objectMapper.writeValueAsString does not properly serialize Kotlin data class variables that begin with "is"

[skwirskj]

Describe the bug

When calling the objectMapper's writeValueAsString and passing a Kotlin data class to the function, any of the variables that begin with "is" such as "isDayTime" and that is a string type, do not retain its values after serialization. This was observed after using the serialized body in a GET request to a private API where the API threw an error due to missing required parameters. This does work though if the variable is a boolean.

To Reproduce

1. Create a Kotlin data class with at least one variable of String type, starting with "is"
2. Create an instance of the data class and an instance of the Jackson Object Mapper
3. Serialize the data and pass to an API. API should come back with some error if the value is required.

Expected behavior

The objectMapper would serialize the data class as expected and would retain the value, regardless of the naming convention of a variable.

Versions
Kotlin: 1.7.10
Jackson-module-kotlin: 2.13.3
Jackson-databind:

Additional context
Add any other context about the problem here.

[jgorgen]

Reproduced in Jackson-module-kotlin: 2.12.1

import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper

        class TestMapperClass(temp: String){
            val isDebug = "no"
            val a = "a"
            val b = "b"
            val c = "c"
        }
        val objectMapper = jacksonObjectMapper()
        val result = objectMapper.writeValueAsString(TestMapperClass("foo"))
        System.out.println(result)

produces

{"a":"a","b":"b","c":"c"}

instead of the expected

{"isDebug":"no","a":"a","b":"b","c":"c"}

[cowtowncoder]

@jgorgen 2.12.1 is an old version, so at very least should test with 2.12.7 (latest from 2.12).
But ideally with latest release 2.13.4.

I assume reproduction is still valid but this is an important consideration in many other cases.

[zigzago]

reproduced with 2.13.4 ;)

[Richie94]

I had a look at it and it is filtered out here https://github.com/FasterXML/jackson-databind/blob/2.14/src/main/java/com/fasterxml/jackson/databind/introspect/DefaultAccessorNamingStrategy.java#L73

Seems like the getters starting with is are only allowed with boolean type.

I am not sure, on what is the desired behaviour for this...

[cowtowncoder]

Thank you @Richie94. Yes, that would be problematic; for original Java Beans there is this limitation -- but Kotlin does not impose it so ideally Jackson would not either, in case of Kotlin classes.
The challenge is that of how to configure this part of behavior: unfortunately it is deep within code and does not get configuration/context passed.

[cowtowncoder]

Another thought here: does Kotlin module provide custom AccessorNamingStrategy (added in Jackson 2.12)?
If it doesn't (and I think it doesn't), it could provide one to customize handling for Kotlin types specifically.
This is done by registering AccessorNamingStrategy.Provider which could apply separate strategy just to Kotlin types.
But that could implement whatever logic Kotlin specifically defines.

This could be complementary to:

FasterXML/jackson-databind#3609

which would allow exposing non-boolean-returning-is-getters more generally.

The main reason why I think it'd be good to do this Kotlin-specific override is if it allows solving other Kotlin-naming issues (wrt Java POJO naming Jackson assumes).
General allow of all "is-setters" can be useful for others tho so it need not be either-or case.

[Richie94]

I also think your suggestion doing it for all would be the better way, because you could also build these combinations with java code.

[cowtowncoder]

The problem with doing it for all is that semantics of JAva POJOs, Scala Objects and Kotlin Objects vary to some degree. So the applies-to-all solution will never work exactly right by default for everything.

That said, the basic logic for whether strict "Boolean-returning" is needed for is-getter is something that makes enough sense. It's just that if anything more fine-grained is needed, that should go via AccessorNamingStrategy (or some other pluggable mechanism).

[k163377]

This issue will be fixed by #630 and therefore closed as a duplicate.