DATACMNS-1197 - Resolve Kotlin interface properties for nullability inspection.

We now resolve Kotlin interface properties to inspect these for nullability. Kotlin-reflect does not resolve interface property accessors yet so we need to handle this aspect ourselves.

Related ticket: https://youtrack.jetbrains.com/issue/KT-20768.
Original pull request: #254.

src/main/java/org/springframework/data/util/ReflectionUtils.java

@@ -15,7 +15,12 @@
  */
 package org.springframework.data.util;
 
+import kotlin.jvm.JvmClassMappingKt;
+import kotlin.reflect.KCallable;
+import kotlin.reflect.KClass;
 import kotlin.reflect.KFunction;
+import kotlin.reflect.KMutableProperty;
+import kotlin.reflect.KProperty;
 import kotlin.reflect.KType;
 import kotlin.reflect.jvm.ReflectJvmMapping;
 import kotlin.reflect.jvm.internal.impl.load.kotlin.header.KotlinClassHeader;
@@ -402,11 +407,37 @@ public class ReflectionUtils {
 		}
 
 		if (isSupportedKotlinClass(parameter.getDeclaringClass())) {
+			return KotlinReflectionUtils.isNullable(parameter);
+		}
 
-			KFunction<?> kotlinFunction = ReflectJvmMapping.getKotlinFunction(parameter.getMethod());
+		return !parameter.getParameterType().isPrimitive();
+	}
+
+	/**
+	 * Reflection utility methods specific to Kotlin reflection.
+	 */
+	static class KotlinReflectionUtils {
+
+		/**
+		 * Returns {@literal} whether the given {@link MethodParameter} is nullable. Its declaring method can reference a
+		 * Kotlin function, property or interface property.
+		 *
+		 * @return {@literal true} if {@link MethodParameter} is nullable.
+		 * @since 2.0.1
+		 */
+		static boolean isNullable(MethodParameter parameter) {
+
+			Method method = parameter.getMethod();
+			KFunction<?> kotlinFunction = ReflectJvmMapping.getKotlinFunction(method);
 
 			if (kotlinFunction == null) {
-				throw new IllegalArgumentException(String.format("Cannot resolve %s to a Kotlin function!", parameter));
+
+				// Fallback to own lookup because there's no public Kotlin API for that kind of lookup until
+				// https://youtrack.jetbrains.com/issue/KT-20768 gets resolved.
+				Optional<? extends KFunction> first = findKFunction(method);
+
+				kotlinFunction = first.orElseThrow(
+						() -> new IllegalArgumentException(String.format("Cannot resolve %s to a Kotlin function!", parameter)));
 			}
 
 			KType type = parameter.getParameterIndex() == -1 ? kotlinFunction.getReturnType()
@@ -415,6 +446,46 @@ public class ReflectionUtils {
 			return type.isMarkedNullable();
 		}
 
-		return !parameter.getParameterType().isPrimitive();
+		/**
+		 * Lookup a {@link Method} to a {@link KFunction}.
+		 * 
+		 * @param method the JVM {@link Method} to look up.
+		 * @return {@link Optional} wrapping a possibly existing {@link KFunction}.
+		 */
+		private static Optional<? extends KFunction> findKFunction(Method method) {
+
+			KClass<?> kotlinClass = JvmClassMappingKt.getKotlinClass(method.getDeclaringClass());
+
+			return kotlinClass.getMembers() //
+					.stream() //
+					.flatMap(KotlinReflectionUtils::toKFunctionStream) //
+					.filter(it -> {
+
+						Method javaMethod = ReflectJvmMapping.getJavaMethod(it);
+						return javaMethod != null && javaMethod.equals(method);
+					}) //
+					.findFirst();
+		}
+
+		private static Stream<? extends KFunction> toKFunctionStream(KCallable<?> it) {
+
+			if (it instanceof KMutableProperty<?>) {
+
+				KMutableProperty property = (KMutableProperty<?>) it;
+				return Stream.of(property.getGetter(), property.getSetter());
+			}
+
+			if (it instanceof KProperty<?>) {
+
+				KProperty<?> property = (KProperty<?>) it;
+				return Stream.of(property.getGetter());
+			}
+
+			if (it instanceof KFunction<?>) {
+				return Stream.of((KFunction<?>) it);
+			}
+
+			return Stream.empty();
+		}
 	}
 }