removed or the variable is changed to an immutable val (as opposed to mutable var) then the test passes again.
When a var is used, then the class will have a setter method added (implicitly) as well as the constructor param. Presumably, the setter method may be missing the JsonProperty annotation. Later in the week, I'll try experimenting with the annotation introspector in jackson-module-scala to see if I can hack something that will fix the test.

[cowtowncoder]

Having multiple accessors for same logical property is fine, but there are sanity checks to try to see that there are not naming conflicts -- and in this case seemingly finding one. May be due to synthetic accessors added by Scala compiler or something.

[pjfanning]

I haven't gotten to the bottom of this yet but the issue seems to stem us deriving 2 explicit names for the test/t property.
In Jackson 2, we just get the 't' name but in Jackson 3, we get the 'test' name from a setter. The setter is created by Scala and this is not new but us deriving 'test' as the explicit name for this setter is the diff. I need to work out why this changed.
The derived setter method is called:
AnnotatedVarTestConstructor#test_$eq(int)

[pjfanning]

#782 seems to fix this

I'm not sure why this doesn't cause issues in Jackson 2 but removing the line in PR782 doesn't break any tests and fixes this use case so I think it's an ok fix.

[pjfanning]

The 782 fix will appear in 3.0.3 or 3.1.0 if that appears first.