Add support for Kotlin value classes in BeanUtils

This commit adds support for Kotlin value classes annotated
with @JvmInline to BeanUtils#findPrimaryConstructor.

This is only the first step, more refinements are expected
to be needed to achieve a comprehensive support of Kotlin
values classes in Spring Framework.

Closes gh-28638

Author: sdeleuze

Diff for: spring-beans/src/main/java/org/springframework/beans/BeanUtils.java

@@ -31,8 +31,11 @@
 import java.util.Set;
 
 import kotlin.jvm.JvmClassMappingKt;
+import kotlin.jvm.JvmInline;
+import kotlin.reflect.KClass;
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
+import kotlin.reflect.full.KAnnotatedElements;
 import kotlin.reflect.full.KClasses;
 import kotlin.reflect.jvm.KCallablesJvm;
 import kotlin.reflect.jvm.ReflectJvmMapping;
@@ -835,13 +838,22 @@ private static class KotlinDelegate {
 		 * @see <a href="https://kotlinlang.org/docs/reference/classes.html#constructors">
 		 * https://kotlinlang.org/docs/reference/classes.html#constructors</a>
 		 */
+		@SuppressWarnings("unchecked")
 		@Nullable
 		public static <T> Constructor<T> findPrimaryConstructor(Class<T> clazz) {
 			try {
-				KFunction<T> primaryCtor = KClasses.getPrimaryConstructor(JvmClassMappingKt.getKotlinClass(clazz));
+				KClass<T> kClass = JvmClassMappingKt.getKotlinClass(clazz);
+				KFunction<T> primaryCtor = KClasses.getPrimaryConstructor(kClass);
 				if (primaryCtor == null) {
 					return null;
 				}
+				if (kClass.isValue() && !KAnnotatedElements
+						.findAnnotations(kClass, JvmClassMappingKt.getKotlinClass(JvmInline.class)).isEmpty()) {
+					Constructor<?>[] constructors = clazz.getDeclaredConstructors();
+					Assert.state(constructors.length == 1,
+							"Kotlin value classes annotated with @JvmInline are expected to have a single JVM constructor");
+					return (Constructor<T>) constructors[0];
+				}
 				Constructor<T> constructor = ReflectJvmMapping.getJavaConstructor(primaryCtor);
 				if (constructor == null) {
 					throw new IllegalStateException(

Diff for: spring-beans/src/test/kotlin/org/springframework/beans/BeanUtilsKotlinTests.kt

@@ -90,6 +90,45 @@ class BeanUtilsKotlinTests {
 		BeanUtils.instantiateClass(PrivateClass::class.java.getDeclaredConstructor())
 	}
 
+	@Test
+	fun `Instantiate value class`() {
+		val constructor = BeanUtils.findPrimaryConstructor(ValueClass::class.java)!!
+		assertThat(constructor).isNotNull()
+		val value = "Hello value class!"
+		val instance = BeanUtils.instantiateClass(constructor, value)
+		assertThat(instance).isEqualTo(ValueClass(value))
+	}
+
+	@Test
+	fun `Instantiate value class with multiple constructors`() {
+		val constructor = BeanUtils.findPrimaryConstructor(ValueClassWithMultipleConstructors::class.java)!!
+		assertThat(constructor).isNotNull()
+		val value = "Hello value class!"
+		val instance = BeanUtils.instantiateClass(constructor, value)
+		assertThat(instance).isEqualTo(ValueClassWithMultipleConstructors(value))
+	}
+
+	@Test
+	fun `Instantiate class with value class parameter`() {
+		val constructor = BeanUtils.findPrimaryConstructor(OneConstructorWithValueClass::class.java)!!
+		assertThat(constructor).isNotNull()
+		val value = ValueClass("Hello value class!")
+		val instance = BeanUtils.instantiateClass(constructor, value)
+		assertThat(instance).isEqualTo(OneConstructorWithValueClass(value))
+	}
+
+	@Test
+	fun `Instantiate class with nullable value class parameter`() {
+		val constructor = BeanUtils.findPrimaryConstructor(OneConstructorWithNullableValueClass::class.java)!!
+		assertThat(constructor).isNotNull()
+		val value = ValueClass("Hello value class!")
+		var instance = BeanUtils.instantiateClass(constructor, value)
+		assertThat(instance).isEqualTo(OneConstructorWithNullableValueClass(value))
+		instance = BeanUtils.instantiateClass(constructor, null)
+		assertThat(instance).isEqualTo(OneConstructorWithNullableValueClass(null))
+	}
+
+
 	class Foo(val param1: String, val param2: Int)
 
 	class Bar(val param1: String, val param2: Int = 12)
@@ -128,4 +167,17 @@ class BeanUtilsKotlinTests {
 
 	private class PrivateClass
 
+	@JvmInline
+	value class ValueClass(private val value: String)
+
+	@JvmInline
+	value class ValueClassWithMultipleConstructors(private val value: String) {
+		constructor() : this("Fail")
+		constructor(part1: String, part2: String) : this("Fail")
+	}
+
+	data class OneConstructorWithValueClass(val value: ValueClass)
+
+	data class OneConstructorWithNullableValueClass(val value: ValueClass?)
+
 }