Coercion shouldn't be necessary for Enums specifying an empty string

[joaocanaverde-blue]

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Describe the bug

Trying to deserialise:

{ "example": "" }

Results in:

Cannot coerce empty String ("") to `YesOrNoOrEmpty` value (but could if coercion was enabled using `CoercionConfig`)

Despite an empty string ("") being listed as a value in the enum.

enum class YesOrNoOrEmpty(val value: kotlin.String) {
    @JsonProperty(value = "")
    EMPTY(""),

    @JsonProperty(value = "yes")
    YES("yes"),

    @JsonProperty(value = "no")
    NO("no");
}

I'm trying to deserialise historical values, so this is a value I have to support. Ideally I'd still like to deserialise to an enum rather than just into a string, and would like to avoid coercion if possible.

Given that I'm specifying the empty string explicitly with @JsonProperty(value = ""), I'm assuming encountering this exception is a bug rather than intended behaviour?

Version Information

2.18.2

Reproduction

Read:

{ "example": "" }

into an Example object like:

data class Example(
    val example: YesOrNo
)

enum class YesOrNo(val value: kotlin.String) {
    @JsonProperty(value = "")
    EMPTY(""),

    @JsonProperty(value = "yes")
    YES("yes"),

    @JsonProperty(value = "no")
    NO("no");
}

Expected behavior

Deserialisation works.

Additional context

A workaround does exist (as the exception message suggests):

import com.fasterxml.jackson.databind.cfg.CoercionAction
import com.fasterxml.jackson.databind.cfg.CoercionInputShape
import com.fasterxml.jackson.databind.type.LogicalType
import org.springframework.boot.autoconfigure.jackson.Jackson2ObjectMapperBuilderCustomizer
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder

@Configuration
class JacksonConfig {
    @Bean
    fun jsonCustomizer(): Jackson2ObjectMapperBuilderCustomizer {
        return Jackson2ObjectMapperBuilderCustomizer { builder: Jackson2ObjectMapperBuilder ->
            builder.postConfigurer { objectMapper ->
                objectMapper.coercionConfigFor(LogicalType.Enum)
                    .setCoercion(CoercionInputShape.EmptyString, CoercionAction.TryConvert)
            }

        }
    }
}

[cowtowncoder]

@joaocanaverde-blue I agree, this sounds like a bug; edge case handled incorrectly.

[cowtowncoder]

Hmmmh. Interesting... the problem here is actually handling of @JsonProperty("") -- empty String is the default, "unspecified" value. So actual name/id will be "EMPTY" because @JsonProperty is essentially ignored.
Part of the problem is that these two are the same

@JsonProperty("")
@JsonProperty

due to JsonProperty having:

String value() default "";

Will need to think of how to tackle this.

[cowtowncoder]

Ok: this is fixable, and I will do so.

However, this does change semantics of following case a bit:

public enum MyEnum {
   @JsonProperty // or with ("")
   DEFAULT
}

which would formerly (before fix) use id DEFAULT -- similar to regular POJO properties, where empty/missing value simply means "use whatever default name this property has" (Field name, modified name of Method etc).

Given this, I think I cannot change this in a patch but fix it for 2.19.0 (minor version).
So in case someone somewhere was relying on this behavior (possible accidentally) at least patch won't break that. At the same time, I do not think this is Major-version-only kind of change.

[joaocanaverde-blue]

Amazing, thanks @cowtowncoder!

[cowtowncoder]

@joaocanaverde-blue Thank you for your help here, good to have this edge case covered.