bean validation not working with kotlin (JSR 380)

Question

so first of all i could not think of a better title for this question so i\'m open for changes.

I am trying to validate a bean using the bean validation mechanism (JSR-3

Answer 1

Try adding ? like this:

data class User(
    @field:Valid
    @field:NotEmpty
    var roles: MutableSet<@NotNull Role?> = HashSet()
)

Then the kotlin compiler should realise roles could be null, and it might honor the validation, I know little about JSR380 so i'm just guessing though.

Answer 2

Answer (Kotlin 1.3.70)

Make sure to compile the kotlin code with jvm target 1.8 or greater and enable this feature by providing the -Xemit-jvm-type-annotations when compiling.

For Spring Boot projects you only have to do the following changes (tested with Spring Boot 2.3.3 and Kotlin 1.4.0):

1. In your pom set the following property:

   <properties>
       <java.version>11</java.version>
       <kotlin.version>1.4.0</kotlin.version>
   </properties>
   

2. Add <arg>-Xemit-jvm-type-annotations</arg> to the kotlin-maven-plugin:

   <build>
       <plugin>
           <artifactId>kotlin-maven-plugin</artifactId>
           <groupId>org.jetbrains.kotlin</groupId>
           <configuration>
               <args>
                   <arg>-Xjsr305=strict</arg>
                   <arg>-Xemit-jvm-type-annotations</arg>
               </args>
               <compilerPlugins>
                   <plugin>spring</plugin>
               </compilerPlugins>
           </configuration>
           <dependencies>
               <dependency>
                   <groupId>org.jetbrains.kotlin</groupId>
                   <artifactId>kotlin-maven-allopen</artifactId>
                   <version>${kotlin.version}</version>
               </dependency>
           </dependencies>
       </plugin>
   </build>
   

Sample Project

---

Workaround (pre Kotlin 1.3.70)

Rafal G. already pointed out that we could use a custom validator as a workaround. So here's some code:

The Annotation:

import javax.validation.Constraint
import javax.validation.Payload
import kotlin.annotation.AnnotationTarget.ANNOTATION_CLASS
import kotlin.annotation.AnnotationTarget.CONSTRUCTOR
import kotlin.annotation.AnnotationTarget.FIELD
import kotlin.annotation.AnnotationTarget.FUNCTION
import kotlin.annotation.AnnotationTarget.TYPE_PARAMETER
import kotlin.annotation.AnnotationTarget.VALUE_PARAMETER
import kotlin.reflect.KClass

@MustBeDocumented
@Constraint(validatedBy = [NoNullElementsValidator::class])
@Target(allowedTargets = [FUNCTION, FIELD, ANNOTATION_CLASS, CONSTRUCTOR, VALUE_PARAMETER, TYPE_PARAMETER])
@Retention(AnnotationRetention.RUNTIME)
annotation class NoNullElements(
    val message: String = "must not contain null elements",
    val groups: Array<KClass<out Any>> = [],
    val payload: Array<KClass<out Payload>> = []
)

The ConstraintValidator:

import javax.validation.ConstraintValidator
import javax.validation.ConstraintValidatorContext

class NoNullElementsValidator : ConstraintValidator<NoNullElements, Collection<Any>> {
    override fun isValid(value: Collection<Any>?, context: ConstraintValidatorContext): Boolean {
        // null values are valid
        if (value == null) {
            return true
        }
        return value.stream().noneMatch { it == null }
    }
}

And finally the updated User class:

data class User(
    @field:NotEmpty
    @field:NoNullElements
    var roles: MutableSet<Role> = HashSet()
)

---

Altough validation works now, the resulting ConstrainViolation is slightly different. For example the elementType and propertyPath differs as you can see below.

Java:

Kotlin:

Source is available here: https://github.com/DarkAtra/jsr380-kotlin-issue/tree/workaround

Thanks again for your help Rafal G.