Introduce KotlinDetector#hasSerializableAnnotation

This commit introduces a KotlinDetector#hasSerializableAnnotation
utility method designed to detect types annotated with
`@Serializable` at type or generics level.

See gh-35761

spring-core/src/main/java/org/springframework/core/KotlinDetector.java

@@ -38,6 +38,8 @@ public abstract class KotlinDetector {
 
 	private static final @Nullable Class<? extends Annotation> KOTLIN_JVM_INLINE;
 
+	private static final @Nullable Class<? extends Annotation> KOTLIN_SERIALIZABLE;
+
 	private static final @Nullable Class<?> KOTLIN_COROUTINE_CONTINUATION;
 
 	// For ConstantFieldFeature compliance, otherwise could be deduced from kotlinMetadata
@@ -49,6 +51,7 @@ public abstract class KotlinDetector {
 		ClassLoader classLoader = KotlinDetector.class.getClassLoader();
 		Class<?> metadata = null;
 		Class<?> jvmInline = null;
+		Class<?> serializable = null;
 		Class<?> coroutineContinuation = null;
 		try {
 			metadata = ClassUtils.forName("kotlin.Metadata", classLoader);
@@ -58,6 +61,12 @@ public abstract class KotlinDetector {
 			catch (ClassNotFoundException ex) {
 				// JVM inline support not available
 			}
+			try {
+				serializable = ClassUtils.forName("kotlinx.serialization.Serializable", classLoader);
+			}
+			catch (ClassNotFoundException ex) {
+				// Kotlin Serialization not available
+			}
 			try {
 				coroutineContinuation = ClassUtils.forName("kotlin.coroutines.Continuation", classLoader);
 			}
@@ -72,6 +81,7 @@ public abstract class KotlinDetector {
 		KOTLIN_PRESENT = (KOTLIN_METADATA != null);
 		KOTLIN_REFLECT_PRESENT = ClassUtils.isPresent("kotlin.reflect.full.KClasses", classLoader);
 		KOTLIN_JVM_INLINE = (Class<? extends Annotation>) jvmInline;
+		KOTLIN_SERIALIZABLE = (Class<? extends Annotation>) serializable;
 		KOTLIN_COROUTINE_CONTINUATION = coroutineContinuation;
 	}
 
@@ -125,4 +135,26 @@ public abstract class KotlinDetector {
 		return (KOTLIN_JVM_INLINE != null && clazz.getDeclaredAnnotation(KOTLIN_JVM_INLINE) != null);
 	}
 
+	/**
+	 * Determine whether the given {@code ResolvableType} is annotated with {@code @kotlinx.serialization.Serializable}
+	 * at type or generics level.
+	 * @since 7.0
+	 */
+	public static boolean hasSerializableAnnotation(ResolvableType type) {
+		Class<?> resolvedClass = type.resolve();
+		if (KOTLIN_SERIALIZABLE == null || resolvedClass == null) {
+			return false;
+		}
+		if (resolvedClass.isAnnotationPresent(KOTLIN_SERIALIZABLE)) {
+			return true;
+		}
+		@Nullable Class<?>[] resolvedGenerics = type.resolveGenerics();
+		for (Class<?> resolvedGeneric : resolvedGenerics) {
+			if (resolvedGeneric != null && resolvedGeneric.isAnnotationPresent(KOTLIN_SERIALIZABLE)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }

spring-core/src/test/kotlin/org/springframework/core/KotlinDetectorTests.kt

@@ -15,6 +15,7 @@
  */
 package org.springframework.core
 
+import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions
 import org.junit.jupiter.api.Test
 
@@ -46,7 +47,26 @@ class KotlinDetectorTests {
 		Assertions.assertThat(KotlinDetector.isInlineClass(KotlinDetectorTests::class.java)).isFalse()
 	}
 
+	@Test
+	fun hasSerializableAnnotation() {
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClass(WithoutSerializable::class.java))).isFalse()
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClass(WithSerializable::class.java))).isTrue()
+
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClassWithGenerics(List::class.java, WithoutSerializable::class.java))).isFalse()
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClassWithGenerics(List::class.java, WithSerializable::class.java))).isTrue()
+
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClassWithGenerics(Map::class.java, String::class.java, WithoutSerializable::class.java))).isFalse()
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.forClassWithGenerics(Map::class.java, String::class.java, WithSerializable::class.java))).isTrue()
+
+		Assertions.assertThat(KotlinDetector.hasSerializableAnnotation(ResolvableType.NONE)).isFalse()
+	}
+
 	@JvmInline
 	value class ValueClass(val value: String)
 
+	data class WithoutSerializable(val value: String)
+
+	@Serializable
+	data class WithSerializable(val value: String)
+
 }