Implementing a Java Annotation in Kotlin

Question

I am implementing the dropwizard example application in Kotlin and have an issue implementing the DateRequiredFeature. The Java code code is as follows:

    @Provider
    public class DateRequiredFeature implements DynamicFeature {
        @Override
        public void configure(ResourceInfo resourceInfo, FeatureContext context) {
            if (resourceInfo.getResourceMethod().getAnnotation(DateRequired.class) != null) {
                context.register(DateNotSpecifiedFilter.class);
            }
        }
    }

with the annotation defined as:

    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.METHOD)
    public @interface DateRequired {}

For the Kotlin side I have for the feature:

    @Provider
    class DateRequiredFeature : DynamicFeature(){
        override fun configure(resourceInfo: ResourceInfo, context: FeatureContext) {
            if (resourceInfo.resourceMethod.getAnnotation(DateRequired::class.java) != null) {
                context.register(DateNotSpecifiedFilter::class.java)
            }
        }
    }

but is is not clear how to implement the corresponding DateRequired annotation so that getAnnotation() is valid. The (Java) signature for getAnnotation() is

public <T extends Annotation> T getAnnotation(Class<T> annotationClass)

Note

Looks like an IntelliJ bug, I need to explicitly import the annotation although it is in the same package:

...
import DateRequired

@Provider
class DateRequiredFeature : DynamicFeature {
    override fun configure(resourceInfo: ResourceInfo, context: FeatureContext) {
        if (resourceInfo.resourceMethod.getAnnotation(DateRequired::class.java) != null) {
            context.register(DateNotSpecifiedFilter::class.java)
        }
    }
}

then all is well using standard Kotlin annotation annotation class DateRequired

I'm not sure what you want to do. Do you want to recode the annotation to Kotlin? — Mibac
– Mibac, Commented Jun 25, 2017 at 21:01
I would like to implement DateRequired annotation in Kotlin so that the call resourceInfo.resourceMethod.getAnnotation(DateRequired..) works. — David Soroko
– David Soroko, Commented Jun 25, 2017 at 21:05

Mibac · Accepted Answer · 2017-06-25 22:16:43Z

4

Implementing a annotation in Kotlin is simpler than in Java. The default RetentionPolicy in Kotlin is RUNTIME. The FUNCTION,PROPERTY_GETTER and PROPERTY_SETTER are mapped to Java's ElementType.METHOD, for example:

import kotlin.annotation.AnnotationTarget.*

@Target(FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER)
annotation class DateRequired;

Secondly, I have also discovered you used a Java platform class ResourceInfo which returns a java.reflect.Method instance. It might return null resulting in NullPointerException in Kotlin. You must use the NPE-lovers operator !! before calling the getAnnotation method:

resourceInfo.resourceMethod!!.getAnnotation(DateRequired::class.java)
//                         ^--- force to call

Last, the DynamicFeature is an interface. implements an interface in kotlin you must remove the parentheses ():

@Provider
class DateRequiredFeature : DynamicFeature{...}

You can see more details about annotations here.

edited Jun 25, 2017 at 22:16

Mibac

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answered Jun 25, 2017 at 21:04

holi-java

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4 Comments

David Soroko Over a year ago

Yes, that is what I tried. I am unable to refer to the class of DateRequired in the getAnnotation() call.

holi-java Over a year ago

@DavidSoroko it has reported any error? indeed, the kotlin annotation class is transformed to java @interface like as yours. maybe your logic wrong?

David Soroko Over a year ago

see my note, looks like an IntelliJ issue.

holi-java Over a year ago

@DavidSoroko hi, how about it after using the !! operator?

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