Unable to use Kotlin enums with @Validated beans [SPR-16931]

[spring-projects-issues]

Daniel Jones opened SPR-16931 and commented

I have a configuration properties kotlin class with a kotlin enum property. If the class is annotated with @Validated an exception occurs in hibernate-validator's ParameterMetadata class due to an IndexOutOfBoundsException.

This seems to happen due to the underlying ParameterNameDiscoverer, KotlinReflectionParameterNameDiscoverer.

I think this is due to the validator class expecting a 2-parameter constructor (name, ordinal) but KotlinReflectionParameterNameDiscoverer returns an empty array.

Example configuration properties class:

@ConfigurationProperties("my.prefix")
@Validated
data class MyProperties(        
        var enumProp: MyEnum = MyEnum.ONE

) {
    enum class MyEnum {
        ONE, TWO
    }
} 

---

Affects: 5.0.6

Referenced from: commits 2c5a1af, 73db208

Backported to: 5.0.11

[spring-projects-issues]

Sébastien Deleuze commented

Indeed, I have been able to reproduce this issue. Not sure what we do wrong here, it could be a Kotlin reflection bug or a conceptual difference between Java and Kotlin. Any thoughts Daniel Jones?

[spring-projects-issues]

Sébastien Deleuze commented

I have created KT-25165.

[spring-projects-issues]

Daniel Jones commented

Sorry, missed the notification from 6 days ago. I'm not really sure other than I remembering getting different results in the debugger if I used Kotlin's reflection API over Java's.

A quick go on the Kotlin REPL in IntelliJ gives me the following differences:

Using the Kotlin API, this returns an empty array

// toTypeArray().size == 1
MyEnum::class.constructors.toTypedArray()[0].parameters
>> []

Using the Java API, I get the expected 2-arg constructor

// declaredConstructors.size == 1
MyEnum::class.java.declaredConstructors[0]
>> protected my.package.MyEnum(java.lang.String,int)

So I think you're on the right track that the issue is with Kotlin.

[spring-projects-issues]

ruslanys commented

Sébastien Deleuze, 

I'm sorry for disturbing you, but I have the same issue and tried to solve it.

So, can we tweak KotlinReflectionParameterNameDiscoverer to solve this issue?

In my perspective, looks that all in our hands and we don't need to wait for something from Kotlin.

Looks, like if we add checking for enums, it will solve the issue.
I changed the method with the following and it works for me.

@Override
@Nullable
public String[] getParameterNames(Constructor<?> ctor) {
	if (ctor.getDeclaringClass().isEnum() || !KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
		return null;
	}

	try {
		KFunction<?> function = ReflectJvmMapping.getKotlinFunction(ctor);
		return (function != null ? getParameterNames(function.getParameters()) : null);
	}
	catch (UnsupportedOperationException ex) {
		return null;
	}
}

According to PrioritizedParameterNameDiscoverer.getParameterNames(Constructor<?> ctor) method, ParameterNameDiscoverer s will be applied until one of them won't return null.

So, when KotlinReflectionParameterNameDiscoverer returns null, then the following StandardReflectionParameterNameDiscoverer will be applied and return "$enum$name", "$enum$oridnal" parameters for default enum and this won't cause an error.

And looks like Alexander Udalov from JetBrains had clear clarification: for Kotlin language, Enum doesn't have undeclared parameters.

So, because of this, don't you think that for Kotlin Enums we should delegate parameters discovering to Java?
Can we use the suggested approach?

[spring-projects-issues]

ruslanys commented

I've added a PR-1985, please take a look.

[spring-projects-issues]

Sébastien Deleuze commented

Thanks I will have a look asap.

[spring-projects-issues]

Sébastien Deleuze commented

Fix in master and 5.0.x branches, I think indeed this is the correct approach. Thanks for your feedback and help on that issue.